On 2019/9/12 10:46, Ming Lei wrote:
On Sat, Sep 07, 2019 at 06:24:50PM +0800, Yufen Yu wrote:
There is a race condition between timeout check and completion for
flush request as follow:
timeout_work issue flush issue flush
blk_insert_flush
blk_insert_flush
blk_mq_timeout_work
blk_kick_flush
blk_mq_queue_tag_busy_iter
blk_mq_check_expired(flush_rq)
__blk_mq_end_request
flush_end_io
blk_kick_flush
blk_rq_init(flush_rq)
memset(flush_rq, 0)
Not see there is memset(flush_rq, 0) in block/blk-flush.c
Call path as follow:
blk_kick_flush
blk_rq_init
memset(rq, 0, sizeof(*rq));
blk_mq_timed_out
BUG_ON flush_rq->q->mq_ops
flush_rq->q won't be changed by blk_rq_init(), and either READ or WRITE
on variable with machine WORD length are atomic, so how can the BUG_ON()
be triggered? Do you have the actual BUG log?
Also now it is driver's responsibility for avoiding race between normal
completion and timeout.
I have reproduced the bug by adding time delay in timeout handle and memset.
BUG_ON log as follow:
[ 108.825472] BUG: kernel NULL pointer dereference, address:
0000000000000040
[ 108.826091] #PF: supervisor read access in kernel mode
[ 108.826543] #PF: error_code(0x0000) - not-present page
[ 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
[ 108.828326] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS ?-20180531_142017-buildhw-08.phx2.fedoraproject.org-1.fc28 04/01/2014
[ 108.829503] Workqueue: kblockd blk_mq_timeout_work
[ 108.829913] RIP: 0010:blk_mq_check_expired+0x258/0x330
[ 108.830439] Code: 01 e9 0a ff ff ff 48 83 05 34 45 dd 02 01 4c 39 63
40 0f 84 8a 00 00 00 0d 00 00 20 00 40 0f b6 f5 41 89 44 24 1c 49 8b 04
24 <48> 8b 40 40 48 8b 40 20 48 85 c0 0f 84 90 00 00 00 48 83 05 2f 44
[ 108.832246] RSP: 0018:ffffbf7ac18b7db0 EFLAGS: 00010206
[ 108.832756] RAX: 0000000000000000 RBX: ffffffffb56e0250 RCX:
0000000000000000
[ 108.833444] RDX: 0000000000000000 RSI: 0000000000000000 RDI:
ffff9ab7fbb96538
[ 108.834149] RBP: 0000000000000000 R08: 000000000000024b R09:
0000000000000030
[ 108.834840] R10: 000000000000004e R11: ffffbf7ac18b7c40 R12:
ffff9ab7f756e000
[ 108.835531] R13: ffffbf7ac18b7e70 R14: 0000000000000017 R15:
ffff9ab7f6ead0a0
[ 108.836228] FS: 0000000000000000(0000) GS:ffff9ab7fbb80000(0000)
knlGS:0000000000000000
[ 108.837026] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 108.837588] CR2: 0000000000000040 CR3: 000000013544c000 CR4:
00000000000006e0
[ 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
[ 108.843455] Modules linked in:
[ 108.843758] CR2: 0000000000000040
[ 108.844090] ---[ end trace e0ac552505fa1b95 ]---
blk_mq_rq_timed_out() attempt to read 'req->q->mq_ops->timeout', but 'q
== 0' currently,
which triggers BUG_ON.
For normal request, we need to get a tag and then allocate corresponding request.
Thus, the request cannot be reallocated before the tag have been free.
Commit 1d9bd5161ba ("blk-mq: replace timeout synchronization with a RCU and
generation based scheme") and commit 12f5b93145 ("blk-mq: Remove generation
seqeunce") can guarantee the consistency of timeout handle and completion.
However, 'flush_rq' have been forgotten. 'flush_rq' allocation management
dependents on flush implemention mechanism. Each hctx has only one 'flush_rq'.
When a flush request have completed, the next flush request will hold the 'flush_rq'.
In the end, timeout handle may access the cleared 'flush_rq'.
We fix this problem by checking request refcount 'rq->ref', as normal request.
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().
Signed-off-by: Yufen Yu <yuyufen@xxxxxxxxxx>
---
block/blk-flush.c | 8 ++++++++
block/blk-mq.c | 7 +++++--
block/blk.h | 5 +++++
include/linux/blkdev.h | 2 ++
4 files changed, 20 insertions(+), 2 deletions(-)
diff --git a/block/blk-flush.c b/block/blk-flush.c
index aedd9320e605..359a7e1a0925 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -212,6 +212,14 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
struct blk_flush_queue *fq = blk_get_flush_queue(q, flush_rq->mq_ctx);
struct blk_mq_hw_ctx *hctx;
+ if (!refcount_dec_and_test(&flush_rq->ref)) {
+ flush_rq->rq_status = error;
+ return;
+ }
+
+ if (flush_rq->rq_status != BLK_STS_OK)
+ error = flush_rq->rq_status;
+
/* release the tag's ownership to the req cloned from */
spin_lock_irqsave(&fq->mq_flush_lock, flags);
hctx = flush_rq->mq_hctx;
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0835f4d8d42e..3d2b2c2e9cdf 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -905,9 +905,12 @@ static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
*/
if (blk_mq_req_expired(rq, next))
blk_mq_rq_timed_out(rq, reserved);
- if (refcount_dec_and_test(&rq->ref))
- __blk_mq_free_request(rq);
+ if (is_flush_rq(rq, hctx)) {
+ rq->end_io(rq, 0);
+ } else if (refcount_dec_and_test(&rq->ref)) {
+ __blk_mq_free_request(rq);
+ }
return true;
}
diff --git a/block/blk.h b/block/blk.h
index de6b2e146d6e..f503ef9ad3e6 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -47,6 +47,11 @@ static inline void __blk_get_queue(struct request_queue *q)
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);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 1ef375dafb1c..b1d05077e03f 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -237,6 +237,8 @@ struct request {
*/
rq_end_io_fn *end_io;
void *end_io_data;
+
+ blk_status_t rq_status;
};
'rq_status' is only for flush request, so it may be added to 'struct
blk_flush_queue' instead of 'struct request'.
That's a good idea.
Thanks,
Yufen
