blktrace: re-write setting q->blk_trace (cdea01b2) · Commits · 戴 / test

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>

kernel/trace/blktrace.c

+5 −11

Original line number	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 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;
		if (cmpxchg(&q->blk_trace, NULL, bt))
		goto free_bt;
		}

		if (atomic_inc_return(&blk_probes_ref) == 1)
		blk_register_tracepoints();

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