linux-kernel - [tip:perf/core] uprobes: Fix the waitqueue_active() check in xol_free_insn

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Date:	Fri, 31 Jul 2015 07:02:07 -0700
From:	tip-bot for Oleg Nesterov <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	oleg@...hat.com, srikar@...ux.vnet.ibm.com, panand@...hat.com,
	torvalds@...ux-foundation.org, tglx@...utronix.de,
	luto@...capital.net, linux-kernel@...r.kernel.org,
	mingo@...nel.org, peterz@...radead.org, hpa@...or.com
Subject: [tip:perf/core] uprobes: Fix the waitqueue_active()
  check in xol_free_insn_slot()

Commit-ID:  2a742cedcf13572999436676cbe36c3a9b733b0f
Gitweb:     http://git.kernel.org/tip/2a742cedcf13572999436676cbe36c3a9b733b0f
Author:     Oleg Nesterov <oleg@...hat.com>
AuthorDate: Tue, 21 Jul 2015 15:40:36 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 31 Jul 2015 10:38:07 +0200

uprobes: Fix the waitqueue_active() check in xol_free_insn_slot()

The xol_free_insn_slot()->waitqueue_active() check is buggy. We
need mb() after we set the conditon for wait_event(), or
xol_take_insn_slot() can miss the wakeup.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Pratyush Anand <panand@...hat.com>
Cc: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20150721134036.GA4799@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/events/uprobes.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 2d5b7bd..4e5e979 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1337,6 +1337,7 @@ static void xol_free_insn_slot(struct task_struct *tsk)
 
 		clear_bit(slot_nr, area->bitmap);
 		atomic_dec(&area->slot_count);
+		smp_mb__after_atomic(); /* pairs with prepare_to_wait() */
 		if (waitqueue_active(&area->wq))
 			wake_up(&area->wq);
 
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