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

[Luis Chamberlain <mcgrof@kernel.org>]

On Thu, May 28, 2020 at 08:31:52PM +0200, Jan Kara wrote:
> 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.

If you run a blktrace against a different partition the check does have
an effect today. This is because the request_queue is shared between
partitions implicitly, even though they end up using a different struct
dentry. So the check is actually still needed, however my change adds
this check early as well so we don't do a memory allocation just to
throw it away.

Even then, the last final check is neede then.

Try these:

https://github.com/mcgrof/break-blktrace

  Luis