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

[Jan Kiszka]

On 2012-05-09 08:15, Peter Maydell wrote:
> On 9 May 2012 11:11, Kevin Wolf <address@hidden> 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.

Jan

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