[3/5] memrlimit fix sleep inside sleeplock in mm_update_next_owner()

[Balbir Singh]

We have a sleep inside a spinlock (read side locking of tasklist_lock). We
try to acquire mmap_sem without releasing the read_lock. Since we have
the task_struct of the new process, we can release the read_lock, before
acquiring the task_lock of the chosen one.

Reported-by: Hugh Dickins <hugh@xxxxxxxxxxx>



Signed-off-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
---

 kernel/exit.c |   10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff -puN kernel/exit.c~memrlimit-fix-sleep-in-spinlock-bug kernel/exit.c
--- linux-2.6.26-rc5/kernel/exit.c~memrlimit-fix-sleep-in-spinlock-bug	2008-06-26 14:48:21.000000000 +0530
+++ linux-2.6.26-rc5-balbir/kernel/exit.c	2008-06-26 14:48:21.000000000 +0530
@@ -636,28 +636,24 @@ retry:
 assign_new_owner:
 	BUG_ON(c == p);
 	get_task_struct(c);
+	read_unlock(&tasklist_lock);
 	down_write(&mm->mmap_sem);
 	/*
 	 * The task_lock protects c->mm from changing.
 	 * We always want mm->owner->mm == mm
 	 */
 	task_lock(c);
-	/*
-	 * Delay read_unlock() till we have the task_lock()
-	 * to ensure that c does not slip away underneath us
-	 */
-	read_unlock(&tasklist_lock);
 	if (c->mm != mm) {
 		task_unlock(c);
-		put_task_struct(c);
 		up_write(&mm->mmap_sem);
+		put_task_struct(c);
 		goto retry;
 	}
 	cgroup_mm_owner_callbacks(mm->owner, c);
 	mm->owner = c;
 	task_unlock(c);
-	put_task_struct(c);
 	up_write(&mm->mmap_sem);
+	put_task_struct(c);
 }
 #endif /* CONFIG_MM_OWNER */
 
_

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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