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From:   Tejun Heo <tj@kernel.org>
To:     torvalds@linux-foundation.org, jannh@google.com,
        paulmck@linux.vnet.ibm.com, bcrl@kvack.org,
        viro@zeniv.linux.org.uk, kent.overstreet@gmail.com
Cc:     security@kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, Tejun Heo <tj@kernel.org>,
        Bart Van Assche <bart.vanassche@wdc.com>,
        Jens Axboe <axboe@kernel.dk>
Subject: [PATCH 5/7] block: Remove superflous rcu_read_[un]lock_sched() in blk_queue_enter()
Date:   Tue,  6 Mar 2018 09:33:14 -0800
Message-Id: <20180306173316.3088458-5-tj@kernel.org>
In-Reply-To: <20180306173316.3088458-1-tj@kernel.org>
References: <20180306172657.3060270-1-tj@kernel.org>
 <20180306173316.3088458-1-tj@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@kernel.org>
Cc: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-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