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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