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

[Jan Kara <jack@suse.cz>]

On Thu 28-05-20 18:43:33, Luis Chamberlain wrote:
> 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.

I'm not sure we are speaking about the same check but I might be missing
something. blk_trace_setup_queue() is only called from
sysfs_blk_trace_attr_store(). That does:

        mutex_lock(&q->blk_trace_mutex);

        bt = rcu_dereference_protected(q->blk_trace,
                                       lockdep_is_held(&q->blk_trace_mutex));
        if (attr == &dev_attr_enable) {
                if (!!value == !!bt) {
                        ret = 0;
                        goto out_unlock_bdev;
                }

		^^^ So if 'bt' is non-NULL, and we are enabling, we bail
instead of calling blk_trace_setup_queue().

Similarly later:

        if (bt == NULL) {
                ret = blk_trace_setup_queue(q, bdev);
	...
so we again call blk_trace_setup_queue() only if bt is NULL. So IMO the
cmpxchg() in blk_trace_setup_queue() could never fail to set the value.
Am I missing something?

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