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qemu-e2k/include/qemu/atomic.h
Emilio G. Cota 15487aa132 atomics: do not emit consume barrier for atomic_rcu_read 
Currently we emit a consume-load in atomic_rcu_read.  Because of
limitations in current compilers, this is overkill for non-Alpha hosts
and it is only useful to make Thread Sanitizer work.

This patch leaves the consume-load in atomic_rcu_read when
compiling with Thread Sanitizer enabled, and resorts to a
relaxed load + smp_read_barrier_depends otherwise.

On an RMO host architecture, such as aarch64, the performance
improvement of this change is easily measurable. For instance,
qht-bench performs an atomic_rcu_read on every lookup. Performance
before and after applying this patch:

$ tests/qht-bench -d 5 -n 1
Before: 9.78 MT/s
After:  10.96 MT/s

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1464120374-8950-4-git-send-email-cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2016-05-29 09:11:11 +02:00

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/*
* Simple interface for atomic operations.
*
* Copyright (C) 2013 Red Hat, Inc.
*
* Author: Paolo Bonzini <pbonzini@redhat.com>
*
* This work is licensed under the terms of the GNU GPL, version 2 or later.
* See the COPYING file in the top-level directory.
*
* See docs/atomics.txt for discussion about the guarantees each
* atomic primitive is meant to provide.
*/
#ifndef __QEMU_ATOMIC_H
#define __QEMU_ATOMIC_H 1
/* Compiler barrier */
#define barrier() ({ asm volatile("" ::: "memory"); (void)0; })
#ifdef __ATOMIC_RELAXED
/* For C11 atomic ops */
/* Manual memory barriers
*
*__atomic_thread_fence does not include a compiler barrier; instead,
* the barrier is part of __atomic_load/__atomic_store's "volatile-like"
* semantics. If smp_wmb() is a no-op, absence of the barrier means that
* the compiler is free to reorder stores on each side of the barrier.
* Add one here, and similarly in smp_rmb() and smp_read_barrier_depends().
*/
#define smp_mb() ({ barrier(); __atomic_thread_fence(__ATOMIC_SEQ_CST); barrier(); })
#define smp_wmb() ({ barrier(); __atomic_thread_fence(__ATOMIC_RELEASE); barrier(); })
#define smp_rmb() ({ barrier(); __atomic_thread_fence(__ATOMIC_ACQUIRE); barrier(); })
/* Most compilers currently treat consume and acquire the same, but really
* no processors except Alpha need a barrier here. Leave it in if
* using Thread Sanitizer to avoid warnings, otherwise optimize it away.
*/
#if defined(__SANITIZE_THREAD__)
#define smp_read_barrier_depends() ({ barrier(); __atomic_thread_fence(__ATOMIC_CONSUME); barrier(); })
#elsif defined(__alpha__)
#define smp_read_barrier_depends() asm volatile("mb":::"memory")
#else
#define smp_read_barrier_depends() barrier()
#endif
/* Weak atomic operations prevent the compiler moving other
* loads/stores past the atomic operation load/store. However there is
* no explicit memory barrier for the processor.
*/
#define atomic_read(ptr) \
({ \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _val; \
__atomic_load(ptr, &_val, __ATOMIC_RELAXED); \
_val; \
})
#define atomic_set(ptr, i) do { \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _val = (i); \
__atomic_store(ptr, &_val, __ATOMIC_RELAXED); \
} while(0)
/* See above: most compilers currently treat consume and acquire the
* same, but this slows down atomic_rcu_read unnecessarily.
*/
#ifdef __SANITIZE_THREAD__
#define atomic_rcu_read__nocheck(ptr, valptr) \
__atomic_load(ptr, valptr, __ATOMIC_CONSUME);
#else
#define atomic_rcu_read__nocheck(ptr, valptr) \
__atomic_load(ptr, valptr, __ATOMIC_RELAXED); \
smp_read_barrier_depends();
#endif
#define atomic_rcu_read(ptr) \
({ \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _val; \
atomic_rcu_read__nocheck(ptr, &_val); \
_val; \
})
#define atomic_rcu_set(ptr, i) do { \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _val = (i); \
__atomic_store(ptr, &_val, __ATOMIC_RELEASE); \
} while(0)
/* atomic_mb_read/set semantics map Java volatile variables. They are
* less expensive on some platforms (notably POWER & ARMv7) than fully
* sequentially consistent operations.
*
* As long as they are used as paired operations they are safe to
* use. See docs/atomic.txt for more discussion.
*/
#if defined(_ARCH_PPC)
#define atomic_mb_read(ptr) \
({ \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _val; \
__atomic_load(ptr, &_val, __ATOMIC_RELAXED); \
smp_rmb(); \
_val; \
})
