[PATCH] cpuset oom lock fix

The problem, reported in:

  http://bugzilla.kernel.org/show_bug.cgi?id=5859

and by various other email messages and lkml posts is that the cpuset hook
in the oom (out of memory) code can try to take a cpuset semaphore while
holding the tasklist_lock (a spinlock).

One must not sleep while holding a spinlock.

The fix seems easy enough - move the cpuset semaphore region outside the
tasklist_lock region.

This required a few lines of mechanism to implement.  The oom code where
the locking needs to be changed does not have access to the cpuset locks,
which are internal to kernel/cpuset.c only.  So I provided a couple more
cpuset interface routines, available to the rest of the kernel, which
simple take and drop the lock needed here (cpusets callback_sem).

Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 4748b906aff2..14bd4ec79597 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -274,6 +274,7 @@ void out_of_memory(gfp_t gfp_mask, int order)
                 show_mem();
         }
 
+        cpuset_lock();
         read_lock(&tasklist_lock);
 retry:
         p = select_bad_process();
@@ -284,6 +285,7 @@ retry:
         /* Found nothing?!?! Either we hang forever, or we panic. */
         if (!p) {
                 read_unlock(&tasklist_lock);
+                cpuset_unlock();
                 panic("Out of memory and no killable processes...\n");
         }
 
@@ -293,6 +295,7 @@ retry:
 
  out:
         read_unlock(&tasklist_lock);
+        cpuset_unlock();
         if (mm)
                 mmput(mm);