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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