#define atomic_mb_set(ptr, i) do { \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _val = (i); \
smp_wmb(); \
__atomic_store(ptr, &_val, __ATOMIC_RELAXED); \
smp_mb(); \
} while(0)
#else
#define atomic_mb_read(ptr) \
({ \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _val; \
__atomic_load(ptr, &_val, __ATOMIC_SEQ_CST); \
_val; \
})
#define atomic_mb_set(ptr, i) do { \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _val = (i); \
__atomic_store(ptr, &_val, __ATOMIC_SEQ_CST); \
} while(0)
#endif
/* All the remaining operations are fully sequentially consistent */
#define atomic_xchg(ptr, i) ({ \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _new = (i), _old; \
__atomic_exchange(ptr, &_new, &_old, __ATOMIC_SEQ_CST); \
_old; \
})
/* Returns the eventual value, failed or not */
#define atomic_cmpxchg(ptr, old, new) \
({ \
QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
typeof(*ptr) _old = (old), _new = (new); \
__atomic_compare_exchange(ptr, &_old, &_new, false, \
__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); \
_old; \
})
/* Provide shorter names for GCC atomic builtins, return old value */
#define atomic_fetch_inc(ptr) __atomic_fetch_add(ptr, 1, __ATOMIC_SEQ_CST)
#define atomic_fetch_dec(ptr) __atomic_fetch_sub(ptr, 1, __ATOMIC_SEQ_CST)
#define atomic_fetch_add(ptr, n) __atomic_fetch_add(ptr, n, __ATOMIC_SEQ_CST)
#define atomic_fetch_sub(ptr, n) __atomic_fetch_sub(ptr, n, __ATOMIC_SEQ_CST)
#define atomic_fetch_and(ptr, n) __atomic_fetch_and(ptr, n, __ATOMIC_SEQ_CST)
#define atomic_fetch_or(ptr, n) __atomic_fetch_or(ptr, n, __ATOMIC_SEQ_CST)
/* And even shorter names that return void. */
#define atomic_inc(ptr) ((void) __atomic_fetch_add(ptr, 1, __ATOMIC_SEQ_CST))
#define atomic_dec(ptr) ((void) __atomic_fetch_sub(ptr, 1, __ATOMIC_SEQ_CST))
#define atomic_add(ptr, n) ((void) __atomic_fetch_add(ptr, n, __ATOMIC_SEQ_CST))
#define atomic_sub(ptr, n) ((void) __atomic_fetch_sub(ptr, n, __ATOMIC_SEQ_CST))
#define atomic_and(ptr, n) ((void) __atomic_fetch_and(ptr, n, __ATOMIC_SEQ_CST))
#define atomic_or(ptr, n) ((void) __atomic_fetch_or(ptr, n, __ATOMIC_SEQ_CST))
#else /* __ATOMIC_RELAXED */
/*
* We use GCC builtin if it's available, as that can use mfence on
* 32-bit as well, e.g. if built with -march=pentium-m. However, on
* i386 the spec is buggy, and the implementation followed it until
* 4.3 (http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36793).
*/
#if defined(__i386__) || defined(__x86_64__)
#if !QEMU_GNUC_PREREQ(4, 4)
#if defined __x86_64__
#define smp_mb() ({ asm volatile("mfence" ::: "memory"); (void)0; })
#else
#define smp_mb() ({ asm volatile("lock; addl $0,0(%%esp) " ::: "memory"); (void)0; })
#endif
#endif
#endif
#ifdef __alpha__
#define smp_read_barrier_depends() asm volatile("mb":::"memory")
#endif
#if defined(__i386__) || defined(__x86_64__) || defined(__s390x__)
/*
* Because of the strongly ordered storage model, wmb() and rmb() are nops
* here (a compiler barrier only). QEMU doesn't do accesses to write-combining
* qemu memory or non-temporal load/stores from C code.
*/
#define smp_wmb() barrier()
#define smp_rmb() barrier()
/*
* __sync_lock_test_and_set() is documented to be an acquire barrier only,
* but it is a full barrier at the hardware level. Add a compiler barrier
* to make it a full barrier also at the compiler level.
*/
#define atomic_xchg(ptr, i) (barrier(), __sync_lock_test_and_set(ptr, i))
/*
* Load/store with Java volatile semantics.
*/
#define atomic_mb_set(ptr, i) ((void)atomic_xchg(ptr, i))
#elif defined(_ARCH_PPC)
/*
* We use an eieio() for wmb() on powerpc. This assumes we don't
* need to order cacheable and non-cacheable stores with respect to
* each other.
*
* smp_mb has the same problem as on x86 for not-very-new GCC
* (http://patchwork.ozlabs.org/patch/126184/, Nov 2011).
*/
#define smp_wmb() ({ asm volatile("eieio" ::: "memory"); (void)0; })
#if defined(__powerpc64__)
#define smp_rmb() ({ asm volatile("lwsync" ::: "memory"); (void)0; })
#else
#define smp_rmb() ({ asm volatile("sync" ::: "memory"); (void)0; })
#endif
#define smp_mb() ({ asm volatile("sync" ::: "memory"); (void)0; })
#endif /* _ARCH_PPC */
/*
* For (host) platforms we don't have explicit barrier definitions
* for, we use the gcc __sync_synchronize() primitive to generate a
* full barrier. This should be safe on all platforms, though it may
* be overkill for smp_wmb() and smp_rmb().
*/
#ifndef smp_mb
#define smp_mb() __sync_synchronize()
#endif
