From mboxrd@z Thu Jan 1 00:00:00 1970 From: Helge Deller Subject: Re: Aw: Re: [RFC PATCH urcu on mips, parisc] Fix: compat_futex should work-around futex signal-restart kernel bug Date: Sat, 19 Dec 2015 11:37:04 +0100 Message-ID: <56753350.2060906@gmx.de> References: <1450303792-27470-1-git-send-email-mathieu.desnoyers@efficios.com> <663068619.259007.1450356852694.JavaMail.zimbra@efficios.com> <1817225945.264082.1450468696180.JavaMail.zimbra@efficios.com> <56746FC6.10904@gmx.de> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: "Paul E. McKenney" , Jon Bernard , Michael Jeanson , Ralf Baechle , linux-mips , linux-kernel@vger.kernel.org, "James E.J. Bottomley" , linux-parisc , Ed Swierk , Greg Kroah-Hartman To: Mathieu Desnoyers Return-path: In-Reply-To: <56746FC6.10904@gmx.de> List-ID: List-Id: linux-parisc.vger.kernel.org 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