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

NeilBrown
2014-04-16T04:18:33Z

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@suse.de>
---
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);
}