[RFC] cpuset oom lock fix

[Paul Jackson <pj@sgi.com>]

The following is not adequately tested yet, so it
probably shouldn't go in *-mm.

I'm posting it so that the people who reported this
bug can have the opportunity to test this 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 too
the rest of the kernel, which simple take and drop
the lock needed here (cpusets callback_sem).

The initial build and boot and poke at it once test
passes ok.  But I still need to:
 1) build some other arch's (crosstool), and
 2) test with the various CONFIG_*DEBUG* and CONFIG_*LOCK*_
    options enabled.

It will be about one more day before I complete this
and submit this patch for consideration in *-mm.

If those who have stress tests that hit this could
give this patch a trial, that would be appreciated.

Signed-off-by: Paul Jackson

---

 include/linux/cpuset.h |    6 ++++++
 kernel/cpuset.c        |   33 ++++++++++++++++++++++++++++-----
 mm/oom_kill.c          |    3 +++
 3 files changed, 37 insertions(+), 5 deletions(-)

--- 2.6.15-mm2.orig/include/linux/cpuset.h	2006-01-10 16:00:21.190309415 -0800
+++ 2.6.15-mm2/include/linux/cpuset.h	2006-01-10 23:07:59.648091868 -0800
@@ -48,6 +48,9 @@ extern void __cpuset_memory_pressure_bum
 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_a
 	return buffer;
 }
 
+static inline void cpuset_lock(void) {}
+static inline void cpuset_unlock(void) {}
+
 #endif /* !CONFIG_CPUSETS */
 
 #endif /* _LINUX_CPUSET_H */
--- 2.6.15-mm2.orig/kernel/cpuset.c	2006-01-10 18:26:40.408120365 -0800
+++ 2.6.15-mm2/kernel/cpuset.c	2006-01-10 23:14:28.355492590 -0800
@@ -2150,6 +2150,33 @@ int __cpuset_zone_allowed(struct zone *z
 }
 
 /**
+ * cpuset_lock - lock out any changes to cpuset structures
+ *
+ * The out of memory (oom) code needs to lock down cpusets
+ * from being changed while it scans the tasklist looking for a
+ * task in an overlapping cpuset.  Expose callback_sem via this
+ * cpuset_lock() routine, so the oom code can lock it, before
+ * locking the task list.  The tasklist_lock is a spinlock, so
+ * must be taken inside callback_sem.
+ */
+
+void cpuset_lock(void)
+{
+	down(&callback_sem);
+}
+
+/**
+ * cpuset_unlock - release lock on cpuset changes
+ *
+ * Undo the lock taken in a previous cpuset_lock() call.
+ */
+
+void cpuset_unlock(void)
+{
+	up(&callback_sem);
+}
+
+/**
  * cpuset_excl_nodes_overlap - Do we overlap @p's mem_exclusive ancestors?
  * @p: pointer to task_struct of some other task.
  *
@@ -2158,7 +2185,7 @@ int __cpuset_zone_allowed(struct zone *z
  * determine if task @p's memory usage might impact the memory
  * available to the current task.
  *
- * Acquires callback_sem - not suitable for calling from a fast path.
+ * Call while holding callback_sem.
  **/
 
 int cpuset_excl_nodes_overlap(const struct task_struct *p)
@@ -2166,8 +2193,6 @@ int cpuset_excl_nodes_overlap(const stru
 	const struct cpuset *cs1, *cs2;	/* my and p's cpuset ancestors */
 	int overlap = 0;		/* do cpusets overlap? */
 
-	down(&callback_sem);
-
 	task_lock(current);
 	if (current->flags & PF_EXITING) {
 		task_unlock(current);
@@ -2186,8 +2211,6 @@ int cpuset_excl_nodes_overlap(const stru
 
 	overlap = nodes_intersects(cs1->mems_allowed, cs2->mems_allowed);
 done:
-	up(&callback_sem);
-
 	return overlap;
 }
 
--- 2.6.15-mm2.orig/mm/oom_kill.c	2006-01-10 19:18:25.588685008 -0800
+++ 2.6.15-mm2/mm/oom_kill.c	2006-01-10 23:16:05.936643833 -0800
@@ -274,6 +274,7 @@ void out_of_memory(gfp_t gfp_mask, int o
 		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);
 

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@sgi.com> 1.650.933.1373