[PATCH v2] ocfs2: Cache the mount context for dentry lock callbacks

[Cen Zhang <zzzccc427@gmail.com>]

ocfs2_drop_dentry_lock() drops the inode reference before releasing the
DENTRY lock. This lets inode eviction release the local inode and open
locks before a remote unlink can proceed past the DENTRY lock.

A DLM callback can still arrive before the DENTRY lock has been dropped.
ocfs2_get_dentry_osb() obtains the mount context through dl_inode->i_sb,
which can access a freed inode when the downconvert thread drops the
last inode reference in its post-unlock callback:

  Downconvert thread                     DLM callback thread
  ------------------                     -------------------
  ocfs2_process_blocked_lock()
    ocfs2_dentry_post_unlock()
      ocfs2_dentry_lock_put()
        ocfs2_drop_dentry_lock()
          iput(dl->dl_inode)
          [inode reclaimed after RCU]
                                         ocfs2_blocking_ast()
                                           ocfs2_get_dentry_osb()
                                             dl->dl_inode->i_sb
          ocfs2_simple_drop_lockres()

Store the mount context in the DENTRY lock resource's l_priv field and
use the default mount-context lookup. Recover the containing dentry
lock with container_of(), as already done for refcount locks, and remove
ocfs2_get_dentry_osb(). ocfs2_dismount_volume() shuts down the DLM before
freeing the mount context, so callbacks can use it without accessing the
inode. Preserve the existing iput-before-unlock order to avoid delaying
local inode and open lock release until after the DENTRY lock is released.

Fixes: 8ed6b23709b3 ("ocfs2: revert iput deferring code in ocfs2_drop_dentry_lock")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
Changes in v2:
- Preserve the existing iput-before-unlock order and cache the mount context
  in the lock resource's existing l_priv field instead.
- Recover the containing dentry lock with container_of() and remove the
  inode-dependent mount-context lookup.
- Correct the comment describing the dentry lock's inode reference.

Link to v1: https://lore.kernel.org/r/pm-ocfs2-objects-candidate-0012-v5-a0413a339b72c02fc885@gmail.com

 fs/ocfs2/dcache.h  |  5 ++---
 fs/ocfs2/dlmglue.c | 16 ++++------------
 2 files changed, 6 insertions(+), 15 deletions(-)

diff --git a/fs/ocfs2/dcache.h b/fs/ocfs2/dcache.h
index 7f246c5692d8..046ab9a80769 100644
--- a/fs/ocfs2/dcache.h
+++ b/fs/ocfs2/dcache.h
@@ -17,9 +17,8 @@ struct ocfs2_dentry_lock {
 	u64			dl_parent_blkno;
 
 	/*
-	 * The ocfs2_dentry_lock keeps an inode reference until
-	 * dl_lockres has been destroyed. This is usually done in
-	 * ->d_iput() anyway, so there should be minimal impact.
+	 * The inode reference is dropped before dl_lockres is unlocked.
+	 * Callbacks must not use it to find the mount context during teardown.
 	 */
 	struct inode		*dl_inode;
 	struct ocfs2_lock_res	dl_lockres;
diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
index 6bc4a21cf5f0..d12635c4db95 100644
--- a/fs/ocfs2/dlmglue.c
+++ b/fs/ocfs2/dlmglue.c
@@ -57,7 +57,6 @@ struct ocfs2_mask_waiter {
 #endif
 };
 
-static struct ocfs2_super *ocfs2_get_dentry_osb(struct ocfs2_lock_res *lockres);
 static struct ocfs2_super *ocfs2_get_inode_osb(struct ocfs2_lock_res *lockres);
 static struct ocfs2_super *ocfs2_get_file_osb(struct ocfs2_lock_res *lockres);
 static struct ocfs2_super *ocfs2_get_qinfo_osb(struct ocfs2_lock_res *lockres);
@@ -257,7 +256,6 @@ static const struct ocfs2_lock_res_ops ocfs2_orphan_scan_lops = {
 };
 
 static const struct ocfs2_lock_res_ops ocfs2_dentry_lops = {
-	.get_osb	= ocfs2_get_dentry_osb,
 	.post_unlock	= ocfs2_dentry_post_unlock,
 	.downconvert_worker = ocfs2_dentry_convert_worker,
 	.flags		= 0,
@@ -308,7 +306,7 @@ static inline struct ocfs2_dentry_lock *ocfs2_lock_res_dl(struct ocfs2_lock_res
 {
 	BUG_ON(lockres->l_type != OCFS2_LOCK_TYPE_DENTRY);
 
-	return (struct ocfs2_dentry_lock *)lockres->l_priv;
+	return container_of(lockres, struct ocfs2_dentry_lock, dl_lockres);
 }
 
 static inline struct ocfs2_mem_dqinfo *ocfs2_lock_res_qinfo(struct ocfs2_lock_res *lockres)
@@ -609,13 +607,6 @@ static __u64 ocfs2_get_dentry_lock_ino(struct ocfs2_lock_res *lockres)
 	return be64_to_cpu(inode_blkno_be);
 }
 
-static struct ocfs2_super *ocfs2_get_dentry_osb(struct ocfs2_lock_res *lockres)
-{
-	struct ocfs2_dentry_lock *dl = lockres->l_priv;
-
-	return OCFS2_SB(dl->dl_inode->i_sb);
-}
-
 void ocfs2_dentry_lock_res_init(struct ocfs2_dentry_lock *dl,
 				u64 parent, struct inode *inode)
 {
@@ -623,6 +614,7 @@ void ocfs2_dentry_lock_res_init(struct ocfs2_dentry_lock *dl,
 	u64 inode_blkno = OCFS2_I(inode)->ip_blkno;
 	__be64 inode_blkno_be = cpu_to_be64(inode_blkno);
 	struct ocfs2_lock_res *lockres = &dl->dl_lockres;
+	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb);
 
 	ocfs2_lock_res_init_once(lockres);
 
@@ -651,9 +643,9 @@ void ocfs2_dentry_lock_res_init(struct ocfs2_dentry_lock *dl,
 	memcpy(&lockres->l_name[OCFS2_DENTRY_LOCK_INO_START], &inode_blkno_be,
 	       sizeof(__be64));
 
-	ocfs2_lock_res_init_common(OCFS2_SB(inode->i_sb), lockres,
+	ocfs2_lock_res_init_common(osb, lockres,
 				   OCFS2_LOCK_TYPE_DENTRY, &ocfs2_dentry_lops,
-				   dl);
+				   osb);
 }
 
 static void ocfs2_super_lock_res_init(struct ocfs2_lock_res *res,
-- 
2.43.0