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From: David Howells <dhowells@redhat.com>
To: linux-cachefs@redhat.com
Cc: dhowells@redhat.com, linux-fsdevel@vger.kernel.org,
	linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 05/13] FS-Cache: Synchronise object death state change vs operation submission
Date: Thu, 26 Feb 2015 14:02:27 +0000	[thread overview]
Message-ID: <20150226140227.2387.72446.stgit@warthog.procyon.org.uk> (raw)
In-Reply-To: <20150226140155.2387.70579.stgit@warthog.procyon.org.uk>

When an object is being marked as no longer live, do this under the object
spinlock to prevent a race with operation submission targeted on that object.

The problem occurs due to the following pair of intertwined sequences when the
cache tries to create an object that would take it over the hard available
space limit:

 NETFS INTERFACE
 ===============
 (A) The netfs calls fscache_acquire_cookie().  object creation is deferred to
     the object state machine and the netfs is allowed to continue.

	OBJECT STATE MACHINE KTHREAD
	============================
	(1) The object is looked up on disk by fscache_look_up_object()
	    calling cachefiles_walk_to_object().  The latter finds that the
	    object is not yet represented on disk and calls
	    fscache_object_lookup_negative().

	(2) fscache_object_lookup_negative() sets FSCACHE_COOKIE_NO_DATA_YET
	    and clears FSCACHE_COOKIE_LOOKING_UP, thus allowing the netfs to
	    start queuing read operations.

 (B) The netfs calls fscache_read_or_alloc_pages().  This calls
     fscache_wait_for_deferred_lookup() which sees FSCACHE_COOKIE_LOOKING_UP
     become clear, allowing the read to begin.

 (C) A read operation is set up and passed to fscache_submit_op() to deal
     with.

	(3) cachefiles_walk_to_object() calls cachefiles_has_space(), which
	    fails (or one of the file operations to create stuff fails).
	    cachefiles returns an error to fscache.

	(4) fscache_look_up_object() transits to the LOOKUP_FAILURE state,

	(5) fscache_lookup_failure() sets FSCACHE_OBJECT_LOOKED_UP and
	    FSCACHE_COOKIE_UNAVAILABLE and clears FSCACHE_COOKIE_LOOKING_UP
	    then transits to the KILL_OBJECT state.

	(6) fscache_kill_object() clears FSCACHE_OBJECT_IS_LIVE in an attempt
	    to reject any further requests from the netfs.

	(7) object->n_ops is examined and found to be 0.
	    fscache_kill_object() transits to the DROP_OBJECT state.

 (D) fscache_submit_op() locks the object spinlock, sees if it can dispatch
     the op immediately by calling fscache_object_is_active() - which fails
     since FSCACHE_OBJECT_IS_AVAILABLE has not yet been set.

 (E) fscache_submit_op() then tests FSCACHE_OBJECT_LOOKED_UP - which is set.
     It then queues the object and increments object->n_ops.

	(8) fscache_drop_object() releases the object and eventually
	    fscache_put_object() calls cachefiles_put_object() which suffers
	    an assertion failure here:

		ASSERTCMP(object->fscache.n_ops, ==, 0);

Locking the object spinlock in step (6) around the clearance of
FSCACHE_OBJECT_IS_LIVE ensures that the the decision trees in
fscache_submit_op() and fscache_submit_exclusive_op() don't see the IS_LIVE
flag being cleared mid-decision: either the op is queued before step (7) - in
which case fscache_kill_object() will see n_ops>0 and will deal with the op -
or the op will be rejected.

This, combined with rejecting op submission if the target object is dying, fix
the problem.

