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.
Signed-off-by: Davidlohr Bueso <dbu...@suse.de>
---
v2: sorry I sent a stale version which didn't set EBUSY when the structure is
already initialized.
kernel/trace/blktrace.c | 16 +++++-----------
1 file changed, 5 insertions(+), 11 deletions(-)
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 90e72a0..e3a2618 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();
--
2.1.4
--
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