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

[Jan Kiszka]

On 2012-05-09 14:17, Anthony Liguori wrote:
> 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<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.
> 
> Is there any downside to using SIGUSR1?

You mean for SIG_IPI? I don't think so. But the point is that the, well,
limitation of ucontext will continue to break RT signals, and this in a
very nasty way as only a specific setup is affected. I can't imagine we
want this.

Jan

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