blktrace: re-write setting q->blk_trace

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>

1 files changed, 5 insertions, 11 deletions

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 90e72a0c3047..e3a26188b95e 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -437,7 +437,7 @@ 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;
 
@@ -519,11 +519,8 @@ 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();
@@ -1481,7 +1478,7 @@ 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);
@@ -1497,12 +1494,9 @@ 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();