Re: [PATCH 4/3] xfs: xfs_qm_dqrele mostly doesn't need locking

[Christoph Hellwig <hch@infradead.org>]

On Sat, Dec 14, 2013 at 08:30:06AM +1100, Dave Chinner wrote:
> > 	if (atomic_dec_and_test(&dqp->q_nrefs)) {
> > 		if (list_lru_add(&mp->m_quotainfo->qi_lru, &dqp->q_lru))
> > 			XFS_STATS_INC(xs_qm_dquot_unused);
> > 	}
> > 
> > given that the only locking we need is the internal lru lock?
> 
> Yes, I think it is.
> 
> However, that involves changing all the callers of dqput to not hold
> the dqlock when they call, which is a bigger change than was
> necessary to avoid the lock contention problem. i.e. it doesn't seem
> to be in a fast path that needed immediate fixing, so I didn't touch
> it.

Given that the lru list lock nests inside dqlock we can just turn
dqput into:

void
xfs_qm_dqput(
	struct xfs_dquot        *dqp)
{
	ASSERT(dqp->q_nrefs > 0);
	ASSERT(XFS_DQ_IS_LOCKED(dqp));

	trace_xfs_dqput(dqp);

	xfs_qm_dqrele(dqp);
	xfs_dqunlock(dqp);
}

But with my other patch we can probably replace most callers
with xfs_qm_dqrele, and the remaining ones with an opencoded
version that first drops the lock easily.

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