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Re: [PATCH 07/16] xfs: convert inode cache lookups to use RCU locking

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 07/16] xfs: convert inode cache lookups to use RCU locking
From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Mon, 8 Nov 2010 19:36:28 -0800
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, eric.dumazet@xxxxxxxxx
In-reply-to: <20101108230929.GA13299@xxxxxxxxxxxxx>
References: <1289206519-18377-1-git-send-email-david@xxxxxxxxxxxxx> <1289206519-18377-8-git-send-email-david@xxxxxxxxxxxxx> <20101108230929.GA13299@xxxxxxxxxxxxx>
Reply-to: paulmck@xxxxxxxxxxxxxxxxxx
User-agent: Mutt/1.5.20 (2009-06-14)
On Mon, Nov 08, 2010 at 06:09:29PM -0500, Christoph Hellwig wrote:
> This patch generally looks good to me, but with so much RCU magic I'd prefer
> if Paul & Eric could look over it.

Is there a git tree, tarball, or whatever?  For example, I don't see
how this patch handles the case of an inode being freed just as an RCU
reader gains a reference to it, but then reallocated as some other inode
(so that ->ino is nonzero) before the RCU reader gets a chance to actually
look at the inode.  But such a check might well be in the code that this
patch didn't change...

                                                        Thanx, Paul

> On Mon, Nov 08, 2010 at 07:55:10PM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > 
> > With delayed logging greatly increasing the sustained parallelism of inode
> > operations, the inode cache locking is showing significant read vs write
> > contention when inode reclaim runs at the same time as lookups. There is
> > also a lot more write lock acquistions than there are read locks (4:1 ratio)
> > so the read locking is not really buying us much in the way of parallelism.
> > 
> > To avoid the read vs write contention, change the cache to use RCU locking 
> > on
> > the read side. To avoid needing to RCU free every single inode, use the 
> > built
> > in slab RCU freeing mechanism. This requires us to be able to detect 
> > lookups of
> > freed inodes, so en??ure that ever freed inode has an inode number of zero 
> > and
> > the XFS_IRECLAIM flag set. We already check the XFS_IRECLAIM flag in cache 
> > hit
> > lookup path, but also add a check for a zero inode number as well.
> > 
> > We canthen convert all the read locking lockups to use RCU read side locking
> > and hence remove all read side locking.
> > 
> > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> > Reviewed-by: Alex Elder <aelder@xxxxxxx>
> > ---
> >  fs/xfs/linux-2.6/xfs_iops.c    |    7 +++++-
> >  fs/xfs/linux-2.6/xfs_sync.c    |   13 +++++++++--
> >  fs/xfs/quota/xfs_qm_syscalls.c |    3 ++
> >  fs/xfs/xfs_iget.c              |   44 
> > ++++++++++++++++++++++++++++++---------
> >  fs/xfs/xfs_inode.c             |   22 ++++++++++++-------
> >  5 files changed, 67 insertions(+), 22 deletions(-)
> > 
> > diff --git a/fs/xfs/linux-2.6/xfs_iops.c b/fs/xfs/linux-2.6/xfs_iops.c
> > index 8b46867..909bd9c 100644
> > --- a/fs/xfs/linux-2.6/xfs_iops.c
> > +++ b/fs/xfs/linux-2.6/xfs_iops.c
> > @@ -757,6 +757,8 @@ xfs_diflags_to_iflags(
> >   * We don't use the VFS inode hash for lookups anymore, so make the inode 
> > look
> >   * hashed to the VFS by faking it. This avoids needing to touch inode hash
> >   * locks in this path, but makes the VFS believe the inode is validly 
> > hashed.
> > + * We initialise i_state and i_hash under the i_lock so that we follow the 
> > same
> > + * setup rules that the rest of the VFS follows.
> >   */
> >  void
> >  xfs_setup_inode(
> > @@ -765,10 +767,13 @@ xfs_setup_inode(
> >     struct inode            *inode = &ip->i_vnode;
> >  
> >     inode->i_ino = ip->i_ino;
> > +
> > +   spin_lock(&inode->i_lock);
> >     inode->i_state = I_NEW;
> > +   hlist_nulls_add_fake(&inode->i_hash);
> > +   spin_unlock(&inode->i_lock);
> 
> This screams for another VFS helper, even if it's XFS-specific for now.
