Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1166145AbdD2Ccs (ORCPT ); Fri, 28 Apr 2017 22:32:48 -0400 Received: from mail-pg0-f52.google.com ([74.125.83.52]:33607 "EHLO mail-pg0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1164683AbdD2Cck (ORCPT ); Fri, 28 Apr 2017 22:32:40 -0400 From: Junaid Shahid To: linux-kernel@vger.kernel.org Cc: andreslc@google.com, gthelen@google.com, rusty@rustcorp.com.au Subject: [PATCH] kthread: Fix race condition between kthread_parkme() and kthread_unpark() Date: Fri, 28 Apr 2017 19:32:36 -0700 Message-Id: <20170429023236.60452-1-junaids@google.com> X-Mailer: git-send-email 2.13.0.rc0.306.g87b477812d-goog Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1756 Lines: 50 In general, if kthread_unpark() and kthread_parkme() execute together, the kthread is supposed to be in an unparked state. This is because kthread_unpark() either wakes up the thread if it already got parked, or it cancels a prior kthread_park() call and hence renders the kthread_parkme() moot. However, if kthread_unpark() happens to execute between the time that kthread_parkme() checks the KTHREAD_SHOULD_STOP flag and sets the KTHREAD_IS_PARKED flag, then kthread_unpark() will not wake up the thread and it will remain in a parked state. This is fixed by making the checking of KTHREAD_SHOULD_STOP and the setting of KTHREAD_IS_PARKED atomic via a cmpxchg inside kthread_parkme. Signed-off-by: Junaid Shahid --- kernel/kthread.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index 26db528c1d88..651f03baf62f 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -169,12 +169,20 @@ void *kthread_probe_data(struct task_struct *task) static void __kthread_parkme(struct kthread *self) { + ulong flags; + __set_current_state(TASK_PARKED); - while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) { - if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags)) - complete(&self->parked); - schedule(); + flags = self->flags; + + while (test_bit(KTHREAD_SHOULD_PARK, &flags)) { + if (cmpxchg(&self->flags, flags, + flags | (1 << KTHREAD_IS_PARKED)) == flags) { + if (!test_bit(KTHREAD_IS_PARKED, &flags)) + complete(&self->parked); + schedule(); + } __set_current_state(TASK_PARKED); + flags = self->flags; } clear_bit(KTHREAD_IS_PARKED, &self->flags); __set_current_state(TASK_RUNNING); -- 2.13.0.rc0.306.g87b477812d-goog