The problem shows up as the following oops:

CacheFiles: Assertion failed
CacheFiles: 1 == 0 is false
------------[ cut here ]------------
kernel BUG at ../fs/cachefiles/interface.c:339!
...
RIP: 0010:[<ffffffffa014fd9c>]  [<ffffffffa014fd9c>] cachefiles_put_object+0x2a4/0x301 [cachefiles]
...
Call Trace:
 [<ffffffffa008674b>] fscache_put_object+0x18/0x21 [fscache]
 [<ffffffffa00883e6>] fscache_object_work_func+0x3ba/0x3c9 [fscache]
 [<ffffffff81054dad>] process_one_work+0x226/0x441
 [<ffffffff81055d91>] worker_thread+0x273/0x36b
 [<ffffffff81055b1e>] ? rescuer_thread+0x2e1/0x2e1
 [<ffffffff81059b9d>] kthread+0x10e/0x116
 [<ffffffff81059a8f>] ? kthread_create_on_node+0x1bb/0x1bb
 [<ffffffff815579ac>] ret_from_fork+0x7c/0xb0
 [<ffffffff81059a8f>] ? kthread_create_on_node+0x1bb/0x1bb

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

 fs/fscache/object.c |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/fscache/object.c b/fs/fscache/object.c
index 9b79fc9a1464..40049f7505f0 100644
--- a/fs/fscache/object.c
+++ b/fs/fscache/object.c
@@ -328,6 +328,17 @@ void fscache_object_init(struct fscache_object *object,
 EXPORT_SYMBOL(fscache_object_init);
 
 /*
+ * Mark the object as no longer being live, making sure that we synchronise
+ * against op submission.
+ */
+static inline void fscache_mark_object_dead(struct fscache_object *object)
+{
+	spin_lock(&object->lock);
+	clear_bit(FSCACHE_OBJECT_IS_LIVE, &object->flags);
+	spin_unlock(&object->lock);
+}
+
+/*
  * Abort object initialisation before we start it.
  */
 static const struct fscache_state *fscache_abort_initialisation(struct fscache_object *object,
@@ -631,7 +642,7 @@ static const struct fscache_state *fscache_kill_object(struct fscache_object *ob
 	_enter("{OBJ%x,%d,%d},%d",
 	       object->debug_id, object->n_ops, object->n_children, event);
 
-	clear_bit(FSCACHE_OBJECT_IS_LIVE, &object->flags);
+	fscache_mark_object_dead(object);
 	object->oob_event_mask = 0;
 
 	if (list_empty(&object->dependents) &&
@@ -976,13 +987,13 @@ static const struct fscache_state *_fscache_invalidate_object(struct fscache_obj
 	return transit_to(UPDATE_OBJECT);
 
 nomem:
-	clear_bit(FSCACHE_OBJECT_IS_LIVE, &object->flags);
+	fscache_mark_object_dead(object);
 	fscache_unuse_cookie(object);
 	_leave(" [ENOMEM]");
 	return transit_to(KILL_OBJECT);
 
 submit_op_failed:
-	clear_bit(FSCACHE_OBJECT_IS_LIVE, &object->flags);
+	fscache_mark_object_dead(object);
 	spin_unlock(&cookie->lock);
 	fscache_unuse_cookie(object);
 	kfree(op);


WARNING: multiple messages have this Message-ID
From: David Howells <dhowells@redhat.com>
To: linux-cachefs@redhat.com
Cc: linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: [PATCH 05/13] FS-Cache: Synchronise object death state change vs operation submission
Date: Thu, 26 Feb 2015 14:02:27 +0000	[thread overview]
Message-ID: <20150226140227.2387.72446.stgit@warthog.procyon.org.uk> (raw)
In-Reply-To: <20150226140155.2387.70579.stgit@warthog.procyon.org.uk>

When an object is being marked as no longer live, do this under the object
spinlock to prevent a race with operation submission targeted on that object.

The problem occurs due to the following pair of intertwined sequences when the
cache tries to create an object that would take it over the hard available
space limit:

 NETFS INTERFACE
 ===============
 (A) The netfs calls fscache_acquire_cookie().  object creation is deferred to
     the object state machine and the netfs is allowed to continue.

	OBJECT STATE MACHINE KTHREAD
	============================
	(1) The object is looked up on disk by fscache_look_up_object()
	    calling cachefiles_walk_to_object().  The latter finds that the
	    object is not yet represented on disk and calls
	    fscache_object_lookup_negative().

