qemu-e2k/include/qemu/coroutine.h

328 lines
10 KiB
C
Raw Normal View History

coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
/*
* QEMU coroutine implementation
*
* Copyright IBM, Corp. 2011
*
* Authors:
* Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
* Kevin Wolf <kwolf@redhat.com>
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
*
* This work is licensed under the terms of the GNU LGPL, version 2 or later.
* See the COPYING.LIB file in the top-level directory.
*
*/
#ifndef QEMU_COROUTINE_H
#define QEMU_COROUTINE_H
#include "qemu/queue.h"
#include "qemu/timer.h"
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
/**
* Coroutines are a mechanism for stack switching and can be used for
* cooperative userspace threading. These functions provide a simple but
* useful flavor of coroutines that is suitable for writing sequential code,
* rather than callbacks, for operations that need to give up control while
* waiting for events to complete.
*
* These functions are re-entrant and may be used outside the global mutex.
*/
/**
* Mark a function that executes in coroutine context
*
* Functions that execute in coroutine context cannot be called directly from
* normal functions. In the future it would be nice to enable compiler or
* static checker support for catching such errors. This annotation might make
* it possible and in the meantime it serves as documentation.
*
* For example:
*
* static void coroutine_fn foo(void) {
* ....
* }
*/
#define coroutine_fn
typedef struct Coroutine Coroutine;
/**
* Coroutine entry point
*
* When the coroutine is entered for the first time, opaque is passed in as an
* argument.
*
* When this function returns, the coroutine is destroyed automatically and
* execution continues in the caller who last entered the coroutine.
*/
typedef void coroutine_fn CoroutineEntry(void *opaque);
/**
* Create a new coroutine
*
* Use qemu_coroutine_enter() to actually transfer control to the coroutine.
* The opaque argument is passed as the argument to the entry point.
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
*/
Coroutine *qemu_coroutine_create(CoroutineEntry *entry, void *opaque);
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
/**
* Transfer control to a coroutine
*/
void qemu_coroutine_enter(Coroutine *coroutine);
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
/**
* Transfer control to a coroutine if it's not active (i.e. part of the call
* stack of the running coroutine). Otherwise, do nothing.
*/
void qemu_coroutine_enter_if_inactive(Coroutine *co);
/**
* Transfer control to a coroutine and associate it with ctx
*/
void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co);
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
/**
* Transfer control back to a coroutine's caller
*
* This function does not return until the coroutine is re-entered using
* qemu_coroutine_enter().
*/
void coroutine_fn qemu_coroutine_yield(void);
/**
* Get the AioContext of the given coroutine
*/
AioContext *coroutine_fn qemu_coroutine_get_aio_context(Coroutine *co);
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
/**
* Get the currently executing coroutine
*/
Coroutine *coroutine_fn qemu_coroutine_self(void);
/**
* Return whether or not currently inside a coroutine
*
* This can be used to write functions that work both when in coroutine context
* and when not in coroutine context. Note that such functions cannot use the
* coroutine_fn annotation since they work outside coroutine context.
*/
bool qemu_in_coroutine(void);
/**
* Return true if the coroutine is currently entered
*
* A coroutine is "entered" if it has not yielded from the current
* qemu_coroutine_enter() call used to run it. This does not mean that the
* coroutine is currently executing code since it may have transferred control
* to another coroutine using qemu_coroutine_enter().
*
* When several coroutines enter each other there may be no way to know which
* ones have already been entered. In such situations this function can be
* used to avoid recursively entering coroutines.
*/
bool qemu_coroutine_entered(Coroutine *co);
/**
* Provides a mutex that can be used to synchronise coroutines
*/
struct CoWaitRecord;
struct CoMutex {
/* Count of pending lockers; 0 for a free mutex, 1 for an
* uncontended mutex.
*/
unsigned locked;
/* Context that is holding the lock. Useful to avoid spinning
* when two coroutines on the same AioContext try to get the lock. :)
*/
AioContext *ctx;
/* A queue of waiters. Elements are added atomically in front of
* from_push. to_pop is only populated, and popped from, by whoever
* is in charge of the next wakeup. This can be an unlocker or,
* through the handoff protocol, a locker that is about to go to sleep.
