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Re: [PATCH 1/4] xfs: fix locking in xfs_iget_cache_hit

To: Felix Blyakher <felixb@xxxxxxx>
Subject: Re: [PATCH 1/4] xfs: fix locking in xfs_iget_cache_hit
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Mon, 10 Aug 2009 13:09:40 -0400
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <24CFF78E-BB3F-47A1-8D6C-49EA198CCD94@xxxxxxx>
References: <20090804141554.992235000@xxxxxxxxxxxxxxxxxxxxxx> <20090804141834.526088000@xxxxxxxxxxxxxxxxxxxxxx> <24CFF78E-BB3F-47A1-8D6C-49EA198CCD94@xxxxxxx>
User-agent: Mutt/1.5.19 (2009-01-05)
On Fri, Aug 07, 2009 at 12:25:47PM -0500, Felix Blyakher wrote:
>> +    if (ip->i_flags & XFS_INEW) {
>
> Another case when we find XFS_INEW set is the race with the
> cache miss, which just set up a new inode. Would the proposed
> code be still sensible in that case? If yes, at least comments
> should be updated.

>> +            wait_on_inode(inode);
>
> It's possible to have XFS_INEW set, but no I_LOCK|I_NEW yet.
> Then the wait_on_inode() would return quickly even before the
> linux inode is reinitialized. Though, that was the case with
> the old code as well.

The wait_on_inode is only sensible for the non-recycle case.  But it's
not actually very useful with our flags scheme, so for now I've reverted
to do the old-style polling and just added an XXX comment that we'll
eventually look into a better scheme.

>> +    /*
>> +     * If lookup is racing with unlink, then we should return an
>> +     * error immediately so we don't remove it from the reclaim
>> +     * list and potentially leak the inode.
>> +     */
>> +    if (ip->i_d.di_mode == 0 && !(flags & XFS_IGET_CREATE)) {
>
> Previously the conclusion of the race with unlink was based on
> XFS_IRECLAIMABLE i_flag set in addition to the test above.
> Is is no longer a case, or not really necessary?

We actually had the test two times in the old code, once for the
reclaim case, and once after the igrab succeeded.  I just moved it into
a single command place.  I've updated the comment to match that.


New version below:

-- 

Subject: xfs: fix locking in xfs_iget_cache_hit
From: Christoph Hellwig <hch@xxxxxx>

The locking in xfs_iget_cache_hit currently has numerous problems:

 - we clear the reclaim tag without i_flags_lock which protects modifications
   to it
 - we call inode_init_always which can sleep with pag_ici_lock held
   (this is oss.sgi.com BZ #819)
 - we acquire and drop i_flags_lock a lot and thus provide no consistency
   between the various flags we set/clear under it

This patch fixes all that with a major revamp of the locking in the function.
The new version acquires i_flags_lock early and only drops it once we need to
call into inode_init_always or before calling xfs_ilock.

This patch fixes a bug seen in the wild where we race modifying the reclaim tag.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>

Index: linux-2.6/fs/xfs/xfs_iget.c
===================================================================
--- linux-2.6.orig/fs/xfs/xfs_iget.c    2009-08-10 13:10:19.141974933 -0300
+++ linux-2.6/fs/xfs/xfs_iget.c 2009-08-10 13:19:06.913056731 -0300
@@ -191,80 +191,83 @@ xfs_iget_cache_hit(
        int                     flags,
        int                     lock_flags) __releases(pag->pag_ici_lock)
 {
+       struct inode            *inode = VFS_I(ip);
        struct xfs_mount        *mp = ip->i_mount;
-       int                     error = EAGAIN;
+       int                     error;
+
+       spin_lock(&ip->i_flags_lock);
 
        /*
-        * If INEW is set this inode is being set up
-        * If IRECLAIM is set this inode is being torn down
-        * Pause and try again.
+        * If we are racing with another cache hit that is currently
+        * instantiating this inode or currently recycling it out of
+        * reclaimabe state, wait for the initialisation to complete
+        * before continuing.
+        *
+        * XXX(hch): eventually we should do something equivalent to
+        *           wait_on_inode to wait for these flags to be cleared
+        *           instead of polling for it.
         */
-       if (xfs_iflags_test(ip, (XFS_INEW|XFS_IRECLAIM))) {
+       if (ip->i_flags & (XFS_INEW|XFS_IRECLAIM)) {
                XFS_STATS_INC(xs_ig_frecycle);
+               error = EAGAIN;
                goto out_error;
        }
 
-       /* If IRECLAIMABLE is set, we've torn down the vfs inode part */
-       if (xfs_iflags_test(ip, XFS_IRECLAIMABLE)) {
-
-               /*
-                * If lookup is racing with unlink, then we should return an
-                * error immediately so we don't remove it from the reclaim
-                * list and potentially leak the inode.
-                */
-               if ((ip->i_d.di_mode == 0) && !(flags & XFS_IGET_CREATE)) {
-                       error = ENOENT;
-                       goto out_error;
-               }
+       /*
+        * If lookup is racing with unlink return an error immediately.
+        */
+       if (ip->i_d.di_mode == 0 && !(flags & XFS_IGET_CREATE)) {
+               error = ENOENT;
+               goto out_error;
+       }
 
+       /*
+        * If IRECLAIMABLE is set, we've torn down the VFS inode already.
+        * Need to carefully get it back into useable state.
+        */
+       if (ip->i_flags & XFS_IRECLAIMABLE) {
                xfs_itrace_exit_tag(ip, "xfs_iget.alloc");
 
