Re: Aw: Re: [RFC PATCH urcu on mips, parisc] Fix: compat_futex should work-around futex signal-restart kernel bug

[Helge Deller <deller@gmx.de>]

Hi Mathieu,

On 18.12.2015 21:42, Helge Deller wrote:
> On 18.12.2015 20:58, Mathieu Desnoyers wrote:
>>>>> When testing liburcu on a 3.18 Linux kernel, 2-core MIPS (cpu model :
>>>>> Ingenic JZRISC V4.15  FPU V0.0), we notice that a blocked sys_futex
>>>>> FUTEX_WAIT returns -1, errno=ENOSYS when interrupted by a SA_RESTART
>>>>> signal handler. This spurious ENOSYS behavior causes hangs in liburcu
>>>>> 0.9.x. Running a MIPS 3.18 kernel under a QEMU emulator exhibits the
>>>>> same behavior. This might affect earlier kernels.
>>>>>
>>>>> This issue appears to be fixed in 3.18.y stable kernels and 3.19, but
>>>>> nevertheless, we should try to handle this kernel bug more gracefully
>>>>> than a user-space hang due to unexpected spurious ENOSYS return value.
>>>>
>>>> It's actually fixed in 3.19, but not in 3.18.y stable kernels. The
>>>> Linux kernel upstream fix commit is:
>>>> e967ef02 "MIPS: Fix restart of indirect syscalls"

>> Looks like parisc has an issue very similar to the one that
>> has been fixed on MIPS by e967ef02 "MIPS: Fix restart of indirect syscalls".

Yes, parisc is affected the same way.
I've posted a patch to the parisc mailing list which fixes this issue for
parisc and which I plan to push into stable kernels:
http://thread.gmane.org/gmane.linux.ports.parisc/26243

Regarding your patch for liburcu:

>>>>> Therefore, fallback on the "async-safe" version of compat_futex in those
>>>>> situations where FUTEX_WAIT returns ENOSYS. This async-safe fallback has
>>>>> the nice property of being OK to use concurrently with other FUTEX_WAKE
>>>>> and FUTEX_WAIT futex() calls, because it's simply a busy-wait scheme.

I've tested your patch. It does not produce any regressions on parisc, but I can't
say for sure if it really works. ENOSYS is returned randomly, so maybe I didn't
faced a situation where your patch actually was used.

Helge