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
---
Andrew - this should be ready for *-mm now.
It's the same change I sent yesterday labeled [RFC].
It passed my testing fine.
This is a bug fix that I would recommend for 2.6.16
in a few days.
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);
