Re: [PATCH] blk-mq: fix is_flush_rq

[Ming Lei <ming.lei@redhat.com>]

On Tue, Aug 17, 2021 at 08:02:58PM -0700, Bart Van Assche wrote:
> On 8/17/21 6:09 PM, Ming Lei wrote:
> > +bool is_flush_rq(struct request *rq)
> > +{
> > +	return rq->end_io == flush_end_io;
> > +}
> 
> My understanding is that calling is_flush_rq() is only safe if the
> caller guarantees that the request refcount >= 1. How about adding a
> comment that explains this?

Yeah, we can add the following words, but it isn't something urgent
since is_flush_rq() is one block layer internal helper.

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 4201728bf3a5..babf6262120e 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -262,6 +262,11 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
 	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
 }
 
+/*
+ * Caller has to grab refcount of this request, otherwise the flush request
+ * may be re-cycled, then rq->end_io is cleared and kernel panic is caused,
+ * see blk_mq_put_rq_ref().
+ */
 bool is_flush_rq(struct request *rq)
 {
 	return rq->end_io == flush_end_io;




Thanks,
Ming