linux-kernel - [PATCH 3/7] staging: sync: Fix a race condition between release_obj and print

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Message-Id: <1391558920-31590-4-git-send-email-john.stultz@linaro.org>
Date:	Tue,  4 Feb 2014 16:08:36 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Alistair Strachan <alistair.strachan@...tec.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Colin Cross <ccross@...roid.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	John Stultz <john.stultz@...aro.org>
Subject: [PATCH 3/7] staging: sync: Fix a race condition between release_obj and print_obj

From: Alistair Strachan <alistair.strachan@...tec.com>

Before this change, a timeline would only be removed from the timeline
list *after* the sync driver had its release_obj() called. However, the
driver's release_obj() may free resources needed by print_obj().

Although the timeline list is locked when print_obj() is called, it is
not locked when release_obj() is called. If one CPU was in print_obj()
when another was in release_obj(), the print_obj() may make unsafe
accesses.

It is not actually necessary to hold the timeline list lock when calling
release_obj() if the call is made after the timeline is unlinked from
the list, since there is no possibility another thread could be in --
or enter -- print_obj() for that timeline.

This change moves the release_obj() call to after the timeline is
unlinked, preventing the above race from occurring.

Cc: Greg KH <gregkh@...uxfoundation.org>
Cc: Colin Cross <ccross@...roid.com>
Cc: Android Kernel Team <kernel-team@...roid.com>
Signed-off-by: Alistair Strachan <alistair.strachan@...tec.com>
[jstultz: minor commit subject tweak]
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 drivers/staging/android/sync.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/staging/android/sync.c b/drivers/staging/android/sync.c
index fec2d1c..3d05f662 100644
--- a/drivers/staging/android/sync.c
+++ b/drivers/staging/android/sync.c
@@ -79,13 +79,13 @@ static void sync_timeline_free(struct kref *kref)
 		container_of(kref, struct sync_timeline, kref);
 	unsigned long flags;

-	if (obj->ops->release_obj)
-		obj->ops->release_obj(obj);
-
 	spin_lock_irqsave(&sync_timeline_list_lock, flags);
 	list_del(&obj->sync_timeline_list);
 	spin_unlock_irqrestore(&sync_timeline_list_lock, flags);

+	if (obj->ops->release_obj)
+		obj->ops->release_obj(obj);
+
 	kfree(obj);
 }

-- 
1.8.3.2

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