On Mon, Jan 25, 2021 at 01:03:05PM +0100, 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(+)

I slightly prefer Laszlo's patch: since the signal disposition is
process-wide it's cleaner to set it up globally and once only. That
said, this patch is okay too.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>