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
