Re: [PATCH] blktrace: Avoid sparse warnings when assigning q->blk_trace

[Jan Kara <jack@suse.cz>]

On Thu 28-05-20 07:44:38, Bart Van Assche wrote:
> (+Luis)
> 
> On 2020-05-28 02:29, Jan Kara wrote:
> > 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. 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@intel.com>
> > Signed-off-by: Jan Kara <jack@suse.cz>
> 
> How about adding a reference to commit c780e86dd48e ("blktrace: Protect
> q->blk_trace with RCU") in the description of this patch?

Yes, that's probably a good idea.

> > @@ -1669,10 +1672,7 @@ 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;
> 
> This changes a conditional assignment of q->blk_trace into an
> unconditional assignment. Shouldn't q->blk_trace only be assigned if
> q->blk_trace == NULL?

Yes but both callers of blk_trace_setup_queue() actually check that
q->blk_trace is NULL before calling blk_trace_setup_queue() and since we
hold blk_trace_mutex all the time, the value of q->blk_trace cannot change.
So the conditional assignment was just bogus.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR