queue/blktrace-Avoid-sparse-warnings-when-assigning-q-blk_.patch

From 30fe4076e26b4f17fbe63def291f06bba5f2b54f Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Fri, 5 Jun 2020 16:58:37 +0200
Subject: [PATCH] blktrace: Avoid sparse warnings when assigning q->blk_trace

commit c3dbe541ef77754729de5e82be2d6e5d267c6c8c upstream.

Mostly for historical reasons, q->blk_trace is assigned through xchg()
and cmpxchg() atomic operations. Although this is correct, sparse
complains about this because it violates rcu annotations since commit
c780e86dd48e ("blktrace: Protect q->blk_trace with RCU") which started
to use rcu for accessing q->blk_trace. Furthermore there's no real need
for atomic operations anymore since all changes to q->blk_trace happen
under q->blk_trace_mutex and since it also makes more sense to check if
q->blk_trace is set with the mutex held earlier.

So let's just replace xchg() with rcu_replace_pointer() and cmpxchg()
with explicit check and rcu_assign_pointer(). This makes the code more
efficient and sparse happy.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index eaee960153e1..9bedf962e162 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -348,7 +348,8 @@ static int __blk_trace_remove(struct request_queue *q)
 {
 	struct blk_trace *bt;
 
-	bt = xchg(&q->blk_trace, NULL);
+	bt = rcu_replace_pointer(q->blk_trace, NULL,
+				 lockdep_is_held(&q->blk_trace_mutex));
 	if (!bt)
 		return -EINVAL;
 
@@ -502,7 +503,8 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 	 * bdev can be NULL, as with scsi-generic, this is a helpful as
 	 * we can be.
 	 */
-	if (q->blk_trace) {
+	if (rcu_dereference_protected(q->blk_trace,
+				      lockdep_is_held(&q->blk_trace_mutex))) {
 		pr_warn("Concurrent blktraces are not allowed on %s\n",
 			buts->name);
 		return -EBUSY;
@@ -557,10 +559,7 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 	bt->pid = buts->pid;
 	bt->trace_state = Blktrace_setup;
 
-	ret = -EBUSY;
-	if (cmpxchg(&q->blk_trace, NULL, bt))
-		goto err;
-
+	rcu_assign_pointer(q->blk_trace, bt);
 	get_probe_ref();
 
 	ret = 0;
@@ -1634,7 +1633,8 @@ static int blk_trace_remove_queue(struct request_queue *q)
 {
 	struct blk_trace *bt;
 
-	bt = xchg(&q->blk_trace, NULL);
+	bt = rcu_replace_pointer(q->blk_trace, NULL,
+				 lockdep_is_held(&q->blk_trace_mutex));
 	if (bt == NULL)
 		return -EINVAL;
 
@@ -1666,10 +1666,7 @@ static int blk_trace_setup_queue(struct request_queue *q,
 
 	blk_trace_setup_lba(bt, bdev);
 
-	ret = -EBUSY;
-	if (cmpxchg(&q->blk_trace, NULL, bt))
-		goto free_bt;
-
+	rcu_assign_pointer(q->blk_trace, bt);
 	get_probe_ref();
 	return 0;
 
-- 
2.27.0