> Having to duplicate inode.c-private locking rules in XFS seems a bit
> nasty to me.
> 
> >  
> >     inode_sb_list_add(inode);
> > -   hlist_nulls_add_fake(&inode->i_hash);
> >  
> >     inode->i_mode   = ip->i_d.di_mode;
> >     inode->i_nlink  = ip->i_d.di_nlink;
> > diff --git a/fs/xfs/linux-2.6/xfs_sync.c b/fs/xfs/linux-2.6/xfs_sync.c
> > index afb0d7c..9a53cc9 100644
> > --- a/fs/xfs/linux-2.6/xfs_sync.c
> > +++ b/fs/xfs/linux-2.6/xfs_sync.c
> > @@ -53,6 +53,10 @@ xfs_inode_ag_walk_grab(
> >  {
> >     struct inode            *inode = VFS_I(ip);
> >  
> > +   /* check for stale RCU freed inode */
> > +   if (!ip->i_ino)
> > +           return ENOENT;
> 
> Assuming i_ino is never 0 is fine for XFS, unlike for the generic VFS
> code, so ACK.
> 
> >     /* nothing to sync during shutdown */
> >     if (XFS_FORCED_SHUTDOWN(ip->i_mount))
> >             return EFSCORRUPTED;
> > @@ -98,12 +102,12 @@ restart:
> >             int             error = 0;
> >             int             i;
> >  
> > -           read_lock(&pag->pag_ici_lock);
> > +           rcu_read_lock();
> >             nr_found = radix_tree_gang_lookup(&pag->pag_ici_root,
> >                                     (void **)batch, first_index,
> >                                     XFS_LOOKUP_BATCH);
> >             if (!nr_found) {
> > -                   read_unlock(&pag->pag_ici_lock);
> > +                   rcu_read_unlock();
> >                     break;
> >             }
> >  
> > @@ -129,7 +133,7 @@ restart:
> >             }
> >  
> >             /* unlock now we've grabbed the inodes. */
> > -           read_unlock(&pag->pag_ici_lock);
> > +           rcu_read_unlock();
> >  
> >             for (i = 0; i < nr_found; i++) {
> >                     if (!batch[i])
> > @@ -639,6 +643,9 @@ xfs_reclaim_inode_grab(
> >     struct xfs_inode        *ip,
> >     int                     flags)
> >  {
> > +   /* check for stale RCU freed inode */
> > +   if (!ip->i_ino)
> > +           return 1;
> >  
> >     /*
> >      * do some unlocked checks first to avoid unnecceary lock traffic.
> > diff --git a/fs/xfs/quota/xfs_qm_syscalls.c b/fs/xfs/quota/xfs_qm_syscalls.c
> > index bdebc18..8b207fc 100644
> > --- a/fs/xfs/quota/xfs_qm_syscalls.c
> > +++ b/fs/xfs/quota/xfs_qm_syscalls.c
> > @@ -875,6 +875,9 @@ xfs_dqrele_inode(
> >     struct xfs_perag        *pag,
> >     int                     flags)
> >  {
> > +   if (!ip->i_ino)
> > +           return ENOENT;
> > +
> 
> Why do we need the check here again?  Having it in
> xfs_inode_ag_walk_grab should be enough.