*/
QSLIST_HEAD(, CoWaitRecord) from_push, to_pop;
unsigned handoff, sequence;
Coroutine *holder;
};
/**
* Initialises a CoMutex. This must be called before any other operation is used
* on the CoMutex.
*/
void qemu_co_mutex_init(CoMutex *mutex);
/**
* Locks the mutex. If the lock cannot be taken immediately, control is
* transferred to the caller of the current coroutine.
*/
void coroutine_fn qemu_co_mutex_lock(CoMutex *mutex);
/**
* Unlocks the mutex and schedules the next coroutine that was waiting for this
* lock to be run.
*/
void coroutine_fn qemu_co_mutex_unlock(CoMutex *mutex);
/**
* Assert that the current coroutine holds @mutex.
*/
static inline coroutine_fn void qemu_co_mutex_assert_locked(CoMutex *mutex)
{
/*
* mutex->holder doesn't need any synchronisation if the assertion holds
* true because the mutex protects it. If it doesn't hold true, we still
* don't mind if another thread takes or releases mutex behind our back,
* because the condition will be false no matter whether we read NULL or
* the pointer for any other coroutine.
*/
assert(qatomic_read(&mutex->locked) &&
mutex->holder == qemu_coroutine_self());
}
/**
* CoQueues are a mechanism to queue coroutines in order to continue executing
* them later. They are similar to condition variables, but they need help
* from an external mutex in order to maintain thread-safety.
*/
typedef struct CoQueue {
QSIMPLEQ_HEAD(, Coroutine) entries;
} CoQueue;
/**
* Initialise a CoQueue. This must be called before any other operation is used
* on the CoQueue.
*/
void qemu_co_queue_init(CoQueue *queue);
/**
* Adds the current coroutine to the CoQueue and transfers control to the
* caller of the coroutine. The mutex is unlocked during the wait and
* locked again afterwards.
*/
#define qemu_co_queue_wait(queue, lock) \
qemu_co_queue_wait_impl(queue, QEMU_MAKE_LOCKABLE(lock))
void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock);
/**
* Removes the next coroutine from the CoQueue, and wake it up.
* Returns true if a coroutine was removed, false if the queue is empty.
*/
bool coroutine_fn qemu_co_queue_next(CoQueue *queue);
/**
* Empties the CoQueue; all coroutines are woken up.
*/
void coroutine_fn qemu_co_queue_restart_all(CoQueue *queue);
/**
* Removes the next coroutine from the CoQueue, and wake it up. Unlike
* qemu_co_queue_next, this function releases the lock during aio_co_wake
* because it is meant to be used outside coroutine context; in that case, the
* coroutine is entered immediately, before qemu_co_enter_next returns.
*
* If used in coroutine context, qemu_co_enter_next is equivalent to
* qemu_co_queue_next.
*/
#define qemu_co_enter_next(queue, lock) \
qemu_co_enter_next_impl(queue, QEMU_MAKE_LOCKABLE(lock))
bool qemu_co_enter_next_impl(CoQueue *queue, QemuLockable *lock);
/**
* Checks if the CoQueue is empty.
