Yufen Yu yuyufen@huawei.com 于2019年9月26日周四 上午11:30写道：
We got a null pointer deference BUG_ON in blk_mq_rq_timed_out() as following:
[ 108.825472] BUG: kernel NULL pointer dereference, address: 0000000000000040 [ 108.827059] PGD 0 P4D 0 [ 108.827313] Oops: 0000 [#1] SMP PTI [ 108.827657] CPU: 6 PID: 198 Comm: kworker/6:1H Not tainted 5.3.0-rc8+ #431 [ 108.829503] Workqueue: kblockd blk_mq_timeout_work [ 108.829913] RIP: 0010:blk_mq_check_expired+0x258/0x330 [ 108.838191] Call Trace: [ 108.838406] bt_iter+0x74/0x80 [ 108.838665] blk_mq_queue_tag_busy_iter+0x204/0x450 [ 108.839074] ? __switch_to_asm+0x34/0x70 [ 108.839405] ? blk_mq_stop_hw_queue+0x40/0x40 [ 108.839823] ? blk_mq_stop_hw_queue+0x40/0x40 [ 108.840273] ? syscall_return_via_sysret+0xf/0x7f [ 108.840732] blk_mq_timeout_work+0x74/0x200 [ 108.841151] process_one_work+0x297/0x680 [ 108.841550] worker_thread+0x29c/0x6f0 [ 108.841926] ? rescuer_thread+0x580/0x580 [ 108.842344] kthread+0x16a/0x1a0 [ 108.842666] ? kthread_flush_work+0x170/0x170 [ 108.843100] ret_from_fork+0x35/0x40
The bug is caused by the race between timeout handle and completion for flush request.
When timeout handle function blk_mq_rq_timed_out() try to read 'req->q->mq_ops', the 'req' have completed and reinitiated by next flush request, which would call blk_rq_init() to clear 'req' as 0.
After commit 12f5b93145 ("blk-mq: Remove generation seqeunce"), normal requests lifetime are protected by refcount. Until 'rq->ref' drop to zero, the request can really be free. Thus, these requests cannot been reused before timeout handle finish.
However, flush request has defined .end_io and rq->end_io() is still called even if 'rq->ref' doesn't drop to zero. After that, the 'flush_rq' can be reused by the next flush request handle, resulting in null pointer deference BUG ON.
We fix this problem by covering flush request with 'rq->ref'. If the refcount is not zero, flush_end_io() return and wait the last holder recall it. To record the request status, we add a new entry 'rq_status', which will be used in flush_end_io().
Cc: Ming Lei ming.lei@redhat.com Cc: Christoph Hellwig hch@infradead.org Cc: Keith Busch keith.busch@intel.com Cc: Bart Van Assche bvanassche@acm.org Cc: stable@vger.kernel.org # v4.18+ Signed-off-by: Yufen Yu yuyufen@huawei.com
Hi Yufen,
Can you share your reproducer, I think the bug was there for long time, we hit it in kernel 4.4. We also need to fix it for older LTS kernel.
Do you have an idea, how should we fix it for older LTS kernel?
Regards, Jack Wang