linux-kernel - [PATCH 5/7] block: Remove superflous rcu_read_[un]lock_sched() in blk_queue

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Date:   Tue,  6 Mar 2018 09:33:14 -0800
From:   Tejun Heo <tj@...nel.org>
To:     torvalds@...ux-foundation.org, jannh@...gle.com,
        paulmck@...ux.vnet.ibm.com, bcrl@...ck.org,
        viro@...iv.linux.org.uk, kent.overstreet@...il.com
Cc:     security@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, Tejun Heo <tj@...nel.org>,
        Bart Van Assche <bart.vanassche@....com>,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 5/7] block: Remove superflous rcu_read_[un]lock_sched() in blk_queue_enter()

3a0a529971ec ("block, scsi: Make SCSI quiesce and resume work
reliably") added rcu_read_[un]lock_sched() to blk_queue_enter() along
with other changes but it doesn't seem to be doing anything.

blk_queue_enter() is called with @q - the pointer to the target
request_queue, so the caller obviously has to guarantee that @q can be
dereferenced, and inside the RCU-sched protected area, there's nothing
which needs further protection.

Let's remove the superflous rcu_read_[un]lock_sched().

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Bart Van Assche <bart.vanassche@....com>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
---
Hello, Bart.

This came up while auditing percpu_ref users for problematic RCU
usages.  I couldn't understand what the RCU protection was doing.
It'd be great if you can take a look and tell me whether I missed
something.

Thanks.

 block/blk-core.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 6d82c4f..e3b4d1849 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -827,7 +827,6 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
 		bool success = false;
 		int ret;

-		rcu_read_lock_sched();
 		if (percpu_ref_tryget_live(&q->q_usage_counter)) {
 			/*
 			 * The code that sets the PREEMPT_ONLY flag is
@@ -840,7 +839,6 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
 				percpu_ref_put(&q->q_usage_counter);
 			}
 		}
-		rcu_read_unlock_sched();

 		if (success)
 			return 0;
-- 
2.9.5