Re: [Qemu-devel] [PATCH 1.1] coroutine: Avoid ucontext usage on i386 Linux host

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-05-09 16:27, Michael Tokarev wrote:
> On 09.05.2012 23:21, Jan Kiszka wrote:
>> On i386, glibc only saves/restores the signal mask via sigprocmask,
>> excluding RT signal. A Linux bug in the compat version of this syscall
>> corrupts the RT signal state, which will cause lockups of QEMU's VCPU
>> threads.
> 
> This should obviously be fixed in kernel, for benefit of all (not only
> qemu), do you have any details here?

compat_sys_sigprocmask reads 32-bit sigmask from user space, i.e.
excluding RT signal, but calls sys_sigprocmask that takes a 64-bit
sigset. So the RT signals are unblocked. I'm testing a simple patch ATM,
will post it to LKML once this works.

> 
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>> ---
>>
>> I'm not sure where to fall back to. The existing code uses gthread,
>> likely because it is the safer harbor. So I picked it as well.
> 
> Can't we resort to the SIGUSR1 workaround for the time being, while
> no RT signals are in actual use, and just have the time to let the
> kernel side to fix the things up before some actual RTsig user will
> emerge in qemu?  I think it is a bit more conservative approach,
> especially having in mind the minority of users this issue affects
> (only 32/64 mixed environment).  I'd favor for this variant, and
> it looks like I'm the "main" 32/64bit user of qemu in this world :)

Most conservative is definitely this patch, not switching to SIGUSR1,
hoping that no other RT signal user shows up until current kernel are no
longer in use.

Jan

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