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[PATCH 06/19] nfsd: set PF_FSTRANS for nfsd threads.

To: linux-mm@xxxxxxxxx, linux-nfs@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
Subject: [PATCH 06/19] nfsd: set PF_FSTRANS for nfsd threads.
From: NeilBrown <neilb@xxxxxxx>
Date: Wed, 16 Apr 2014 14:03:36 +1000
Cc: xfs@xxxxxxxxxxx
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140416033623.10604.69237.stgit@xxxxxxxxxxxxxx>
References: <20140416033623.10604.69237.stgit@xxxxxxxxxxxxxx>
User-agent: StGit/0.16
If a localhost mount is present, then it is easy to deadlock NFS by
nfsd entering direct reclaim and calling nfs_release_page() which
requires nfsd to perform an fsync() (which it cannot do because it is
reclaiming memory).

By setting PF_FSTRANS we stop the memory allocator from ever
attempting any FS operation would could deadlock.

We need this flag set for any thread which is handling a request from
the local host, but we also need to always have it for at least 1 or 2
threads so that we don't end up with all threads blocked in allocation.

When we set PF_FSTRANS we also tell lockdep that we are handling
reclaim so that it can detect deadlocks for us.

Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
 fs/nfsd/nfssvc.c           |   18 ++++++++++++++++++
 include/linux/sunrpc/svc.h |    1 +
 net/sunrpc/svc.c           |    6 ++++++
 3 files changed, 25 insertions(+)

diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index 9a4a5f9e7468..6af8bc2daf7d 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -565,6 +565,8 @@ nfsd(void *vrqstp)
        struct svc_xprt *perm_sock = 
list_entry(rqstp->rq_server->sv_permsocks.next, typeof(struct svc_xprt), 
xpt_list);
        struct net *net = perm_sock->xpt_net;
        int err;
+       unsigned int pflags = 0;
+       gfp_t reclaim_state = 0;
 
        /* Lock module and set up kernel thread */
        mutex_lock(&nfsd_mutex);
@@ -611,14 +613,30 @@ nfsd(void *vrqstp)
                        ;
                if (err == -EINTR)
                        break;
+               if (rqstp->rq_local && !current_test_flags(PF_FSTRANS)) {
+                       current_set_flags_nested(&pflags, PF_FSTRANS);
+                       atomic_inc(&rqstp->rq_pool->sp_nr_fstrans);
+                       reclaim_state = 
lockdep_set_current_reclaim_state(GFP_KERNEL);
+               }
                validate_process_creds();
                svc_process(rqstp);
                validate_process_creds();
+               if (current_test_flags(PF_FSTRANS) &&
+                   atomic_dec_if_positive(&rqstp->rq_pool->sp_nr_fstrans) >= 
0) {
+                       current_restore_flags_nested(&pflags, PF_FSTRANS);
+                       lockdep_restore_current_reclaim_state(reclaim_state);
+               }
        }
 
        /* Clear signals before calling svc_exit_thread() */
        flush_signals(current);
 
+       if (current_test_flags(PF_FSTRANS)) {
+               current_restore_flags_nested(&pflags, PF_FSTRANS);
+               lockdep_restore_current_reclaim_state(reclaim_state);
+               atomic_dec(&rqstp->rq_pool->sp_nr_fstrans);
+       }
+
        mutex_lock(&nfsd_mutex);
        nfsdstats.th_cnt --;
 
diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index a0dbbd1e00e9..4b274aba51dd 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -48,6 +48,7 @@ struct svc_pool {
        struct list_head        sp_threads;     /* idle server threads */
        struct list_head        sp_sockets;     /* pending sockets */
        unsigned int            sp_nrthreads;   /* # of threads in pool */
+       atomic_t                sp_nr_fstrans;  /* # threads with PF_FSTRANS */
        struct list_head        sp_all_threads; /* all server threads */
        struct svc_pool_stats   sp_stats;       /* statistics on pool operation 
*/
        int                     sp_task_pending;/* has pending task */
diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 5de6801cd924..8b13f35b6cbb 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -477,6 +477,12 @@ __svc_create(struct svc_program *prog, unsigned int 
bufsize, int npools,
                INIT_LIST_HEAD(&pool->sp_threads);
                INIT_LIST_HEAD(&pool->sp_sockets);
                INIT_LIST_HEAD(&pool->sp_all_threads);
+               /* The number of threads with PF_FSTRANS set
+                * should never be reduced below 2, except when
+                * threads exit.  So we use atomic_dec_if_positive()
+                * on this value.
+                */
+               atomic_set(&pool->sp_nr_fstrans, -2);
                spin_lock_init(&pool->sp_lock);
        }
 


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