[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, 6 insertions, 0 deletions

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index c472f972bd6d..3bc606927116 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -48,6 +48,9 @@ extern void __cpuset_memory_pressure_bump(void);
 extern struct file_operations proc_cpuset_operations;
 extern char *cpuset_task_status_allowed(struct task_struct *task, char *buffer);
 
+extern void cpuset_lock(void);
+extern void cpuset_unlock(void);
+
 #else /* !CONFIG_CPUSETS */
 
 static inline int cpuset_init_early(void) { return 0; }
@@ -93,6 +96,9 @@ static inline char *cpuset_task_status_allowed(struct task_struct *task,
         return buffer;
 }
 
+static inline void cpuset_lock(void) {}
+static inline void cpuset_unlock(void) {}
+
 #endif /* !CONFIG_CPUSETS */
 
 #endif /* _LINUX_CPUSET_H */