	(2) fscache_object_lookup_negative() sets FSCACHE_COOKIE_NO_DATA_YET
	    and clears FSCACHE_COOKIE_LOOKING_UP, thus allowing the netfs to
	    start queuing read operations.

 (B) The netfs calls fscache_read_or_alloc_pages().  This calls
     fscache_wait_for_deferred_lookup() which sees FSCACHE_COOKIE_LOOKING_UP
     become clear, allowing the read to begin.

 (C) A read operation is set up and passed to fscache_submit_op() to deal
     with.

	(3) cachefiles_walk_to_object() calls cachefiles_has_space(), which
	    fails (or one of the file operations to create stuff fails).
	    cachefiles returns an error to fscache.

	(4) fscache_look_up_object() transits to the LOOKUP_FAILURE state,

	(5) fscache_lookup_failure() sets FSCACHE_OBJECT_LOOKED_UP and
	    FSCACHE_COOKIE_UNAVAILABLE and clears FSCACHE_COOKIE_LOOKING_UP
	    then transits to the KILL_OBJECT state.

	(6) fscache_kill_object() clears FSCACHE_OBJECT_IS_LIVE in an attempt
	    to reject any further requests from the netfs.

	(7) object->n_ops is examined and found to be 0.
	    fscache_kill_object() transits to the DROP_OBJECT state.

 (D) fscache_submit_op() locks the object spinlock, sees if it can dispatch
     the op immediately by calling fscache_object_is_active() - which fails
     since FSCACHE_OBJECT_IS_AVAILABLE has not yet been set.

 (E) fscache_submit_op() then tests FSCACHE_OBJECT_LOOKED_UP - which is set.
     It then queues the object and increments object->n_ops.

	(8) fscache_drop_object() releases the object and eventually
	    fscache_put_object() calls cachefiles_put_object() which suffers
	    an assertion failure here:

		ASSERTCMP(object->fscache.n_ops, ==, 0);

Locking the object spinlock in step (6) around the clearance of
FSCACHE_OBJECT_IS_LIVE ensures that the the decision trees in
fscache_submit_op() and fscache_submit_exclusive_op() don't see the IS_LIVE
flag being cleared mid-decision: either the op is queued before step (7) - in
which case fscache_kill_object() will see n_ops>0 and will deal with the op -
or the op will be rejected.

This, combined with rejecting op submission if the target object is dying, fix
the problem.

The problem shows up as the following oops:

CacheFiles: Assertion failed
CacheFiles: 1 == 0 is false
------------[ cut here ]------------
kernel BUG at ../fs/cachefiles/interface.c:339!
...
RIP: 0010:[<ffffffffa014fd9c>]  [<ffffffffa014fd9c>] cachefiles_put_object+0x2a4/0x301 [cachefiles]
...
Call Trace:
 [<ffffffffa008674b>] fscache_put_object+0x18/0x21 [fscache]
 [<ffffffffa00883e6>] fscache_object_work_func+0x3ba/0x3c9 [fscache]
 [<ffffffff81054dad>] process_one_work+0x226/0x441
 [<ffffffff81055d91>] worker_thread+0x273/0x36b
 [<ffffffff81055b1e>] ? rescuer_thread+0x2e1/0x2e1
 [<ffffffff81059b9d>] kthread+0x10e/0x116
 [<ffffffff81059a8f>] ? kthread_create_on_node+0x1bb/0x1bb
 [<ffffffff815579ac>] ret_from_fork+0x7c/0xb0
 [<ffffffff81059a8f>] ? kthread_create_on_node+0x1bb/0x1bb

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

 fs/fscache/object.c |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/fscache/object.c b/fs/fscache/object.c
index 9b79fc9a1464..40049f7505f0 100644
--- a/fs/fscache/object.c
+++ b/fs/fscache/object.c
@@ -328,6 +328,17 @@ void fscache_object_init(struct fscache_object *object,
 EXPORT_SYMBOL(fscache_object_init);
 
