linux-kernel - [PATCH] nfsd: decrease sc_count directly if fail to queue dl

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Message-ID: <20250410015708.2036236-1-lilingfeng3@huawei.com>
Date: Thu, 10 Apr 2025 09:57:08 +0800
From: Li Lingfeng <lilingfeng3@...wei.com>
To: <chuck.lever@...cle.com>, <jlayton@...nel.org>, <neilb@...e.de>,
	<okorniev@...hat.com>, <Dai.Ngo@...cle.com>, <tom@...pey.com>,
	<linux-nfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC: <yukuai1@...weicloud.com>, <houtao1@...wei.com>, <yi.zhang@...wei.com>,
	<yangerkun@...wei.com>, <lilingfeng@...weicloud.com>,
	<lilingfeng3@...wei.com>
Subject: [PATCH] nfsd: decrease sc_count directly if fail to queue dl_recall

A deadlock warning occurred when invoking nfs4_put_stid following a failed
dl_recall queue operation:
            T1                            T2
                                nfs4_laundromat
                                 nfs4_get_client_reaplist
                                  nfs4_anylock_blockers
__break_lease
 spin_lock // ctx->flc_lock
                                   spin_lock // clp->cl_lock
                                   nfs4_lockowner_has_blockers
                                    locks_owner_has_blockers
                                     spin_lock // flctx->flc_lock
 nfsd_break_deleg_cb
  nfsd_break_one_deleg
   nfs4_put_stid
    refcount_dec_and_lock
     spin_lock // clp->cl_lock

When a file is opened, an nfs4_delegation is allocated with sc_count
initialized to 1, and the file_lease holds a reference to the delegation.
The file_lease is then associated with the file through kernel_setlease.

The disassociation is performed in nfsd4_delegreturn via the following
call chain:
nfsd4_delegreturn --> destroy_delegation --> destroy_unhashed_deleg -->
nfs4_unlock_deleg_lease --> kernel_setlease --> generic_delete_lease
The corresponding sc_count reference will be released after this
disassociation.

Since nfsd_break_one_deleg executes while holding the flc_lock, the
disassociation process becomes blocked when attempting to acquire flc_lock
in generic_delete_lease. This means:
1) sc_count in nfsd_break_one_deleg will not be decremented to 0;
2) The nfs4_put_stid called by nfsd_break_one_deleg will not attempt to
acquire cl_lock;
3) Consequently, no deadlock condition is created.

Given that sc_count in nfsd_break_one_deleg remains non-zero, we can
safely perform refcount_dec on sc_count directly. This approach
effectively avoids triggering deadlock warnings.

Fixes: 230ca758453c ("nfsd: put dl_stid if fail to queue dl_recall")
Signed-off-by: Li Lingfeng <lilingfeng3@...wei.com>
---
 fs/nfsd/nfs4state.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 2041268b398a..59a693f22452 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -5430,7 +5430,7 @@ static void nfsd_break_one_deleg(struct nfs4_delegation *dp)
 	queued = nfsd4_run_cb(&dp->dl_recall);
 	WARN_ON_ONCE(!queued);
 	if (!queued)
-		nfs4_put_stid(&dp->dl_stid);
+		refcount_dec(&dp->dl_stid.sc_count);
 }

 /* Called from break_lease() with flc_lock held. */
-- 
2.31.1