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blktrace: Fix potential deadlock between delete & sysfs ops

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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: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Acked-by: Steven 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: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Waiman Long 2017-09-20 13:12:20 -06:00 committed by Jens Axboe
parent 1dae69bede
commit 5acb3cc2c2
3 changed files with 16 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
block/blk-core.c
View File

@ -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);

1
include/linux/blkdev.h
View File

@ -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

18
kernel/trace/blktrace.c
View File

@ -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: