GFS2: Fix inode deallocation race

This area of the code has always been a bit delicate due to the
subtleties of lock ordering. The problem is that for "normal"
alloc/dealloc, we always grab the inode locks first and the rgrp lock
later.

In order to ensure no races in looking up the unlinked, but still
allocated inodes, we need to hold the rgrp lock when we do the lookup,
which means that we can't take the inode glock.

The solution is to borrow the technique already used by NFS to solve
what is essentially the same problem (given an inode number, look up
the inode carefully, checking that it really is in the expected
state).

We cannot do that directly from the allocation code (lock ordering
again) so we give the job to the pre-existing delete workqueue and
carry on with the allocation as normal.

If we find there is no space, we do a journal flush (required anyway
if space from a deallocation is to be released) which should block
against the pending deallocations, so we should always get the space
back.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

1 files changed, 10 insertions, 11 deletions

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index 87778857f099..f92c17704169 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -686,21 +686,20 @@ static void delete_work_func(struct work_struct *work)
 {
         struct gfs2_glock *gl = container_of(work, struct gfs2_glock, gl_delete);
         struct gfs2_sbd *sdp = gl->gl_sbd;
-        struct gfs2_inode *ip = NULL;
+        struct gfs2_inode *ip;
         struct inode *inode;
-        u64 no_addr = 0;
+        u64 no_addr = gl->gl_name.ln_number;
+
+        ip = gl->gl_object;
+        /* Note: Unsafe to dereference ip as we don't hold right refs/locks */
 
-        spin_lock(&gl->gl_spin);
-        ip = (struct gfs2_inode *)gl->gl_object;
         if (ip)
-                no_addr = ip->i_no_addr;
-        spin_unlock(&gl->gl_spin);
-        if (ip) {
                 inode = gfs2_ilookup(sdp->sd_vfs, no_addr);
-                if (inode) {
-                        d_prune_aliases(inode);
-                        iput(inode);
-                }
+        else
+                inode = gfs2_lookup_by_inum(sdp, no_addr, NULL, GFS2_BLKST_UNLINKED);
+        if (inode && !IS_ERR(inode)) {
+                d_prune_aliases(inode);
+                iput(inode);
         }
         gfs2_glock_put(gl);
 }