CacheFiles: Catch an overly long wait for an old active object

Catch an overly long wait for an old, dying active object when we want to
replace it with a new one.  The probability is that all the slow-work threads
are hogged, and the delete can't get a look in.

What we do instead is:

 (1) if there's nothing in the slow work queue, we sleep until either the dying
     object has finished dying or there is something in the slow work queue
     behind which we can queue our object.

 (2) if there is something in the slow work queue, we return ETIMEDOUT to
     fscache_lookup_object(), which then puts us back on the slow work queue,
     presumably behind the deletion that we're blocked by.  We are then
     deferred for a while until we work our way back through the queue -
     without blocking a slow-work thread unnecessarily.

A backtrace similar to the following may appear in the log without this patch:

	INFO: task kslowd004:5711 blocked for more than 120 seconds.
	"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
	kslowd004     D 0000000000000000     0  5711      2 0x00000080
	 ffff88000340bb80 0000000000000046 ffff88002550d000 0000000000000000
	 ffff88002550d000 0000000000000007 ffff88000340bfd8 ffff88002550d2a8
	 000000000000ddf0 00000000000118c0 00000000000118c0 ffff88002550d2a8
	Call Trace:
	 [<ffffffff81058e21>] ? trace_hardirqs_on+0xd/0xf
	 [<ffffffffa011c4d8>] ? cachefiles_wait_bit+0x0/0xd [cachefiles]
	 [<ffffffffa011c4e1>] cachefiles_wait_bit+0x9/0xd [cachefiles]
	 [<ffffffff81353153>] __wait_on_bit+0x43/0x76
	 [<ffffffff8111ae39>] ? ext3_xattr_get+0x1ec/0x270
	 [<ffffffff813531ef>] out_of_line_wait_on_bit+0x69/0x74
	 [<ffffffffa011c4d8>] ? cachefiles_wait_bit+0x0/0xd [cachefiles]
	 [<ffffffff8104c125>] ? wake_bit_function+0x0/0x2e
	 [<ffffffffa011bc79>] cachefiles_mark_object_active+0x203/0x23b [cachefiles]
	 [<ffffffffa011c209>] cachefiles_walk_to_object+0x558/0x827 [cachefiles]
	 [<ffffffffa011a429>] cachefiles_lookup_object+0xac/0x12a [cachefiles]
	 [<ffffffffa00aa1e9>] fscache_lookup_object+0x1c7/0x214 [fscache]
	 [<ffffffffa00aafc5>] fscache_object_state_machine+0xa5/0x52d [fscache]
	 [<ffffffffa00ab4ac>] fscache_object_slow_work_execute+0x5f/0xa0 [fscache]
	 [<ffffffff81082093>] slow_work_execute+0x18f/0x2d1
	 [<ffffffff8108239a>] slow_work_thread+0x1c5/0x308
	 [<ffffffff8104c0f1>] ? autoremove_wake_function+0x0/0x34
	 [<ffffffff810821d5>] ? slow_work_thread+0x0/0x308
	 [<ffffffff8104be91>] kthread+0x7a/0x82
	 [<ffffffff8100beda>] child_rip+0xa/0x20
	 [<ffffffff8100b87c>] ? restore_args+0x0/0x30
	 [<ffffffff8104be17>] ? kthread+0x0/0x82
	 [<ffffffff8100bed0>] ? child_rip+0x0/0x20
	1 lock held by kslowd004/5711:
	 #0:  (&sb->s_type->i_mutex_key#7/1){+.+.+.}, at: [<ffffffffa011be64>] cachefiles_walk_to_object+0x1b3/0x827 [cachefiles]

Signed-off-by: David Howells <dhowells@redhat.com>

3 files changed, 13 insertions, 2 deletions

diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index 5b49a373689b..0ca2566e038c 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -215,6 +215,7 @@ extern atomic_t fscache_n_object_no_alloc;
 extern atomic_t fscache_n_object_lookups;
 extern atomic_t fscache_n_object_lookups_negative;
 extern atomic_t fscache_n_object_lookups_positive;
+extern atomic_t fscache_n_object_lookups_timed_out;
 extern atomic_t fscache_n_object_created;
 extern atomic_t fscache_n_object_avail;
 extern atomic_t fscache_n_object_dead;
diff --git a/fs/fscache/object.c b/fs/fscache/object.c
index f3f952cf887e..e513ac599c8e 100644
--- a/fs/fscache/object.c
+++ b/fs/fscache/object.c
@@ -468,6 +468,7 @@ static void fscache_lookup_object(struct fscache_object *object)
 {
         struct fscache_cookie *cookie = object->cookie;
         struct fscache_object *parent;
+        int ret;
 
         _enter("");
 
@@ -493,12 +494,19 @@ static void fscache_lookup_object(struct fscache_object *object)
 
         fscache_stat(&fscache_n_object_lookups);
         fscache_stat(&fscache_n_cop_lookup_object);
-        object->cache->ops->lookup_object(object);
+        ret = object->cache->ops->lookup_object(object);
         fscache_stat_d(&fscache_n_cop_lookup_object);
 
         if (test_bit(FSCACHE_OBJECT_EV_ERROR, &object->events))
                 set_bit(FSCACHE_COOKIE_UNAVAILABLE, &cookie->flags);
 
+        if (ret == -ETIMEDOUT) {
+                /* probably stuck behind another object, so move this one to
+                 * the back of the queue */
+                fscache_stat(&fscache_n_object_lookups_timed_out);
+                set_bit(FSCACHE_OBJECT_EV_REQUEUE, &object->events);
+        }
+
         _leave("");
 }
 
diff --git a/fs/fscache/stats.c b/fs/fscache/stats.c
index 05f77caf4a2d..46435f3aae68 100644
--- a/fs/fscache/stats.c
+++ b/fs/fscache/stats.c
@@ -98,6 +98,7 @@ atomic_t fscache_n_object_no_alloc;
 atomic_t fscache_n_object_lookups;
 atomic_t fscache_n_object_lookups_negative;
 atomic_t fscache_n_object_lookups_positive;
+atomic_t fscache_n_object_lookups_timed_out;
 atomic_t fscache_n_object_created;
 atomic_t fscache_n_object_avail;
 atomic_t fscache_n_object_dead;
@@ -160,10 +161,11 @@ static int fscache_stats_show(struct seq_file *m, void *v)
                    atomic_read(&fscache_n_acquires_nobufs),
                    atomic_read(&fscache_n_acquires_oom));
 
-        seq_printf(m, "Lookups: n=%u neg=%u pos=%u crt=%u\n",
+        seq_printf(m, "Lookups: n=%u neg=%u pos=%u crt=%u tmo=%u\n",
                    atomic_read(&fscache_n_object_lookups),
                    atomic_read(&fscache_n_object_lookups_negative),
                    atomic_read(&fscache_n_object_lookups_positive),
+                   atomic_read(&fscache_n_object_lookups_timed_out),
                    atomic_read(&fscache_n_object_created));
 
         seq_printf(m, "Updates: n=%u nul=%u run=%u\n",