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.

The patch itself looks ok, but it doesn't seem to apply to a recent kernel tree. It appears as though it is white-space damaged. Would you mind re-sending it?

Quoted 3 lines

The patch itself looks ok, but it doesn't seem to apply to a recent kernel tree. It appears as though it is white-space damaged. Would you mind re-sending it?

Hmm sorry about that. I thought I had based it against that day's tip/master. Anyway, here is v3 which is against tip/master as of 338e29ba93639138fafb9fb5ba946fd99a512aae.

8<--------------------------------------------------------------------------------
From: Davidlohr Bueso <[email protected]>
Date: Thu, 29 Oct 2015 12:13:24 -0700
Subject: [PATCH -tip v3] 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 Davidlohr Bueso <[email protected]>

---
v3: rebased ontop of today's -tip.
minor changelog addition.

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

kernel/trace/blktrace.c +5 -12

index 90e72a0..e3a2618 100644
@@ -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

Quoted 4 lines

The patch itself looks ok, but it doesn't seem to apply to a recent kernel tree. It appears as though it is white-space damaged. Would you mind re-sending it?

Hmm sorry about that. I thought I had based it against that day's tip/master.
Anyway, here is v3 which is against tip/master as of 338e29ba93639138fafb9fb5ba946fd99a512aae.

Quoted 5 lines · Davidlohr Bueso

Quoted 4 lines

The patch itself looks ok, but it doesn't seem to apply to a recent kernel tree. It appears as though it is white-space damaged. Would you mind re-sending it?

Quoted 4 lines

Hmm sorry about that. I thought I had based it against that day's tip/master.
Anyway, here is v3 which is against tip/master as of 338e29ba93639138fafb9fb5ba946fd99a512aae.

Thanks, this is definitely cleaner. Your patch is still pretty mangled, though. I had to hand apply it. Please check the result:

http://git.kernel.dk/cgit/linux-block/commit/?h=for-4.4/core&id=cdea01b2bf98affb7e9c44530108a4a28535eee8

Quoted 11 lines · Jens Axboe

Quoted 9 lines · Davidlohr Bueso

Quoted 4 lines

The patch itself looks ok, but it doesn't seem to apply to a recent kernel tree. It appears as though it is white-space damaged. Would you mind re-sending it?

Hmm sorry about that. I thought I had based it against that day's tip/master.
Anyway, here is v3 which is against tip/master as of 338e29ba93639138fafb9fb5ba946fd99a512aae.

Quoted 2 lines

Thanks, this is definitely cleaner. Your patch is still pretty mangled, though. I had to hand apply it. Please check the result:

Heh, and here I just assumed the problem was on my end.

Quoted 1 line

http://git.kernel.dk/cgit/linux-block/commit/?h=for-4.4/core&id=cdea01b2bf98affb7e9c44530108a4a28535eee8

That looks fine, except you're missing the 'R' in Reviewed-by.

Quoted 16 lines · Jeff Moyer

Quoted 13 lines · Jens Axboe

Quoted 9 lines · Davidlohr Bueso

Quoted 4 lines

The patch itself looks ok, but it doesn't seem to apply to a recent kernel tree. It appears as though it is white-space damaged. Would you mind re-sending it?

Hmm sorry about that. I thought I had based it against that day's tip/master.
Anyway, here is v3 which is against tip/master as of 338e29ba93639138fafb9fb5ba946fd99a512aae.

Thanks, this is definitely cleaner. Your patch is still pretty mangled, though. I had to hand apply it. Please check the result:

Quoted 5 lines

Heh, and here I just assumed the problem was on my end.

Quoted 1 line

http://git.kernel.dk/cgit/linux-block/commit/?h=for-4.4/core&id=cdea01b2bf98affb7e9c44530108a4a28535eee8

That looks fine, except you're missing the 'R' in Reviewed-by.

e-viewed. Electronically viewed by jeff?

Thanks, this is definitely cleaner. Your patch is still pretty mangled, though. I had to hand apply it. Please check the result:

http://git.kernel.dk/cgit/linux-block/commit/?h=for-4.4/core&id=cdea01b2bf98affb7e9c44530108a4a28535eee8