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linux-user: Fix locking order in fork_start()

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Our locking order is that the tb lock should be taken
inside the mmap_lock, but fork_start() grabs locks the
other way around. This means that if a heavily multithreaded
guest process (such as Java) calls fork() it can deadlock,
with the thread that called fork() stuck in fork_start()
with the tb lock and waiting for the mmap lock, but some
other thread in tb_find() with the mmap lock and waiting
for the tb lock. The cpu_list_lock() should also always be
taken last, not first.

Fix this by making fork_start() grab the locks in the
right order. The order in which we drop locks doesn't
matter, so we leave fork_end() the way it is.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-stable@nongnu.org
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <1512397331-15238-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
This commit is contained in:
Peter Maydell 2017-12-04 14:22:11 +00:00 committed by Laurent Vivier
parent 52483b067c
commit 024949caf3
1 changed files with 2 additions and 2 deletions
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4
linux-user/main.c
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@ -127,9 +127,9 @@ int cpu_get_pic_interrupt(CPUX86State *env)
/* Make sure everything is in a consistent state for calling fork(). */
void fork_start(void)
{
cpu_list_lock();
qemu_mutex_lock(&tb_ctx.tb_lock);
mmap_fork_start();
qemu_mutex_lock(&tb_ctx.tb_lock);
cpu_list_lock();
}
void fork_end(int child)