From: Yufen Yu yuyufen@huawei.com
commit 8d6996630c03d7ceeabe2611378fea5ca1c3f1b3 upstream.
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: 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+ Reviewed-by: Ming Lei ming.lei@redhat.com Reviewed-by: Bob Liu bob.liu@oracle.com Signed-off-by: Yufen Yu yuyufen@huawei.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
------- v2: - move rq_status from struct request to struct blk_flush_queue v3: - remove unnecessary '{}' pair. v4: - let spinlock to protect 'fq->rq_status' v5: - move rq_status after flush_running_idx member of struct blk_flush_queue Signed-off-by: Jens Axboe axboe@kernel.dk
--- block/blk-flush.c | 10 ++++++++++ block/blk-mq.c | 5 ++++- block/blk.h | 7 +++++++ 3 files changed, 21 insertions(+), 1 deletion(-)
--- a/block/blk-flush.c +++ b/block/blk-flush.c @@ -232,6 +232,16 @@ static void flush_end_io(struct request
/* release the tag's ownership to the req cloned from */ spin_lock_irqsave(&fq->mq_flush_lock, flags); + + if (!refcount_dec_and_test(&flush_rq->ref)) { + fq->rq_status = error; + spin_unlock_irqrestore(&fq->mq_flush_lock, flags); + return; + } + + if (fq->rq_status != BLK_STS_OK) + error = fq->rq_status; + hctx = blk_mq_map_queue(q, flush_rq->mq_ctx->cpu); if (!q->elevator) { blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq); --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -844,7 +844,10 @@ static void blk_mq_check_expired(struct */ if (blk_mq_req_expired(rq, next)) blk_mq_rq_timed_out(rq, reserved); - if (refcount_dec_and_test(&rq->ref)) + + if (is_flush_rq(rq, hctx)) + rq->end_io(rq, 0); + else if (refcount_dec_and_test(&rq->ref)) __blk_mq_free_request(rq); }
--- a/block/blk.h +++ b/block/blk.h @@ -23,6 +23,7 @@ struct blk_flush_queue { unsigned int flush_queue_delayed:1; unsigned int flush_pending_idx:1; unsigned int flush_running_idx:1; + blk_status_t rq_status; unsigned long flush_pending_since; struct list_head flush_queue[2]; struct list_head flush_data_in_flight; @@ -123,6 +124,12 @@ static inline void __blk_get_queue(struc kobject_get(&q->kobj); }
+static inline bool +is_flush_rq(struct request *req, struct blk_mq_hw_ctx *hctx) +{ + return hctx->fq->flush_rq == req; +} + struct blk_flush_queue *blk_alloc_flush_queue(struct request_queue *q, int node, int cmd_size, gfp_t flags); void blk_free_flush_queue(struct blk_flush_queue *q);