Re: [Qemu-devel] [PATCH] linux-user: Fix locking order in fork_start()

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> 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
> ---
>  linux-user/main.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/linux-user/main.c b/linux-user/main.c
> index 6286661..146ee3e 100644
> --- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -128,9 +128,9 @@ 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)

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

--
Alex Bennée