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Re: [PATCH] coroutine-sigaltstack: Add SIGUSR2 mutex

From: Eric Blake
Subject: Re: [PATCH] coroutine-sigaltstack: Add SIGUSR2 mutex
Date: Wed, 27 Jan 2021 13:13:13 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 1/25/21 6:03 AM, 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

I indeed like that one, but also concur that simplicity trumps the
uncertainty of a larger patch.  Let's get things unbroken before we
worry about optimizing things to avoid the mutex.

> 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(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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