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linux-user: wrap fork() in a start/end exclusive section

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When we do a fork() in usermode emulation, we need to be in
a start/end exclusive section, so that we can ensure that no
other thread is in an RCU section. Otherwise you can get this
deadlock:

- fork thread: has mmap_lock, waits for rcu_sync_lock
  (because rcu_init_lock() is registered as a pthread_atfork() hook)
- RCU thread: has rcu_sync_lock, waits for rcu_read_(un)lock
- another CPU thread: in RCU critical section, waits for mmap_lock

This can show up if you have a heavily multithreaded guest program
that does a fork().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Stuart Monteith <stuart.monteith@linaro.org>
Message-Id: <1512650481-1723-1-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
This commit is contained in:
Peter Maydell 2017-12-07 12:41:21 +00:00 committed by Laurent Vivier
parent 024949caf3
commit 06065c451f
1 changed files with 5 additions and 0 deletions
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5
linux-user/main.c
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@ -127,6 +127,7 @@ int cpu_get_pic_interrupt(CPUX86State *env)
/* Make sure everything is in a consistent state for calling fork(). */
void fork_start(void)
{
start_exclusive();
mmap_fork_start();
qemu_mutex_lock(&tb_ctx.tb_lock);
cpu_list_lock();
@ -147,9 +148,13 @@ void fork_end(int child)
qemu_mutex_init(&tb_ctx.tb_lock);
qemu_init_cpu_list();
gdbserver_fork(thread_cpu);
/* qemu_init_cpu_list() takes care of reinitializing the
* exclusive state, so we don't need to end_exclusive() here.
*/
} else {
qemu_mutex_unlock(&tb_ctx.tb_lock);
cpu_list_unlock();
end_exclusive();
}
}