> 
> >     /* skip quota inodes */
> >     if (ip == ip->i_mount->m_quotainfo->qi_uquotaip ||
> >         ip == ip->i_mount->m_quotainfo->qi_gquotaip) {
> > diff --git a/fs/xfs/xfs_iget.c b/fs/xfs/xfs_iget.c
> > index 18991a9..edeb918 100644
> > --- a/fs/xfs/xfs_iget.c
> > +++ b/fs/xfs/xfs_iget.c
> > @@ -69,6 +69,7 @@ xfs_inode_alloc(
> >     ASSERT(atomic_read(&ip->i_pincount) == 0);
> >     ASSERT(!spin_is_locked(&ip->i_flags_lock));
> >     ASSERT(completion_done(&ip->i_flush));
> > +   ASSERT(ip->i_ino == 0);
> >  
> >     mrlock_init(&ip->i_iolock, MRLOCK_BARRIER, "xfsio", ip->i_ino);
> >  
> > @@ -86,9 +87,6 @@ xfs_inode_alloc(
> >     ip->i_new_size = 0;
> >     ip->i_dirty_releases = 0;
> >  
> > -   /* prevent anyone from using this yet */
> > -   VFS_I(ip)->i_state = I_NEW;
> > -
> >     return ip;
> >  }
> >  
> > @@ -135,6 +133,16 @@ xfs_inode_free(
> >     ASSERT(!spin_is_locked(&ip->i_flags_lock));
> >     ASSERT(completion_done(&ip->i_flush));
> >  
> > +   /*
> > +    * because we use SLAB_DESTROY_BY_RCU freeing, ensure the inode
> > +    * always appears to be reclaimed with an invalid inode number
> > +    * when in the free state. The ip->i_flags_lock provides the barrier
> > +    * against lookup races.
> > +    */
> > +   spin_lock(&ip->i_flags_lock);
> > +   ip->i_flags = XFS_IRECLAIM;
> > +   ip->i_ino = 0;
> > +   spin_unlock(&ip->i_flags_lock);
> >     kmem_zone_free(xfs_inode_zone, ip);
> >  }
> >  
> > @@ -146,12 +154,28 @@ xfs_iget_cache_hit(
> >     struct xfs_perag        *pag,
> >     struct xfs_inode        *ip,
> >     int                     flags,
> > -   int                     lock_flags) __releases(pag->pag_ici_lock)
> > +   int                     lock_flags) __releases(RCU)
> >  {
> >     struct inode            *inode = VFS_I(ip);
> >     struct xfs_mount        *mp = ip->i_mount;
> >     int                     error;
> >  
> > +   /*
> > +    * check for re-use of an inode within an RCU grace period due to the
> > +    * radix tree nodes not being updated yet. We monitor for this by
> > +    * setting the inode number to zero before freeing the inode structure.
> > +    * We don't need to recheck this after taking the i_flags_lock because
> > +    * the check against XFS_IRECLAIM will catch a freed inode.
> > +    */
> > +   if (ip->i_ino == 0) {
> > +           trace_xfs_iget_skip(ip);
> > +           XFS_STATS_INC(xs_ig_frecycle);
> > +           rcu_read_unlock();
> > +           /* Expire the grace period so we don't trip over it again. */
> > +           synchronize_rcu();
> > +           return EAGAIN;
> > +   }
> > +
> >     spin_lock(&ip->i_flags_lock);
> >  
> >     /*
> > @@ -195,7 +219,7 @@ xfs_iget_cache_hit(
> >             ip->i_flags |= XFS_IRECLAIM;
> >  
> >             spin_unlock(&ip->i_flags_lock);
> > -           read_unlock(&pag->pag_ici_lock);
> > +           rcu_read_unlock();
> >  
> >             error = -inode_init_always(mp->m_super, inode);
> >             if (error) {
> > @@ -203,7 +227,7 @@ xfs_iget_cache_hit(
> >                      * Re-initializing the inode failed, and we are in deep
> >                      * trouble.  Try to re-add it to the reclaim list.
