----- On Dec 19, 2015, at 5:37 AM, Helge Deller deller@xxxxxx wrote: > 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. If you ran make check and make regtest, and nothing fails/hangs, you should be OK. liburcu runs very heavy stress-tests which makes it likely to hit race conditions repeatedly. Thanks! Mathieu > > Helge -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
