This new header file provides heavy-weight "global" memory barriers that
enforce memory ordering on each running thread belonging to the current
process. For now, use a dummy implementation that issues memory barriers
on both sides (matching what QEMU has been doing so far).
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Prepare for introducing smp_mb_placeholder() and smp_mb_global().
The new smp_mb() in synchronize_rcu() is not strictly necessary, since
the first atomic_mb_set for rcu_gp_ctr provides the required ordering.
However, synchronize_rcu is not performance critical, and it *will* be
necessary to introduce a smp_mb_global before calling wait_for_readers().
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This reverts commit a59629fcc6.
This is not needed anymore because the IOThread mutex is not
"magic" anymore (need not kick the CPU thread)and also because
fork callbacks are only enabled at the very beginning of
QEMU's execution.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Because of -daemonize, system mode QEMU sometimes needs to fork() and
keep RCU enabled in the child. However, there is a possible deadlock
with synchronize_rcu:
- the CPU thread is inside a RCU critical section and wants to take
the BQL in order to do MMIO
- the monitor thread, which is owning the BQL, calls rcu_init_lock
which tries to take the rcu_sync_lock
- the call_rcu thread has taken rcu_sync_lock in synchronize_rcu, but
synchronize_rcu needs the CPU thread to end the critical section
before returning.
This cannot happen for user-mode emulation, because it does not have
a BQL.
To fix it, assume that system mode QEMU only forks in preparation for
exec (except when daemonizing) and disable pthread_atfork as soon as
the double fork has happened.
Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Remove glib.h includes, as it is provided by osdep.h.
This commit was created with scripts/clean-includes.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Clean up includes so that osdep.h is included first and headers
which it implies are not included manually.
This commit was created with scripts/clean-includes.
NB: If this commit breaks compilation for your out-of-tree
patchseries or fork, then you need to make sure you add
#include "qemu/osdep.h" to any new .c files that you have.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
rcu_read_lock cannot change rcu_gp_ongoing from true to false
(the previous value of p_rcu_reader->ctr is zero), hence
there is no need to check p_rcu_reader->waiting and wake up
a concurrent synchronize_rcu.
While at it mark the wakeup as unlikely in rcu_read_unlock.
Reviewed-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1450265542-4323-1-git-send-email-pbonzini@redhat.com>
If QEMU forks after the CPU threads have been created, qemu_mutex_lock_iothread
will not be able to do qemu_cpu_kick_thread. There is no solution other than
assuming that forks after the CPU threads have been created will end up in an
exec. Forks before the CPU threads have been created (such as -daemonize)
have to call rcu_after_fork manually.
Notably, the oxygen theme for GTK+ forks and shows a "No such process" error
without this patch.
This patch can be reverted once the iothread loses the "kick the TCG thread"
magic.
User-mode emulation does not use the iothread, so it can also call
rcu_after_fork.
Reported by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Tested by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Asynchronous callbacks provided by call_rcu are particularly important
for QEMU, because the BQL makes it hard to use synchronize_rcu.
In addition, the current RCU implementation is not particularly friendly
to multiple concurrent synchronize_rcu callers, making call_rcu even
more important.
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This includes a (mangled) copy of the liburcu code. The main changes
are: 1) removing dependencies on many other header files in liburcu; 2)
removing for simplicity the tentative busy waiting in synchronize_rcu,
which has limited performance effects; 3) replacing futexes in
synchronize_rcu with QemuEvents for Win32 portability. The API is
the same as liburcu, so it should be possible in the future to require
liburcu on POSIX systems for example and use our copy only on Windows.
Among the various versions available I chose urcu-mb, which is the
least invasive implementation even though it does not have the
fastest rcu_read_{lock,unlock} implementation. The urcu flavor can
be changed later, after benchmarking.
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