                /*
-                * We need to re-initialise the VFS inode as it has been
-                * 'freed' by the VFS. Do this here so we can deal with
-                * errors cleanly, then tag it so it can be set up correctly
-                * later.
+                * We need to set XFS_INEW atomically with clearing the
+                * reclaimable tag so that we do have an indicator of the
+                * inode still being initialized.
                 */
-               if (inode_init_always(mp->m_super, VFS_I(ip))) {
+               ip->i_flags |= XFS_INEW;
+               ip->i_flags &= ~XFS_IRECLAIMABLE;
+               __xfs_inode_clear_reclaim_tag(mp, pag, ip);
+
+               spin_unlock(&ip->i_flags_lock);
+               read_unlock(&pag->pag_ici_lock);
+
+               if (unlikely(inode_init_always(mp->m_super, inode))) {
+                       /*
+                        * 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);
+                       spin_lock(&ip->i_flags_lock);
+
+                       ip->i_flags &= ~XFS_INEW;
+                       ip->i_flags |= XFS_IRECLAIMABLE;
+                       __xfs_inode_set_reclaim_tag(pag, ip);
+
                        error = ENOMEM;
                        goto out_error;
                }
-
-               /*
-                * We must set the XFS_INEW flag before clearing the
-                * XFS_IRECLAIMABLE flag so that if a racing lookup does
-                * not find the XFS_IRECLAIMABLE above but has the igrab()
-                * below succeed we can safely check XFS_INEW to detect
-                * that this inode is still being initialised.
-                */
-               xfs_iflags_set(ip, XFS_INEW);
-               xfs_iflags_clear(ip, XFS_IRECLAIMABLE);
-
-               /* clear the radix tree reclaim flag as well. */
-               __xfs_inode_clear_reclaim_tag(mp, pag, ip);
-       } else if (!igrab(VFS_I(ip))) {
+               inode->i_state = I_LOCK|I_NEW;
+       } else {
                /* If the VFS inode is being torn down, pause and try again. */
-               XFS_STATS_INC(xs_ig_frecycle);
-               goto out_error;
-       } else if (xfs_iflags_test(ip, XFS_INEW)) {
-               /*
-                * We are racing with another cache hit that is
-                * currently recycling this inode out of the XFS_IRECLAIMABLE
-                * state. Wait for the initialisation to complete before
-                * continuing.
-                */
-               wait_on_inode(VFS_I(ip));
-       }
+               if (!igrab(inode)) {
+                       error = EAGAIN;
+                       goto out_error;
+               }
 
-       if (ip->i_d.di_mode == 0 && !(flags & XFS_IGET_CREATE)) {
-               error = ENOENT;
-               iput(VFS_I(ip));
-               goto out_error;
+               /* We've got a live one. */
+               spin_unlock(&ip->i_flags_lock);
+               read_unlock(&pag->pag_ici_lock);
        }
 
-       /* We've got a live one. */
-       read_unlock(&pag->pag_ici_lock);
-
        if (lock_flags != 0)
                xfs_ilock(ip, lock_flags);
 
@@ -274,6 +277,7 @@ xfs_iget_cache_hit(
        return 0;
 
 out_error:
+       spin_unlock(&ip->i_flags_lock);
        read_unlock(&pag->pag_ici_lock);
        return error;
 }
Index: linux-2.6/fs/xfs/linux-2.6/xfs_sync.c
===================================================================
--- linux-2.6.orig/fs/xfs/linux-2.6/xfs_sync.c  2009-08-10 13:10:19.146974522 
-0300
+++ linux-2.6/fs/xfs/linux-2.6/xfs_sync.c       2009-08-10 13:10:59.958993938 
-0300
@@ -708,6 +708,16 @@ xfs_reclaim_inode(
        return 0;
 }
 
+void
+__xfs_inode_set_reclaim_tag(
+       struct xfs_perag        *pag,
+       struct xfs_inode        *ip)
+{
+       radix_tree_tag_set(&pag->pag_ici_root,
+                          XFS_INO_TO_AGINO(ip->i_mount, ip->i_ino),
+                          XFS_ICI_RECLAIM_TAG);
+}
+
 /*
  * We set the inode flag atomically with the radix tree tag.
  * Once we get tag lookups on the radix tree, this inode flag
@@ -722,8 +732,7 @@ xfs_inode_set_reclaim_tag(
 
        read_lock(&pag->pag_ici_lock);
        spin_lock(&ip->i_flags_lock);
-       radix_tree_tag_set(&pag->pag_ici_root,
-                       XFS_INO_TO_AGINO(mp, ip->i_ino), XFS_ICI_RECLAIM_TAG);
+       __xfs_inode_set_reclaim_tag(pag, ip);
        __xfs_iflags_set(ip, XFS_IRECLAIMABLE);
        spin_unlock(&ip->i_flags_lock);
        read_unlock(&pag->pag_ici_lock);
Index: linux-2.6/fs/xfs/linux-2.6/xfs_sync.h
===================================================================
--- linux-2.6.orig/fs/xfs/linux-2.6/xfs_sync.h  2009-08-10 13:10:19.153974227 
-0300
+++ linux-2.6/fs/xfs/linux-2.6/xfs_sync.h       2009-08-10 13:10:59.962994168 
-0300
@@ -48,6 +48,7 @@ int xfs_reclaim_inode(struct xfs_inode *
 int xfs_reclaim_inodes(struct xfs_mount *mp, int mode);
 
 void xfs_inode_set_reclaim_tag(struct xfs_inode *ip);
+void __xfs_inode_set_reclaim_tag(struct xfs_perag *pag, struct xfs_inode *ip);
 void xfs_inode_clear_reclaim_tag(struct xfs_inode *ip);
 void __xfs_inode_clear_reclaim_tag(struct xfs_mount *mp, struct xfs_perag *pag,
                                struct xfs_inode *ip);

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