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Re: [PATCH] coroutine-sigaltstack: Add SIGUSR2 mutex
From: |
Laszlo Ersek |
Subject: |
Re: [PATCH] coroutine-sigaltstack: Add SIGUSR2 mutex |
Date: |
Mon, 25 Jan 2021 22:55:00 +0100 |
On 01/25/21 13:03, Max Reitz wrote:
> Disposition (action) for any given signal is global for the process.
> When two threads run coroutine-sigaltstack's qemu_coroutine_new()
> concurrently, they may interfere with each other: One of them may revert
> the SIGUSR2 handler to SIG_DFL, between the other thread (a) setting up
> coroutine_trampoline() as the handler and (b) raising SIGUSR2. That
> SIGUSR2 will then terminate the QEMU process abnormally.
>
> We have to ensure that only one thread at a time can modify the
> process-global SIGUSR2 handler. To do so, wrap the whole section where
> that is done in a mutex.
>
> Alternatively, we could for example have the SIGUSR2 handler always be
> coroutine_trampoline(), so there would be no need to invoke sigaction()
> in qemu_coroutine_new(). Laszlo has posted a patch to do so here:
>
> https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg05962.html
>
> However, given that coroutine-sigaltstack is more of a fallback
> implementation for platforms that do not support ucontext, that change
> may be a bit too invasive to be comfortable with it. The mutex proposed
> here may negatively impact performance, but the change is much simpler.
>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
> util/coroutine-sigaltstack.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/util/coroutine-sigaltstack.c b/util/coroutine-sigaltstack.c
> index aade82afb8..e99b8a4f9c 100644
> --- a/util/coroutine-sigaltstack.c
> +++ b/util/coroutine-sigaltstack.c
> @@ -157,6 +157,7 @@ Coroutine *qemu_coroutine_new(void)
> sigset_t sigs;
> sigset_t osigs;
> sigjmp_buf old_env;
> + static pthread_mutex_t sigusr2_mutex = PTHREAD_MUTEX_INITIALIZER;
>
> /* The way to manipulate stack is with the sigaltstack function. We
> * prepare a stack, with it delivering a signal to ourselves and then
> @@ -186,6 +187,12 @@ Coroutine *qemu_coroutine_new(void)
> sa.sa_handler = coroutine_trampoline;
> sigfillset(&sa.sa_mask);
> sa.sa_flags = SA_ONSTACK;
> +
> + /*
> + * sigaction() is a process-global operation. We must not run
> + * this code in multiple threads at once.
> + */
> + pthread_mutex_lock(&sigusr2_mutex);
> if (sigaction(SIGUSR2, &sa, &osa) != 0) {
> abort();
> }
> @@ -234,6 +241,8 @@ Coroutine *qemu_coroutine_new(void)
> * Restore the old SIGUSR2 signal handler and mask
> */
> sigaction(SIGUSR2, &osa, NULL);
> + pthread_mutex_unlock(&sigusr2_mutex);
> +
> pthread_sigmask(SIG_SETMASK, &osigs, NULL);
>
> /*
>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Thanks!
Laszlo