Re: [Qemu-devel] coroutine-ucontext broken for x86-32

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/09/2012 06:38 AM, Jan Kiszka wrote:
> On 2012-05-09 08:15, Peter Maydell wrote:
>> On 9 May 2012 11:11, Kevin Wolf<kwolf@redhat.com>  wrote:
>>> Am 08.05.2012 21:35, schrieb Jan Kiszka:
>>>> I hunted down a fairly subtle corruption of the VCPU thread signal mask
>>>> in KVM mode when using the ucontext version of coroutines:
>>>>
>>>> coroutine_new calls getcontext, makecontext, swapcontext. Those
>>>> functions get/set also the signal mask of the caller. Unfortunately,
>>>> they only use the sigprocmask syscall on i386, not the rt_sigprocmask
>>>> version. So they do not properly save/restore the blocked RT signals,
>>>> namely our SIG_IPI - it becomes unblocke this way.
>>>
>>> If other coroutine backends work (sigaltstack?), we could try to detect
>>> the situation in configure and set the right default. Not sure what the
>>> condition is, glibc + i386?
>>
>> I don't think you can do a compile-time test for this short of
>> just disabling use of the ucontext code on all i386/Linux platforms.
>>
>> I think it's becoming increasingly obvious that the setcontext/getcontext
>> code path is not very well used and prone to nasty libc bugs. Trying
>> to implement coroutines in C is just a really bad idea and I think
>> we should be trying to reduce our use of them if we possibly can,
>> presumably by switching to actually using threads where we really
>> need the parallelism.
>
> I tend to agree.
>
> FWIW, sigaltstack works around the issue here, but I'm still looking s
> bit skeptical at its implementation.

Is there any downside to using SIGUSR1?

Regards,

Anthony Liguori

>
> Jan
>