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, the other bd_fsfreeze_mutex mutex in the block_device structure is now overloaded to protect against concurrent blktrace data access in the blktrace.c file. There is no point in adding one more mutex to the block_device structure just for blktrace. Signed-off-by: Waiman Long <long...@redhat.com> --- include/linux/fs.h | 2 +- kernel/trace/blktrace.c | 26 ++++++++++++++++++++------ 2 files changed, 21 insertions(+), 7 deletions(-) diff --git a/include/linux/fs.h b/include/linux/fs.h index 339e737..330b572 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -448,7 +448,7 @@ struct block_device { /* The counter of freeze processes */ int bd_fsfreeze_count; - /* Mutex for freeze */ + /* Mutex for freeze and blktrace */ struct mutex bd_fsfreeze_mutex; } __randomize_layout; diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c index 2a685b4..7cd5d1d 100644 --- a/kernel/trace/blktrace.c +++ b/kernel/trace/blktrace.c @@ -648,6 +648,20 @@ 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. + * + * Protection from multiple readers and writers accessing blktrace data + * concurrently is still required. The bd_mutex was used for this purpose. + * That could lead to deadlock with concurrent block device deletion and + * sysfs access. Instead, the block device bd_fsfreeze_mutex is now + * overloaded for blktrace data protection. Like freeze/thaw, blktrace + * functionality is not frequently used. There is no point in adding + * one more mutex to the block_device structure just for blktrace. + */ + /** * blk_trace_ioctl: - handle the ioctls associated with tracing * @bdev: the block device @@ -665,7 +679,7 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg) if (!q) return -ENXIO; - mutex_lock(&bdev->bd_mutex); + mutex_lock(&bdev->bd_fsfreeze_mutex); switch (cmd) { case BLKTRACESETUP: @@ -691,7 +705,7 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg) break; } - mutex_unlock(&bdev->bd_mutex); + mutex_unlock(&bdev->bd_fsfreeze_mutex); return ret; } @@ -1727,7 +1741,7 @@ static ssize_t sysfs_blk_trace_attr_show(struct device *dev, if (q == NULL) goto out_bdput; - mutex_lock(&bdev->bd_mutex); + mutex_lock(&bdev->bd_fsfreeze_mutex); if (attr == &dev_attr_enable) { ret = sprintf(buf, "%u\n", !!q->blk_trace); @@ -1746,7 +1760,7 @@ 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(&bdev->bd_fsfreeze_mutex); out_bdput: bdput(bdev); out: @@ -1788,7 +1802,7 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev, if (q == NULL) goto out_bdput; - mutex_lock(&bdev->bd_mutex); + mutex_lock(&bdev->bd_fsfreeze_mutex); if (attr == &dev_attr_enable) { if (value) @@ -1814,7 +1828,7 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev, } out_unlock_bdev: - mutex_unlock(&bdev->bd_mutex); + mutex_unlock(&bdev->bd_fsfreeze_mutex); out_bdput: bdput(bdev); out: -- 1.8.3.1