[PATCH] lockd hangs during host lookup garbage collection

From: Daniel Forrest (forrest@lmcg.wisc.edu)
Date: Wed Jun 12 2002 - 11:27:41 EST

A deadlock occurs under the following sequence:

->nlmsvc_lock calls down(&file->f_sema)
   ->nlm_lookup_host may do garbage collection
      ->nlm_traverse_files action = NLM_ACT_MARK
       ->nlm_inspect_file loops over all files
        ->nlmsvc_traverse_blocks calls down(&file->f_sema)

Under heavy load (i.e. >32 client machines locking/unlocking the same
NFS mounted file repeatedly) this happens within seconds.

I discussed this with Trond Myklebust a couple of months ago, but the
question became one of why f_sema was being used at all. I don't know
the answer to that question. Until someone else removes f_sema, the
following patch will avoid the problem.

Please apply.


--- fs/lockd/svclock.c.ORIG Thu Oct 11 09:52:18 2001
+++ fs/lockd/svclock.c Tue Jun 11 17:06:01 2002
@@ -176,8 +176,14 @@
        struct nlm_rqst *call;
        /* Create host handle for callback */
+ /* We must up the semaphore in case the host lookup does
+ * garbage collection (which calls nlmsvc_traverse_blocks),
+ * but this shouldn't be a problem because nlmsvc_lock has
+ * to retry the lock after this anyway */
+ up(&file->f_sema);
        host = nlmclnt_lookup_host(&rqstp->rq_addr,
                                rqstp->rq_prot, rqstp->rq_vers);
+ down(&file->f_sema);
        if (host == NULL)
                return NULL;

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This archive was generated by hypermail 2b29 : Sat Jun 15 2002 - 22:00:25 EST