Re: [PATCH] cpuset oom lock fix

["Darrick J. Wong" <djwong@us.ibm.com>]

FWIW, the patched kernel hasn't exhibited this crash yet; so far, it
seems that the patch is good.  More results later when the rest of the
batch finish testing.

--D

Paul Jackson wrote:
> 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);
>  
>

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