*/
bool qemu_co_queue_empty(CoQueue *queue);
coroutine-lock: Reimplement CoRwlock to fix downgrade bug An invariant of the current rwlock is that if multiple coroutines hold a reader lock, all must be runnable. The unlock implementation relies on this, choosing to wake a single coroutine when the final read lock holder exits the critical section, assuming that it will wake a coroutine attempting to acquire a write lock. The downgrade implementation violates this assumption by creating a read lock owning coroutine that is exclusively runnable - any other coroutines that are waiting to acquire a read lock are *not* made runnable when the write lock holder converts its ownership to read only. More in general, the old implementation had lots of other fairness bugs. The root cause of the bugs was that CoQueue would wake up readers even if there were pending writers, and would wake up writers even if there were readers. In that case, the coroutine would go back to sleep *at the end* of the CoQueue, losing its place at the head of the line. To fix this, keep the queue of waiters explicitly in the CoRwlock instead of using CoQueue, and store for each whether it is a potential reader or a writer. This way, downgrade can look at the first queued coroutines and wake it only if it is a reader, causing all other readers in line to be released in turn. Reported-by: David Edmondson <david.edmondson@oracle.com> Reviewed-by: David Edmondson <david.edmondson@oracle.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20210325112941.365238-5-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2021-03-25 12:29:39 +01:00
typedef struct CoRwTicket CoRwTicket;
typedef struct CoRwlock {
CoMutex mutex;
coroutine-lock: Reimplement CoRwlock to fix downgrade bug An invariant of the current rwlock is that if multiple coroutines hold a reader lock, all must be runnable. The unlock implementation relies on this, choosing to wake a single coroutine when the final read lock holder exits the critical section, assuming that it will wake a coroutine attempting to acquire a write lock. The downgrade implementation violates this assumption by creating a read lock owning coroutine that is exclusively runnable - any other coroutines that are waiting to acquire a read lock are *not* made runnable when the write lock holder converts its ownership to read only. More in general, the old implementation had lots of other fairness bugs. The root cause of the bugs was that CoQueue would wake up readers even if there were pending writers, and would wake up writers even if there were readers. In that case, the coroutine would go back to sleep *at the end* of the CoQueue, losing its place at the head of the line. To fix this, keep the queue of waiters explicitly in the CoRwlock instead of using CoQueue, and store for each whether it is a potential reader or a writer. This way, downgrade can look at the first queued coroutines and wake it only if it is a reader, causing all other readers in line to be released in turn. Reported-by: David Edmondson <david.edmondson@oracle.com> Reviewed-by: David Edmondson <david.edmondson@oracle.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20210325112941.365238-5-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2021-03-25 12:29:39 +01:00
/* Number of readers, or -1 if owned for writing. */
int owners;
/* Waiting coroutines. */
QSIMPLEQ_HEAD(, CoRwTicket) tickets;
} CoRwlock;
/**
* Initialises a CoRwlock. This must be called before any other operation
* is used on the CoRwlock
*/
void qemu_co_rwlock_init(CoRwlock *lock);
/**
* Read locks the CoRwlock. If the lock cannot be taken immediately because
* of a parallel writer, control is transferred to the caller of the current
* coroutine.
*/
void qemu_co_rwlock_rdlock(CoRwlock *lock);
/**
* Write Locks the CoRwlock from a reader. This is a bit more efficient than
* @qemu_co_rwlock_unlock followed by a separate @qemu_co_rwlock_wrlock.
coroutine-lock: Reimplement CoRwlock to fix downgrade bug An invariant of the current rwlock is that if multiple coroutines hold a reader lock, all must be runnable. The unlock implementation relies on this, choosing to wake a single coroutine when the final read lock holder exits the critical section, assuming that it will wake a coroutine attempting to acquire a write lock. The downgrade implementation violates this assumption by creating a read lock owning coroutine that is exclusively runnable - any other coroutines that are waiting to acquire a read lock are *not* made runnable when the write lock holder converts its ownership to read only. More in general, the old implementation had lots of other fairness bugs. The root cause of the bugs was that CoQueue would wake up readers even if there were pending writers, and would wake up writers even if there were readers. In that case, the coroutine would go back to sleep *at the end* of the CoQueue, losing its place at the head of the line. To fix this, keep the queue of waiters explicitly in the CoRwlock instead of using CoQueue, and store for each whether it is a potential reader or a writer. This way, downgrade can look at the first queued coroutines and wake it only if it is a reader, causing all other readers in line to be released in turn. Reported-by: David Edmondson <david.edmondson@oracle.com> Reviewed-by: David Edmondson <david.edmondson@oracle.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20210325112941.365238-5-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2021-03-25 12:29:39 +01:00
* Note that if the lock cannot be upgraded immediately, control is transferred
* to the caller of the current coroutine; another writer might run while
* @qemu_co_rwlock_upgrade blocks.
