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Commit cdea01b2 authored by Davidlohr Bueso's avatar Davidlohr Bueso Committed by Jens Axboe
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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: default avatarDavidlohr Bueso <dbueso@suse.de>
eviewed-by: default avatarJeff Moyer <jmoyer@redhat.com>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent cfd0c552

kernel/trace/blktrace.c

+5 −11
Original line number Original line Diff line number Diff line
@@ -437,7 +437,7 @@ int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev, 
		       struct block_device *bdev,

		       struct block_device *bdev,

		       struct blk_user_trace_setup *buts)

		       struct blk_user_trace_setup *buts)

{

{

	struct blk_trace *old_bt, *bt = NULL;

	struct blk_trace *bt = NULL;

	struct dentry *dir = NULL;

	struct dentry *dir = NULL;

	int ret;

	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;

	bt->trace_state = Blktrace_setup;





	ret = -EBUSY;

	ret = -EBUSY;

	old_bt = xchg(&q->blk_trace, bt);

	if (cmpxchg(&q->blk_trace, NULL, bt))

	if (old_bt) {

		(void) xchg(&q->blk_trace, old_bt);

		goto err;

		goto err;

	}





	if (atomic_inc_return(&blk_probes_ref) == 1)

	if (atomic_inc_return(&blk_probes_ref) == 1)

		blk_register_tracepoints();

		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,

static int blk_trace_setup_queue(struct request_queue *q,

				 struct block_device *bdev)

				 struct block_device *bdev)

{

{

	struct blk_trace *old_bt, *bt = NULL;

	struct blk_trace *bt = NULL;

	int ret = -ENOMEM;

	int ret = -ENOMEM;





	bt = kzalloc(sizeof(*bt), GFP_KERNEL);

	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);

	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;

		goto free_bt;

	}





	if (atomic_inc_return(&blk_probes_ref) == 1)

	if (atomic_inc_return(&blk_probes_ref) == 1)

		blk_register_tracepoints();

		blk_register_tracepoints();