Commit 574aa0b47994 for kernel

commit 574aa0b4799470ac814479f1138d19efe6262255
Author: Breno Leitao <leitao@debian.org>
Date:   Tue Apr 21 02:41:09 2026 -0700

    9p: skip nlink update in cacheless mode to fix WARN_ON

    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@vger.kernel.org
    Suggested-by: Dominique Martinet <asmadeus@codewreck.org>
    Signed-off-by: Breno Leitao <leitao@debian.org>
    Message-ID: <20260421-9p-v2-1-48762d294fad@debian.org>
    Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
index cdaa5034cbef..3a811db2dc19 100644
--- a/fs/9p/vfs_inode.c
+++ b/fs/9p/vfs_inode.c
@@ -490,10 +490,19 @@ 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);
 }