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Subject: [PATCH v3 13/32] NFSD: WARN when freeing an item still linked via
 nf_lru
From:   Chuck Lever <chuck.lever@oracle.com>
To:     linux-nfs@vger.kernel.org, netdev@vger.kernel.org
Cc:     david@fromorbit.com, jlayton@redhat.com, tgraf@suug.ch
Date:   Fri, 08 Jul 2022 14:25:04 -0400
Message-ID: <165730470467.28142.1460484670621105058.stgit@klimt.1015granger.net>
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References: <165730437087.28142.6731645688073512500.stgit@klimt.1015granger.net>
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Precedence: bulk

Add a guardrail to prevent freeing memory that is still on a list.
This includes either a dispose list or the LRU list.

This is the sign of a bug, but this class of bugs can be detected
so that they don't endanger system stability, especially while
debugging.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/nfsd/filecache.c |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
index 3055a04eeabe..8ade3699664c 100644
--- a/fs/nfsd/filecache.c
+++ b/fs/nfsd/filecache.c
@@ -220,6 +220,14 @@ nfsd_file_free(struct nfsd_file *nf)
 		fput(nf->nf_file);
 		flush = true;
 	}
+
+	/*
+	 * If this item is still linked via nf_lru, that's a bug.
+	 * WARN and leak it to preserve system stability.
+	 */
+	if (WARN_ON_ONCE(!list_empty(&nf->nf_lru)))
+		return flush;
+
 	call_rcu(&nf->nf_rcu, nfsd_file_slab_free);
 	return flush;
 }
@@ -349,7 +357,7 @@ nfsd_file_dispose_list(struct list_head *dispose)
 
 	while(!list_empty(dispose)) {
 		nf = list_first_entry(dispose, struct nfsd_file, nf_lru);
-		list_del(&nf->nf_lru);
+		list_del_init(&nf->nf_lru);
 		nfsd_file_flush(nf);
 		nfsd_file_put_noref(nf);
 	}
@@ -363,7 +371,7 @@ nfsd_file_dispose_list_sync(struct list_head *dispose)
 
 	while(!list_empty(dispose)) {
 		nf = list_first_entry(dispose, struct nfsd_file, nf_lru);
-		list_del(&nf->nf_lru);
+		list_del_init(&nf->nf_lru);
 		nfsd_file_flush(nf);
 		if (!refcount_dec_and_test(&nf->nf_ref))
 			continue;