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

From: Jan Kiszka
Subject: Re: [Qemu-devel] coroutine-ucontext broken for x86-32
Date: Wed, 09 May 2012 07:55:52 -0300
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

On 2012-05-09 07:11, Kevin Wolf wrote:
> Am 08.05.2012 21:35, schrieb Jan Kiszka:
>> Hi,
>> 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?),

Had a brief look at signalstack. Is kill(getpid(), SIGUSR2) in
coroutine_new correct? This should broadcast to the whole process, but
we only block SIGUSR2 in the caller's context. Why not pthread_kill?

I will give it a try nevertheless.

> we could try to detect
> the situation in configure and set the right default. Not sure what the
> condition is, glibc + i386?

So far it looks like a glibc/i386 thing, but maybe it is driven by the
ABI that defines the size of jmpbuf and, thus, the ability to store RT
signals as well. Then it is a generic i386 limitation.


Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux

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