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Commit 505970b9 authored by Paul Jackson's avatar Paul Jackson Committed by Linus Torvalds
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[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: default avatarPaul Jackson <pj@sgi.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent ed68cb36

include/linux/cpuset.h

+6 −0
Original line number Diff line number Diff line
@@ -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 */

kernel/cpuset.c

+28 −5
Original line number Diff line number Diff line
@@ -2149,6 +2149,33 @@ int __cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask) 
	return allowed;

}



/**

 * 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, gfp_t gfp_mask) 
 * 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 struct task_struct *p) 
	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 struct task_struct *p) 


	overlap = nodes_intersects(cs1->mems_allowed, cs2->mems_allowed);

done:

	up(&callback_sem);



	return overlap;

}

mm/oom_kill.c

+3 −0
Original line number Diff line number Diff line
@@ -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 @@ void out_of_memory(gfp_t gfp_mask, int order) 
	/* 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 @@ void out_of_memory(gfp_t gfp_mask, int order) 


 out:

	read_unlock(&tasklist_lock);

	cpuset_unlock();

	if (mm)

		mmput(mm);