 /*
+ * Mark the object as no longer being live, making sure that we synchronise
+ * against op submission.
+ */
+static inline void fscache_mark_object_dead(struct fscache_object *object)
+{
+	spin_lock(&object->lock);
+	clear_bit(FSCACHE_OBJECT_IS_LIVE, &object->flags);
+	spin_unlock(&object->lock);
+}
+
+/*
  * Abort object initialisation before we start it.
  */
 static const struct fscache_state *fscache_abort_initialisation(struct fscache_object *object,
@@ -631,7 +642,7 @@ static const struct fscache_state *fscache_kill_object(struct fscache_object *ob
 	_enter("{OBJ%x,%d,%d},%d",
 	       object->debug_id, object->n_ops, object->n_children, event);
 
-	clear_bit(FSCACHE_OBJECT_IS_LIVE, &object->flags);
+	fscache_mark_object_dead(object);
 	object->oob_event_mask = 0;
 
 	if (list_empty(&object->dependents) &&
@@ -976,13 +987,13 @@ static const struct fscache_state *_fscache_invalidate_object(struct fscache_obj
 	return transit_to(UPDATE_OBJECT);
 
 nomem:
-	clear_bit(FSCACHE_OBJECT_IS_LIVE, &object->flags);
+	fscache_mark_object_dead(object);
 	fscache_unuse_cookie(object);
 	_leave(" [ENOMEM]");
 	return transit_to(KILL_OBJECT);
 
 submit_op_failed:
-	clear_bit(FSCACHE_OBJECT_IS_LIVE, &object->flags);
+	fscache_mark_object_dead(object);
 	spin_unlock(&cookie->lock);
 	fscache_unuse_cookie(object);
 	kfree(op);

  parent reply	other threads:[~2015-02-26 14:02 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-26 14:01 [PATCH 00/13] FS-Cache: Fix a number of bugs that occur when the cache runs out of space David Howells
2015-02-26 14:02 ` [PATCH 01/13] FS-Cache: Count culled objects and objects rejected due to lack " David Howells
2015-02-26 14:02 ` [PATCH 02/13] FS-Cache: Move fscache_report_unexpected_submission() to make it more available David Howells
2015-02-26 14:02 ` [PATCH 03/13] FS-Cache: When submitting an op, cancel it if the target object is dying David Howells
2015-02-26 14:02 ` [PATCH 04/13] FS-Cache: Handle a new operation submitted against a killed object David Howells
2015-02-26 14:02 ` David Howells [this message]
2015-02-26 14:02 ` [PATCH 06/13] FS-Cache: fscache_object_is_dead() has wrong logic, kill it David Howells
2015-02-26 14:02 ` [PATCH 07/13] FS-Cache: Permit fscache_cancel_op() to cancel in-progress operations too David Howells
2015-02-26 14:02 ` [PATCH 08/13] FS-Cache: Out of line fscache_operation_init() David Howells
2015-02-26 14:02 ` [PATCH 09/13] FS-Cache: Count the number of initialised operations David Howells
2015-02-26 14:02 ` [PATCH 10/13] FS-Cache: Fix cancellation of in-progress operation David Howells
2015-02-26 14:03 ` [PATCH 11/13] FS-Cache: Put an aborted initialised op so that it is accounted correctly David Howells
2015-02-26 14:03 ` [PATCH 12/13] FS-Cache: The operation cancellation method needs calling in more places David Howells
2015-02-26 14:03 ` [PATCH 13/13] FS-Cache: Retain the netfs context in the retrieval op earlier David Howells
     [not found] ` <20150226140155.2387.70579.stgit-S6HVgzuS8uM4Awkfq6JHfwNdhmdF6hFW@public.gmane.org>
2015-03-02 23:58   ` [PATCH 00/13] FS-Cache: Fix a number of bugs that occur when the cache runs out of space Jeff Layton
2015-03-02 23:58 ` Jeff Layton

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