*/
void qemu_co_rwlock_upgrade(CoRwlock *lock);
/**
* Downgrades a write-side critical section to a reader. Downgrading with
* @qemu_co_rwlock_downgrade never blocks, unlike @qemu_co_rwlock_unlock
* followed by @qemu_co_rwlock_rdlock. This makes it more efficient, but
* may also sometimes be necessary for correctness.
*/
void qemu_co_rwlock_downgrade(CoRwlock *lock);
/**
* Write Locks the mutex. If the lock cannot be taken immediately because
* of a parallel reader, control is transferred to the caller of the current
* coroutine.
*/
void qemu_co_rwlock_wrlock(CoRwlock *lock);
/**
* Unlocks the read/write lock and schedules the next coroutine that was
* waiting for this lock to be run.
*/
void qemu_co_rwlock_unlock(CoRwlock *lock);
typedef struct QemuCoSleepState QemuCoSleepState;
/**
* Yield the coroutine for a given duration. During this yield, @sleep_state
* (if not NULL) is set to an opaque pointer, which may be used for
* qemu_co_sleep_wake(). Be careful, the pointer is set back to zero when the
* timer fires. Don't save the obtained value to other variables and don't call
* qemu_co_sleep_wake from another aio context.
*/
void coroutine_fn qemu_co_sleep_ns_wakeable(QEMUClockType type, int64_t ns,
QemuCoSleepState **sleep_state);
static inline void coroutine_fn qemu_co_sleep_ns(QEMUClockType type, int64_t ns)
{
qemu_co_sleep_ns_wakeable(type, ns, NULL);
}
/**
* Wake a coroutine if it is sleeping in qemu_co_sleep_ns. The timer will be
* deleted. @sleep_state must be the variable whose address was given to
* qemu_co_sleep_ns() and should be checked to be non-NULL before calling
* qemu_co_sleep_wake().
*/
void qemu_co_sleep_wake(QemuCoSleepState *sleep_state);
/**
* Yield until a file descriptor becomes readable
*
* Note that this function clobbers the handlers for the file descriptor.
*/
void coroutine_fn yield_until_fd_readable(int fd);
#include "qemu/lockable.h"
coroutine: introduce coroutines Asynchronous code is becoming very complex. At the same time synchronous code is growing because it is convenient to write. Sometimes duplicate code paths are even added, one synchronous and the other asynchronous. This patch introduces coroutines which allow code that looks synchronous but is asynchronous under the covers. A coroutine has its own stack and is therefore able to preserve state across blocking operations, which traditionally require callback functions and manual marshalling of parameters. Creating and starting a coroutine is easy: coroutine = qemu_coroutine_create(my_coroutine); qemu_coroutine_enter(coroutine, my_data); The coroutine then executes until it returns or yields: void coroutine_fn my_coroutine(void *opaque) { MyData *my_data = opaque; /* do some work */ qemu_coroutine_yield(); /* do some more work */ } Yielding switches control back to the caller of qemu_coroutine_enter(). This is typically used to switch back to the main thread's event loop after issuing an asynchronous I/O request. The request callback will then invoke qemu_coroutine_enter() once more to switch back to the coroutine. Note that if coroutines are used only from threads which hold the global mutex they will never execute concurrently. This makes programming with coroutines easier than with threads. Race conditions cannot occur since only one coroutine may be active at any time. Other coroutines can only run across yield. This coroutines implementation is based on the gtk-vnc implementation written by Anthony Liguori <anthony@codemonkey.ws> but it has been significantly rewritten by Kevin Wolf <kwolf@redhat.com> to use setjmp()/longjmp() instead of the more expensive swapcontext() and by Paolo Bonzini <pbonzini@redhat.com> for Windows Fibers support. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
2011-01-17 17:08:14 +01:00
#endif /* QEMU_COROUTINE_H */