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

[Jan Kara <jack@suse.cz>]

On Tue 02-06-20 15:10:33, Luis Chamberlain wrote:
> On Tue, Jun 02, 2020 at 02:17:34PM +0000, Luis Chamberlain wrote:
> > On Tue, Jun 02, 2020 at 09:12:05AM +0200, Jan Kara wrote:
> > > 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!
> > 
> > Sure thing, will throw in the pile.
> 
> I've updated the commit log as follows as well, as I think its important
> to annotate that the check for processing of the blktrace only makes
> sense if it was not set. Let me know if this is fine. The commit log
> is below.

Thanks! The changelog looks good to me.

								Honza

> 
> From: Jan Kara <jack@suse.cz>
> Date: Tue, 2 Jun 2020 09:12:05 +0200
> Subject: [PATCH 1/8] blktrace: Avoid sparse warnings when assigning
>  q->blk_trace
> 
> 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 *and* since it also makes more sense to check
> if q->blk_trace is set with the mutex held *earlier* and this is now
> done through the patch titled "blktrace: break out on concurrent calls"
> and was already before on blk_trace_setup_queue().
> 
> 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>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR