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Commit 5acb3cc2 authored by Waiman Long's avatar Waiman Long Committed by Jens Axboe
Browse files

blktrace: Fix potential deadlock between delete & sysfs ops 



The lockdep code had reported the following unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(s_active#228);
                               lock(&bdev->bd_mutex/1);
                               lock(s_active#228);
  lock(&bdev->bd_mutex);

 *** DEADLOCK ***

The deadlock may happen when one task (CPU1) is trying to delete a
partition in a block device and another task (CPU0) is accessing
tracing sysfs file (e.g. /sys/block/dm-1/trace/act_mask) in that
partition.

The s_active isn't an actual lock. It is a reference count (kn->count)
on the sysfs (kernfs) file. Removal of a sysfs file, however, require
a wait until all the references are gone. The reference count is
treated like a rwsem using lockdep instrumentation code.

The fact that a thread is in the sysfs callback method or in the
ioctl call means there is a reference to the opended sysfs or device
file. That should prevent the underlying block structure from being
removed.

Instead of using bd_mutex in the block_device structure, a new
blk_trace_mutex is now added to the request_queue structure to protect
access to the blk_trace structure.

Suggested-by: default avatarChristoph Hellwig <hch@infradead.org>
Signed-off-by: default avatarWaiman Long <longman@redhat.com>
Acked-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>

Fix typo in patch subject line, and prune a comment detailing how
the code used to work.

Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 1dae69be

block/blk-core.c

+3 −0
Original line number Diff line number Diff line
@@ -854,6 +854,9 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id) 


	kobject_init(&q->kobj, &blk_queue_ktype);



#ifdef CONFIG_BLK_DEV_IO_TRACE

	mutex_init(&q->blk_trace_mutex);

#endif

	mutex_init(&q->sysfs_lock);

	spin_lock_init(&q->__queue_lock);

include/linux/blkdev.h

+1 −0
Original line number Diff line number Diff line
@@ -551,6 +551,7 @@ struct request_queue { 
	int			node;

#ifdef CONFIG_BLK_DEV_IO_TRACE

	struct blk_trace	*blk_trace;

	struct mutex		blk_trace_mutex;

#endif

	/*

	 * for flush operations

kernel/trace/blktrace.c

+12 −6
Original line number Diff line number Diff line
@@ -648,6 +648,12 @@ int blk_trace_startstop(struct request_queue *q, int start) 
}

EXPORT_SYMBOL_GPL(blk_trace_startstop);



/*

 * When reading or writing the blktrace sysfs files, the references to the

 * opened sysfs or device files should prevent the underlying block device

 * from being removed. So no further delete protection is really needed.

 */



/**

 * blk_trace_ioctl: - handle the ioctls associated with tracing

 * @bdev:	the block device
@@ -665,7 +671,7 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg) 
	if (!q)

		return -ENXIO;



	mutex_lock(&bdev->bd_mutex);

	mutex_lock(&q->blk_trace_mutex);



	switch (cmd) {

	case BLKTRACESETUP:
@@ -691,7 +697,7 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg) 
		break;

	}



	mutex_unlock(&bdev->bd_mutex);

	mutex_unlock(&q->blk_trace_mutex);

	return ret;

}
@@ -1727,7 +1733,7 @@ static ssize_t sysfs_blk_trace_attr_show(struct device *dev, 
	if (q == NULL)

		goto out_bdput;



	mutex_lock(&bdev->bd_mutex);

	mutex_lock(&q->blk_trace_mutex);



	if (attr == &dev_attr_enable) {

		ret = sprintf(buf, "%u\n", !!q->blk_trace);
@@ -1746,7 +1752,7 @@ static ssize_t sysfs_blk_trace_attr_show(struct device *dev, 
		ret = sprintf(buf, "%llu\n", q->blk_trace->end_lba);



out_unlock_bdev:

	mutex_unlock(&bdev->bd_mutex);

	mutex_unlock(&q->blk_trace_mutex);

out_bdput:

	bdput(bdev);

out:
@@ -1788,7 +1794,7 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev, 
	if (q == NULL)

		goto out_bdput;



	mutex_lock(&bdev->bd_mutex);

	mutex_lock(&q->blk_trace_mutex);



	if (attr == &dev_attr_enable) {

		if (value)
@@ -1814,7 +1820,7 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev, 
	}



out_unlock_bdev:

	mutex_unlock(&bdev->bd_mutex);

	mutex_unlock(&q->blk_trace_mutex);

out_bdput:

	bdput(bdev);

out: