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blktrace: re-write setting q->blk_trace

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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>
This commit is contained in:
Davidlohr Bueso 2015-10-30 05:25:59 +09:00 committed by Jens Axboe
parent cfd0c552a8
commit cdea01b2bf
1 changed files with 5 additions and 11 deletions
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16
kernel/trace/blktrace.c
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@ -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();