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 <mre...@redhat.com> > --- > util/coroutine-sigaltstack.c | 9 +++++++++ > 1 file changed, 9 insertions(+) Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
