On 2019/9/12 18:07, Ming Lei wrote:
On Thu, Sep 12, 2019 at 04:49:15PM +0800, Yufen Yu wrote:
On 2019/9/12 12:16, Ming Lei wrote:
On Thu, Sep 12, 2019 at 11:29:18AM +0800, Yufen Yu wrote:
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));
Looks I miss this one in blk_rq_init(), sorry for that.
Given there are only two users of blk_rq_init(), one simple fix could be
not clearing queue in blk_rq_init(), something like below?
diff --git a/block/blk-core.c b/block/blk-core.c
index 77807a5d7f9e..25e6a045c821 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -107,7 +107,9 @@ EXPORT_SYMBOL_GPL(blk_queue_flag_test_and_set);
void blk_rq_init(struct request_queue *q, struct request *rq)
{
- memset(rq, 0, sizeof(*rq));
+ const int offset = offsetof(struct request, q);
+
+ memset((void *)rq + offset, 0, sizeof(*rq) - offset);
INIT_LIST_HEAD(&rq->queuelist);
rq->q = q;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 1ac790178787..382e71b8787d 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -130,7 +130,7 @@ enum mq_rq_state {
* especially blk_mq_rq_ctx_init() to take care of the added fields.
*/
struct request {
- struct request_queue *q;
+ struct request_queue *q; /* Must be the 1st field */
struct blk_mq_ctx *mq_ctx;
struct blk_mq_hw_ctx *mq_hctx;
Not set req->q as '0' can just avoid BUG_ON for NULL pointer deference.
However, the root problem is that 'flush_rq' have been reused while
timeout function handle it currently. That means mq_ops->timeout() may
access old values remained by the last flush request and make the wrong
decision.
Take the race condition in the patch as an example.
blk_mq_check_expired
blk_mq_rq_timed_out
req->q->mq_ops->timeout // Driver timeout handle may read old data
refcount_dec_and_test(&rq)
__blk_mq_free_request // If rq have been reset has '1' in
blk_rq_init(), it will be free here.
So, I think we should solve this problem completely. Just like normal
request,
we can prevent flush request to call end_io when timeout handle the request.
Seems it isn't specific for 'flush_rq', and it should be one generic issue
for any request which implements .end_io.
For requests without defining .end_io, rq->ref is applied for protecting
its lifetime. However, rq->end_io() is still called even if rq->ref doesn't
drop to zero.
If the above is correct, we need to let rq->ref to cover rq->end_io().
We ignore the fact that we may also need to free 'rq' after calling
rq->end_io(),
such as end_clone_request(), mq_flush_data_end_io().
If we let 'rq->ref' to cover rq->end_io(), 'rq->ref' have been
decreased to '0'
before calling __blk_mq_free_request(). Then, the function will never be
called.
So, I think flush request may need to be fixed individually.
Thanks,
Yufen