> >                      */
> > -                   read_lock(&pag->pag_ici_lock);
> > +                   rcu_read_lock();
> >                     spin_lock(&ip->i_flags_lock);
> >  
> >                     ip->i_flags &= ~XFS_INEW;
> > @@ -231,7 +255,7 @@ xfs_iget_cache_hit(
> >  
> >             /* We've got a live one. */
> >             spin_unlock(&ip->i_flags_lock);
> > -           read_unlock(&pag->pag_ici_lock);
> > +           rcu_read_unlock();
> >             trace_xfs_iget_hit(ip);
> >     }
> >  
> > @@ -245,7 +269,7 @@ xfs_iget_cache_hit(
> >  
> >  out_error:
> >     spin_unlock(&ip->i_flags_lock);
> > -   read_unlock(&pag->pag_ici_lock);
> > +   rcu_read_unlock();
> >     return error;
> >  }
> >  
> > @@ -376,7 +400,7 @@ xfs_iget(
> >  
> >  again:
> >     error = 0;
> > -   read_lock(&pag->pag_ici_lock);
> > +   rcu_read_lock();
> >     ip = radix_tree_lookup(&pag->pag_ici_root, agino);
> >  
> >     if (ip) {
> > @@ -384,7 +408,7 @@ again:
> >             if (error)
> >                     goto out_error_or_again;
> >     } else {
> > -           read_unlock(&pag->pag_ici_lock);
> > +           rcu_read_unlock();
> >             XFS_STATS_INC(xs_ig_missed);
> >  
> >             error = xfs_iget_cache_miss(mp, pag, tp, ino, &ip,
> > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > index 108c7a0..25becb1 100644
> > --- a/fs/xfs/xfs_inode.c
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -2000,13 +2000,14 @@ xfs_ifree_cluster(
> >              */
> >             for (i = 0; i < ninodes; i++) {
> >  retry:
> > -                   read_lock(&pag->pag_ici_lock);
> > +                   rcu_read_lock();
> >                     ip = radix_tree_lookup(&pag->pag_ici_root,
> >                                     XFS_INO_TO_AGINO(mp, (inum + i)));
> >  
> >                     /* Inode not in memory or stale, nothing to do */
> > -                   if (!ip || xfs_iflags_test(ip, XFS_ISTALE)) {
> > -                           read_unlock(&pag->pag_ici_lock);
> > +                   if (!ip || !ip->i_ino ||
> > +                       xfs_iflags_test(ip, XFS_ISTALE)) {
> > +                           rcu_read_unlock();
> >                             continue;
> >                     }
> >  
> > @@ -2019,11 +2020,11 @@ retry:
> >                      */
> >                     if (ip != free_ip &&
> >                         !xfs_ilock_nowait(ip, XFS_ILOCK_EXCL)) {
> > -                           read_unlock(&pag->pag_ici_lock);
> > +                           rcu_read_unlock();
> >                             delay(1);
> >                             goto retry;
> >                     }
> > -                   read_unlock(&pag->pag_ici_lock);
> > +                   rcu_read_unlock();
> >  
> >                     xfs_iflock(ip);
> >                     xfs_iflags_set(ip, XFS_ISTALE);
> > @@ -2629,7 +2630,7 @@ xfs_iflush_cluster(
> >  
> >     mask = ~(((XFS_INODE_CLUSTER_SIZE(mp) >> mp->m_sb.sb_inodelog)) - 1);
> >     first_index = XFS_INO_TO_AGINO(mp, ip->i_ino) & mask;
> > -   read_lock(&pag->pag_ici_lock);
> > +   rcu_read_lock();
> >     /* really need a gang lookup range call here */
> >     nr_found = radix_tree_gang_lookup(&pag->pag_ici_root, (void**)ilist,
> >                                     first_index, inodes_per_cluster);
> > @@ -2640,6 +2641,11 @@ xfs_iflush_cluster(
> >             iq = ilist[i];
> >             if (iq == ip)
> >                     continue;
> > +
> > +           /* check we've got a valid inode */
> > +           if (!iq->i_ino)
> > +                   continue;
> > +
> >             /* if the inode lies outside this cluster, we're done. */
> >             if ((XFS_INO_TO_AGINO(mp, iq->i_ino) & mask) != first_index)
> >                     break;
> > @@ -2692,7 +2698,7 @@ xfs_iflush_cluster(
> >     }
> >  
> >  out_free:
> > -   read_unlock(&pag->pag_ici_lock);
> > +   rcu_read_unlock();
> >     kmem_free(ilist);
> >  out_put:
> >     xfs_perag_put(pag);
> > @@ -2704,7 +2710,7 @@ cluster_corrupt_out:
> >      * Corruption detected in the clustering loop.  Invalidate the
> >      * inode buffer and shut down the filesystem.
> >      */
> > -   read_unlock(&pag->pag_ici_lock);
> > +   rcu_read_unlock();
> >     /*
> >      * Clean up the buffer.  If it was B_DELWRI, just release it --
> >      * brelse can handle it with no problems.  If not, shut down the
> > -- 
> > 1.7.2.3
> > 
> > _______________________________________________
> > xfs mailing list
> > xfs@xxxxxxxxxxx
> > http://oss.sgi.com/mailman/listinfo/xfs
> ---end quoted text---

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