@@ -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;
@@ -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:
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 <longman@redhat.com> --- v5: - Overload the bd_fsfreeze_mutex in block_device structure for blktrace protection. v4: - Use blktrace_mutex in blk_trace_ioctl() as well. v3: - Use a global blktrace_mutex to serialize sysfs attribute accesses instead of the bd_mutex. v2: - Use READ_ONCE() and smp_store_mb() to read and write bd_deleting. - Check for signal in the mutex_trylock loops. - Use usleep() instead of schedule() for RT tasks. include/linux/fs.h | 2 +- kernel/trace/blktrace.c | 26 ++++++++++++++++++++------ 2 files changed, 21 insertions(+), 7 deletions(-)