Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755816AbcDNNg2 (ORCPT ); Thu, 14 Apr 2016 09:36:28 -0400 Received: from down.free-electrons.com ([37.187.137.238]:49545 "EHLO mail.free-electrons.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1755491AbcDNNg0 (ORCPT ); Thu, 14 Apr 2016 09:36:26 -0400 From: Romain Perier To: Arnd Bergmann Cc: Ingo Molnar , Thomas Gleixner , Peter Zijlstra , linux-arch@vger.kernel.org, stable@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Petazzoni Subject: [PATCH] asm-generic/futex: Properly re-enable preemption in futex_atomic_cmpxchg_inatomic() Date: Thu, 14 Apr 2016 15:36:03 +0200 Message-Id: <1460640963-690-1-git-send-email-romain.perier@free-electrons.com> X-Mailer: git-send-email 2.8.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4680 Lines: 124 The futex subsystem relies on futex_atomic_cmpxchg_inatomic() to perform an atomic cmpxchg. Most architectures provide their own implementation. For those who do not, provides a generic software implementations that works on !SMP platforms. To provide an atomicity guarantee on !SMP plaforms, the generic implementation simply disables preemption. However it is not properly re-enabled from conditional branches. The observed consequence of this bug is that the preemption counter is not well balanced. It can be seen at boot time : [ 8.148000] futex hash table entries: 256 (order: -1, 3072 bytes) [ 8.156000] ------------[ cut here ]------------ [ 8.164000] WARNING: CPU: 0 PID: 1 at init/main.c:806 do_one_initcall+0x1ac/0x1d0() [ 8.172000] initcall futex_init+0x0/0xd8 returned with preemption imbalance [ 8.180000] Modules linked in: [ 8.184000] CPU: 0 PID: 1 Comm: swapper Not tainted 4.5.0 #72 [ 8.192000] Stack from d1c1deb8: [ 8.196000] [ 8.196000] d1c1debc [ 8.204000] d1c1defc 00000009 [ 8.204000] d04d8b90 00000326 [ 8.208000] d0000380 d0136484 [ 8.212000] d00093b4 [ 8.216000] 00000326 d0000380 [ 8.220000] d1c1c000 00000000 [ 8.224000] d026c64c 00000000 [ 8.228000] d0515004 d0009410 [ 8.232000] [ 8.232000] d1c1defc [ 8.236000] d04d8d8c d1c1df08 [ 8.240000] d0000380 d026c64c [ 8.244000] d1c1df10 65657270 [ 8.248000] 6974706d [ 8.252000] 69206e6f 6c61626d [ 8.256000] 65636e61 d0780020 [ 8.260000] d05108a0 00000001 [ 8.264000] d00225c8 00000000 [ 8.268000] [ 8.268000] d05103c8 [ 8.272000] 0000003f 00060006 [ 8.276000] d05108a0 00000006 [ 8.280000] 0000003f 00000007 [ 8.284000] 0000003f [ 8.288000] d052501c d0525024 [ 8.288000] d027d480 d0266c78 [ 8.292000] 00000006 00000006 [ 8.296000] 00000000 d026627c [ 8.300000] [ 8.300000] Call Trace: [ 8.308000] [] [] [ 8.312000] [] [] [ 8.316000] [ 8.316000] ---[ end trace 2d3fe47ac46f91f6 ]--- Before commit 8222dbe21e79 ("sched/preempt, mm/fault: Decouple preemption from the page fault logic") , the preemption was disabled/re-enabled by the pagefault_disable()/pagefault_enable() functions. It is no longer the case. Which means that cmpxchg_futex_value_locked(), in kernel/futex.c, no longer disables preemption before calling futex_atomic_cmpxchg_inatomic. Disabling preemption in the generic implementation was added by commit d9b9ff8c1889 ("sched/preempt, futex: Disable preemption in UP futex_atomic_cmpxchg_inatomic() explicitly"). However, this change is bogus: preemption is not re-enabled in the two failure cases. Indeed, At boot time futex_detect_cmpxchg(), in kernel/futex.c, checks if futex_atomic_cmpxchg_inatomic() operation is provided by the current architecture. To do so, it calls futex_atomic_cmpxchg_inatomic() with a NULL pointer as the futex, and expects -EFAULT as a return value. The reasoning if that if -EFAULT is returned, then there is a valid futex_atomic_cmpxchg_inatomic() implementation, while if -ENOSYS is returned, then it means that there is no implementation of this function for the current architecture. By doing this test based on a NULL pointer, we fall into the case where the get_user() in the asm-generic version of futex_atomic_cmpxchg_inatomic() fails, and returns -EFAULT without re-enabling the preemption, causing the preemption unbalance in the futex_init() initcall. This bug was not found, because most of the architectures in the kernel use their own implementation. This is not the case for the nios2 platform. By using a nios2 board without SMP support, this bug can be easily reproduced. Fixes: d9b9ff8c1889 ("sched/preempt, futex: Disable preemption in UP futex_atomic_cmpxchg_inatomic() explicitly") Signed-off-by: Romain Perier --- include/asm-generic/futex.h | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/include/asm-generic/futex.h b/include/asm-generic/futex.h index e56272c..bf2d34c 100644 --- a/include/asm-generic/futex.h +++ b/include/asm-generic/futex.h @@ -108,11 +108,15 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr, u32 val; preempt_disable(); - if (unlikely(get_user(val, uaddr) != 0)) + if (unlikely(get_user(val, uaddr) != 0)) { + preempt_enable(); return -EFAULT; + } - if (val == oldval && unlikely(put_user(newval, uaddr) != 0)) + if (val == oldval && unlikely(put_user(newval, uaddr) != 0)) { + preempt_enable(); return -EFAULT; + } *uval = val; preempt_enable(); -- 2.8.0