linux-kernel - [PATCH v3 14/14] uprobes: fix the waitqueue_active() check in xol_free_insn

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Message-ID: <20150721134036.GA4799@redhat.com>
Date:	Tue, 21 Jul 2015 15:40:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Pratyush Anand <panand@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3 14/14] 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>
---
 kernel/events/uprobes.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

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);
 
-- 
1.5.5.1

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