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

[Peter Maydell]

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.

-- PMM