#ifndef smp_wmb
#define smp_wmb() __sync_synchronize()
#endif
#ifndef smp_rmb
#define smp_rmb() __sync_synchronize()
#endif
#ifndef smp_read_barrier_depends
#define smp_read_barrier_depends() barrier()
#endif
/* These will only be atomic if the processor does the fetch or store
* in a single issue memory operation
*/
#define atomic_read(ptr) (*(__typeof__(*ptr) volatile*) (ptr))
#define atomic_set(ptr, i) ((*(__typeof__(*ptr) volatile*) (ptr)) = (i))
/**
* atomic_rcu_read - reads a RCU-protected pointer to a local variable
* into a RCU read-side critical section. The pointer can later be safely
* dereferenced within the critical section.
*
* This ensures that the pointer copy is invariant thorough the whole critical
* section.
*
* Inserts memory barriers on architectures that require them (currently only
* Alpha) and documents which pointers are protected by RCU.
*
* atomic_rcu_read also includes a compiler barrier to ensure that
* value-speculative optimizations (e.g. VSS: Value Speculation
* Scheduling) does not perform the data read before the pointer read
* by speculating the value of the pointer.
*
* Should match atomic_rcu_set(), atomic_xchg(), atomic_cmpxchg().
*/
#define atomic_rcu_read(ptr) ({ \
typeof(*ptr) _val = atomic_read(ptr); \
smp_read_barrier_depends(); \
_val; \
})
/**
* atomic_rcu_set - assigns (publicizes) a pointer to a new data structure
* meant to be read by RCU read-side critical sections.
*
* Documents which pointers will be dereferenced by RCU read-side critical
* sections and adds the required memory barriers on architectures requiring
* them. It also makes sure the compiler does not reorder code initializing the
* data structure before its publication.
*
* Should match atomic_rcu_read().
*/
#define atomic_rcu_set(ptr, i) do { \
smp_wmb(); \
atomic_set(ptr, i); \
} while (0)
/* These have the same semantics as Java volatile variables.
* See http://gee.cs.oswego.edu/dl/jmm/cookbook.html:
* "1. Issue a StoreStore barrier (wmb) before each volatile store."
* 2. Issue a StoreLoad barrier after each volatile store.
* Note that you could instead issue one before each volatile load, but
* this would be slower for typical programs using volatiles in which
* reads greatly outnumber writes. Alternatively, if available, you
* can implement volatile store as an atomic instruction (for example
* XCHG on x86) and omit the barrier. This may be more efficient if
* atomic instructions are cheaper than StoreLoad barriers.
* 3. Issue LoadLoad and LoadStore barriers after each volatile load."
*
* If you prefer to think in terms of "pairing" of memory barriers,
* an atomic_mb_read pairs with an atomic_mb_set.
*
* And for the few ia64 lovers that exist, an atomic_mb_read is a ld.acq,
* while an atomic_mb_set is a st.rel followed by a memory barrier.
*
* These are a bit weaker than __atomic_load/store with __ATOMIC_SEQ_CST
* (see docs/atomics.txt), and I'm not sure that __ATOMIC_ACQ_REL is enough.
* Just always use the barriers manually by the rules above.
*/
#define atomic_mb_read(ptr) ({ \
typeof(*ptr) _val = atomic_read(ptr); \
smp_rmb(); \
_val; \
})
#ifndef atomic_mb_set
#define atomic_mb_set(ptr, i) do { \
smp_wmb(); \
atomic_set(ptr, i); \
smp_mb(); \
} while (0)
#endif
#ifndef atomic_xchg
#if defined(__clang__)
#define atomic_xchg(ptr, i) __sync_swap(ptr, i)
#else
/* __sync_lock_test_and_set() is documented to be an acquire barrier only. */
#define atomic_xchg(ptr, i) (smp_mb(), __sync_lock_test_and_set(ptr, i))
#endif
#endif
/* Provide shorter names for GCC atomic builtins. */
#define atomic_fetch_inc(ptr) __sync_fetch_and_add(ptr, 1)
#define atomic_fetch_dec(ptr) __sync_fetch_and_add(ptr, -1)
#define atomic_fetch_add __sync_fetch_and_add
#define atomic_fetch_sub __sync_fetch_and_sub
#define atomic_fetch_and __sync_fetch_and_and
#define atomic_fetch_or __sync_fetch_and_or
#define atomic_cmpxchg __sync_val_compare_and_swap
/* And even shorter names that return void. */
#define atomic_inc(ptr) ((void) __sync_fetch_and_add(ptr, 1))
#define atomic_dec(ptr) ((void) __sync_fetch_and_add(ptr, -1))
#define atomic_add(ptr, n) ((void) __sync_fetch_and_add(ptr, n))
#define atomic_sub(ptr, n) ((void) __sync_fetch_and_sub(ptr, n))
#define atomic_and(ptr, n) ((void) __sync_fetch_and_and(ptr, n))
#define atomic_or(ptr, n) ((void) __sync_fetch_and_or(ptr, n))
#endif /* __ATOMIC_RELAXED */
#endif /* __QEMU_ATOMIC_H */
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