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Date: Tue, 10 Jul 2018 17:01:19 +1000
From: Daniel Axtens <dja@...ens.net>
To: NeilBrown <neilb@...e.com>, David Howells <dhowells@...hat.com>,
linux-cachefs@...hat.com
Cc: kiran.modukuri@...il.com, carmark.dlut@...il.com,
vegard.nossum@...il.com, linux-kernel@...r.kernel.org,
aderobertis@...rics.net, dhowells@...hat.com
Subject: Re: [PATCH 1/4] cachefiles: Fix assertion "6 == 5 is false" at fs/fscache/operation.c:494
NeilBrown <neilb@...e.com> writes:
> On Thu, Jul 05 2018, David Howells wrote:
>
>> From: kiran modukuri <kiran.modukuri@...il.com>
>>
>> There is a potential race in fscache operation enqueuing for reading and
>> copying multiple pages from cachefiles to netfs.
>> Under some heavy load system, it will happen very often.
>>
>> If this race occurs, an oops similar to the following is seen:
>>
>> kernel BUG at fs/fscache/operation.c:69!
>> invalid opcode: 0000 [#1] SMP
>> ...
>> #0 [ffff883fff0838d8] machine_kexec at ffffffff81051beb
>> #1 [ffff883fff083938] crash_kexec at ffffffff810f2542
>> #2 [ffff883fff083a08] oops_end at ffffffff8163e1a8
>> #3 [ffff883fff083a30] die at ffffffff8101859b
>> #4 [ffff883fff083a60] do_trap at ffffffff8163d860
>> #5 [ffff883fff083ab0] do_invalid_op at ffffffff81015204
>> #6 [ffff883fff083b60] invalid_op at ffffffff8164701e
>> [exception RIP: fscache_enqueue_operation+246]
>> RIP: ffffffffa0b793c6 RSP: ffff883fff083c18 RFLAGS: 00010046
>> RAX: 0000000000000019 RBX: ffff8832ed1a9ec0 RCX: 0000000000000006
>> RDX: 0000000000000000 RSI: 0000000000000046 RDI: 0000000000000046
>> RBP: ffff883fff083c20 R8: 0000000000000086 R9: 000000000000178f
>> R10: ffffffff816aeb00 R11: ffff883fff08392e R12: ffff8802f0525620
>> R13: ffff88407ffc01d8 R14: 0000000000000000 R15: 0000000000000003
>> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0000
>> #7 [ffff883fff083c10] fscache_enqueue_operation at ffffffffa0b793c6
>> #8 [ffff883fff083c28] cachefiles_read_waiter at ffffffffa0b15a48
>> #9 [ffff883fff083c48] __wake_up_common at ffffffff810af028
>>
>> Reported-by: Lei Xue <carmark.dlut@...il.com>
>> Reported-by: Vegard Nossum <vegard.nossum@...il.com>
>> Reported-by: Anthony DeRobertis <aderobertis@...rics.net>
>> Reported-by: NeilBrown <neilb@...e.com>
>> Reported-by: Daniel Axtens <dja@...ens.net>
>> Reported-by: KiranKumar Modukuri <kiran.modukuri@...il.com>
>> Signed-off-by: David Howells <dhowells@...hat.com>
>> ---
>>
>> fs/cachefiles/rdwr.c | 17 ++++++++++++-----
>> fs/fscache/operation.c | 6 ++++--
>> 2 files changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
>> index 5082c8a49686..40f7595aad10 100644
>> --- a/fs/cachefiles/rdwr.c
>> +++ b/fs/cachefiles/rdwr.c
>> @@ -27,6 +27,7 @@ static int cachefiles_read_waiter(wait_queue_entry_t *wait, unsigned mode,
>> struct cachefiles_one_read *monitor =
>> container_of(wait, struct cachefiles_one_read, monitor);
>> struct cachefiles_object *object;
>> + struct fscache_retrieval *op = monitor->op;
>> struct wait_bit_key *key = _key;
>> struct page *page = wait->private;
>>
>> @@ -51,16 +52,22 @@ static int cachefiles_read_waiter(wait_queue_entry_t *wait, unsigned mode,
>> list_del(&wait->entry);
>>
>> /* move onto the action list and queue for FS-Cache thread pool */
>> - ASSERT(monitor->op);
>> + ASSERT(op);
>>
>> - object = container_of(monitor->op->op.object,
>> - struct cachefiles_object, fscache);
>> + /* We need to temporarily bump the usage count as we don't own a ref
>> + * here otherwise cachefiles_read_copier() may free the op between the
>> + * monitor being enqueued on the op->to_do list and the op getting
>> + * enqueued on the work queue.
>> + */
>> + fscache_get_retrieval(op);
>>
>> + object = container_of(op->op.object, struct cachefiles_object, fscache);
>> spin_lock(&object->work_lock);
>> - list_add_tail(&monitor->op_link, &monitor->op->to_do);
>> + list_add_tail(&monitor->op_link, &op->to_do);
>> spin_unlock(&object->work_lock);
>>
>> - fscache_enqueue_retrieval(monitor->op);
>> + fscache_enqueue_retrieval(op);
>> + fscache_put_retrieval(op);
>> return 0;
>> }
>
> Thanks - I like this approach. Taking the extra reference makes it a
> lot more clear what is happening and why.
I personally preferred the other approach, but I looked at this
approach in great detail and am confident that it is correct.
Reviewed-by: Daniel Axtens <dja@...ens.net>
Regards,
Daniel
>
> Thanks,
> NeilBrown
>
>
>>
>> diff --git a/fs/fscache/operation.c b/fs/fscache/operation.c
>> index e30c5975ea58..8d265790374c 100644
>> --- a/fs/fscache/operation.c
>> +++ b/fs/fscache/operation.c
>> @@ -70,7 +70,8 @@ void fscache_enqueue_operation(struct fscache_operation *op)
>> ASSERT(op->processor != NULL);
>> ASSERT(fscache_object_is_available(op->object));
>> ASSERTCMP(atomic_read(&op->usage), >, 0);
>> - ASSERTCMP(op->state, ==, FSCACHE_OP_ST_IN_PROGRESS);
>> + ASSERTIFCMP(op->state != FSCACHE_OP_ST_IN_PROGRESS,
>> + op->state, ==, FSCACHE_OP_ST_CANCELLED);
>>
>> fscache_stat(&fscache_n_op_enqueue);
>> switch (op->flags & FSCACHE_OP_TYPE) {
>> @@ -499,7 +500,8 @@ void fscache_put_operation(struct fscache_operation *op)
>> struct fscache_cache *cache;
>>
>> _enter("{OBJ%x OP%x,%d}",
>> - op->object->debug_id, op->debug_id, atomic_read(&op->usage));
>> + op->object ? op->object->debug_id : 0,
>> + op->debug_id, atomic_read(&op->usage));
>>
>> ASSERTCMP(atomic_read(&op->usage), >, 0);
>>
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