Mostly for historical reasons, q->blk_trace is assigned through xchg()
and cmpxchg() atomic operations. Although this is correct, sparse
complains about this because it violates rcu annotations since commit
c780e86dd48e ("blktrace: Protect q->blk_trace with RCU") which started
to use rcu for accessing q->blk_trace. Furthermore there's no real need
for atomic operations anymore since all changes to q->blk_trace happen
under q->blk_trace_mutex. So let's just replace xchg() with
rcu_replace_pointer() and cmpxchg() with explicit check and
rcu_assign_pointer(). This makes the code more efficient and sparse
happy.
Reported-by: kbuild test robot <lkp@xxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
kernel/trace/blktrace.c | 19 ++++++++-----------
1 file changed, 8 insertions(+), 11 deletions(-)
Here is version of my patch rebased on top of Luis' blktrace fixes. Luis, if
the patch looks fine, can you perhaps include it in your series since it seems
you'll do another revision of your series due to discussion over patch 5/7?
Thanks!
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index ac6650828d49..13bc09e4594c 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -346,7 +346,8 @@ static int __blk_trace_remove(struct request_queue *q)
{
struct blk_trace *bt;
- bt = xchg(&q->blk_trace, NULL);
+ bt = rcu_replace_pointer(q->blk_trace, NULL,
+ lockdep_is_held(&q->blk_trace_mutex));
if (!bt)
return -EINVAL;
@@ -500,7 +501,8 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
* bdev can be NULL, as with scsi-generic, this is a helpful as
* we can be.
*/
- if (q->blk_trace) {
+ if (rcu_dereference_protected(q->blk_trace,
+ lockdep_is_held(&q->blk_trace_mutex))) {
pr_warn("Concurrent blktraces are not allowed on %s\n",
buts->name);
return -EBUSY;
@@ -570,10 +572,7 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
bt->pid = buts->pid;
bt->trace_state = Blktrace_setup;
- ret = -EBUSY;
- if (cmpxchg(&q->blk_trace, NULL, bt))
- goto err;
-
+ rcu_assign_pointer(q->blk_trace, bt);
get_probe_ref();
ret = 0;
@@ -1662,7 +1661,8 @@ static int blk_trace_remove_queue(struct request_queue *q)
{
struct blk_trace *bt;
- bt = xchg(&q->blk_trace, NULL);
+ bt = rcu_replace_pointer(q->blk_trace, NULL,
+ lockdep_is_held(&q->blk_trace_mutex));
if (bt == NULL)
return -EINVAL;
@@ -1694,10 +1694,7 @@ static int blk_trace_setup_queue(struct request_queue *q,
blk_trace_setup_lba(bt, bdev);
- ret = -EBUSY;
- if (cmpxchg(&q->blk_trace, NULL, bt))
- goto free_bt;
-
+ rcu_assign_pointer(q->blk_trace, bt);
get_probe_ref();
return 0;
--
2.16.4
