[PATCH v2] 9p: skip nlink update in cacheless mode to fix WARN_ON

[Breno Leitao]

v9fs_dec_count() unconditionally calls drop_nlink() on regular files,
even when the inode's nlink is already zero. In cacheless mode the
client refetches inode metadata from the server (the source of truth)
on every operation, so by the time v9fs_remove() returns, the locally
cached nlink may already reflect the post-unlink value:

  1. Client initiates unlink, server processes it and sets nlink to 0
  2. Client refetches inode metadata (nlink=0) before unlink returns
  3. Client's v9fs_remove() completes successfully
  4. Client calls v9fs_dec_count() which calls drop_nlink() on nlink=0

This race is easily triggered under heavy unlink workloads, such as
stress-ng's unlink stressor, producing the following warning:

  WARNING: fs/inode.c:417 at drop_nlink+0x4c/0xc8
  Call trace:
   drop_nlink+0x4c/0xc8
   v9fs_remove+0x1e0/0x250 [9p]
   v9fs_vfs_unlink+0x20/0x38 [9p]
   vfs_unlink+0x13c/0x258
   ...

In cacheless mode the server is authoritative and the inode is on its
way out, so locally adjusting nlink buys nothing. Skip v9fs_dec_count()
entirely when neither CACHE_META nor CACHE_LOOSE is set, which both
avoids the warning and removes a class of nlink races (two concurrent
unlinkers observing nlink > 0 and both calling drop_nlink()) that an
nlink == 0 guard alone would only narrow rather than close.

Fixes: ac89b2ef9b55 ("9p: don't maintain dir i_nlink if the exported fs doesn't either")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
---
Changes in v2:
- Skip v9fs_dec_count() entirely in cacheless mode — server is
  authoritative, so it fixes the WARN and closes the race class.
- Link to v1:
  https://patch.msgid.link/20260126-9p-v1-1-dc234d53ae87@xxxxxxxxxx
---
 fs/9p/vfs_inode.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
index 97abe65bf7c1f..197a155a4e8b5 100644
--- a/fs/9p/vfs_inode.c
+++ b/fs/9p/vfs_inode.c
@@ -488,10 +488,19 @@ static int v9fs_at_to_dotl_flags(int flags)
  * - ext4 (with dir_nlink feature enabled) sets nlink to 1 if a dir has more
  *   than EXT4_LINK_MAX (65000) links.
  *
+ * In cacheless mode the server is the source of truth for nlink and the
+ * inode is going away immediately, so locally adjusting i_nlink buys
+ * nothing and races with concurrent metadata fetches that may already
+ * have observed the post-unlink value (nlink == 0).
+ *
  * @inode: inode whose nlink is being dropped
  */
 static void v9fs_dec_count(struct inode *inode)
 {
+	struct v9fs_session_info *v9ses = v9fs_inode2v9ses(inode);
+
+	if (!(v9ses->cache & (CACHE_META | CACHE_LOOSE)))
+		return;
 	if (!S_ISDIR(inode->i_mode) || inode->i_nlink > 2)
 		drop_nlink(inode);
 }

---
base-commit: ca3a02fda4da8e2c1cb6baee5d72352e9e2cfaea
change-id: 20260126-9p-50d206e2f6f6

Best regards,
--  
Breno Leitao <leitao@xxxxxxxxxx>