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@xxxxxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
v7:
- Add a new blk_trace_mutex in request_queue structure for blk_trace
protection.
v6:
- Add a second patch to rename the bd_fsfreeze_mutex to
bd_fsfreeze_blktrace_mutex.
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.
block/blk-core.c | 3 +++
include/linux/blkdev.h | 1 +
kernel/trace/blktrace.c | 24 ++++++++++++++++++------
3 files changed, 22 insertions(+), 6 deletions(-)
diff --git a/block/blk-core.c b/block/blk-core.c
index aebe676..048be4a 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -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);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 460294b..02fa42d 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -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
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 2a685b4..d5cef05 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -648,6 +648,18 @@ 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. As a result, a new blk_trace_mutex is now added to be
+ * used solely by the blktrace code.
+ */
+
/**
* blk_trace_ioctl: - handle the ioctls associated with tracing
* @bdev: the block device
@@ -665,7 +677,7 @@ 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 +703,7 @@ 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 +1739,7 @@ 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 +1758,7 @@ 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 +1800,7 @@ 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 +1826,7 @@ 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:
--
1.8.3.1
