From: Davidlohr Bueso <dave@stgolabs.ne>
Date: Thu, 29 Oct 2015 20:25:59 +0000 (+0900)
Subject: blktrace: re-write setting q->blk_trace
X-Git-Tag: v3.16.83~37
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=12b91f8bd81a4e97c86bbf4ebbec35fec7c33dce;p=thirdparty%2Fkernel%2Fstable.git

blktrace: re-write setting q->blk_trace

commit cdea01b2bf98affb7e9c44530108a4a28535eee8 upstream.

This is really about simplifying the double xchg patterns into
a single cmpxchg, with the same logic. Other than the immediate
cleanup, there are some subtleties this change deals with:

(i) While the load of the old bt is fully ordered wrt everything,
ie:

        old_bt = xchg(&q->blk_trace, bt);             [barrier]
        if (old_bt)
	     (void) xchg(&q->blk_trace, old_bt);    [barrier]

blk_trace could still be changed between the xchg and the old_bt
load. Note that this description is merely theoretical and afaict
very small, but doing everything in a single context with cmpxchg
closes this potential race.

(ii) Ordering guarantees are obviously kept with cmpxchg.

(iii) Gets rid of the hacky-by-nature (void)xchg pattern.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
eviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 9a9d0288576bd..a72275cbd0e3e 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -448,7 +448,7 @@ int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 		       struct block_device *bdev,
 		       struct blk_user_trace_setup *buts)
 {
-	struct blk_trace *old_bt, *bt = NULL;
+	struct blk_trace *bt = NULL;
 	struct dentry *dir = NULL;
 	int ret, i;
 
@@ -532,11 +532,8 @@ int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 	bt->trace_state = Blktrace_setup;
 
 	ret = -EBUSY;
-	old_bt = xchg(&q->blk_trace, bt);
-	if (old_bt) {
-		(void) xchg(&q->blk_trace, old_bt);
+	if (cmpxchg(&q->blk_trace, NULL, bt))
 		goto err;
-	}
 
 	if (atomic_inc_return(&blk_probes_ref) == 1)
 		blk_register_tracepoints();
@@ -1550,7 +1547,7 @@ static int blk_trace_remove_queue(struct request_queue *q)
 static int blk_trace_setup_queue(struct request_queue *q,
 				 struct block_device *bdev)
 {
-	struct blk_trace *old_bt, *bt = NULL;
+	struct blk_trace *bt = NULL;
 	int ret = -ENOMEM;
 
 	bt = kzalloc(sizeof(*bt), GFP_KERNEL);
@@ -1566,12 +1563,9 @@ static int blk_trace_setup_queue(struct request_queue *q,
 
 	blk_trace_setup_lba(bt, bdev);
 
-	old_bt = xchg(&q->blk_trace, bt);
-	if (old_bt != NULL) {
-		(void)xchg(&q->blk_trace, old_bt);
-		ret = -EBUSY;
+	ret = -EBUSY;
+	if (cmpxchg(&q->blk_trace, NULL, bt))
 		goto free_bt;
-	}
 
 	if (atomic_inc_return(&blk_probes_ref) == 1)
 		blk_register_tracepoints();