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Subject[PATCH 3.16 075/136] blktrace: Fix potential deadlock between delete & sysfs ops
3.16.54-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Waiman Long <longman@redhat.com>

commit 5acb3cc2c2e9d3020a4fee43763c6463767f1572 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
block/blk-core.c | 3 +++
include/linux/blkdev.h | 1 +
kernel/trace/blktrace.c | 18 ++++++++++++------
3 files changed, 16 insertions(+), 6 deletions(-)

--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -611,6 +611,9 @@ struct request_queue *blk_alloc_queue_no

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);

--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -448,6 +448,7 @@ 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
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -634,6 +634,12 @@ int blk_trace_startstop(struct request_q
}
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
@@ -651,7 +657,7 @@ int blk_trace_ioctl(struct block_device
if (!q)
return -ENXIO;

- mutex_lock(&bdev->bd_mutex);
+ mutex_lock(&q->blk_trace_mutex);

switch (cmd) {
case BLKTRACESETUP:
@@ -677,7 +683,7 @@ int blk_trace_ioctl(struct block_device
break;
}

- mutex_unlock(&bdev->bd_mutex);
+ mutex_unlock(&q->blk_trace_mutex);
return ret;
}

@@ -1670,7 +1676,7 @@ static ssize_t sysfs_blk_trace_attr_show
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);
@@ -1689,7 +1695,7 @@ static ssize_t sysfs_blk_trace_attr_show
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:
@@ -1731,7 +1737,7 @@ static ssize_t sysfs_blk_trace_attr_stor
if (q == NULL)
goto out_bdput;

- mutex_lock(&bdev->bd_mutex);
+ mutex_lock(&q->blk_trace_mutex);

if (attr == &dev_attr_enable) {
if (value)
@@ -1757,7 +1763,7 @@ static ssize_t sysfs_blk_trace_attr_stor
}

out_unlock_bdev:
- mutex_unlock(&bdev->bd_mutex);
+ mutex_unlock(&q->blk_trace_mutex);
out_bdput:
bdput(bdev);
out:
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