[PATCH 3.16 211/245] blktrace: re-write setting q->blk_trace

Ben Hutchings
2020-04-23T23:11:02Z

From: Davidlohr Bueso <dave@stgolabs.ne>

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>
---
kernel/trace/blktrace.c | 16 +++++-----------
1 file changed, 5 insertions(+), 11 deletions(-)

--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -448,7 +448,7 @@ int do_blk_trace_setup(struct request_qu
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_qu
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
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

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();