linux-kernel - [tip:locking/core] locking/atomics, net/ipv4/tcp_input.c: Convert ACCESS_ONCE() to READ_ONCE()/WRITE

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Date:   Wed, 25 Oct 2017 03:18:34 -0700
From:   tip-bot for Mark Rutland <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, davem@...emloft.net, hpa@...or.com,
        mingo@...nel.org, tglx@...utronix.de, peterz@...radead.org,
        mark.rutland@....com, torvalds@...ux-foundation.org,
        paulmck@...ux.vnet.ibm.com
Subject: [tip:locking/core] locking/atomics, net/ipv4/tcp_input.c: Convert
 ACCESS_ONCE() to READ_ONCE()/WRITE_ONCE()

Commit-ID:  a9da6f29baf6b61fc94f9768638ee54e9bff173c
Gitweb:     https://git.kernel.org/tip/a9da6f29baf6b61fc94f9768638ee54e9bff173c
Author:     Mark Rutland <mark.rutland@....com>
AuthorDate: Mon, 23 Oct 2017 14:07:18 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 25 Oct 2017 11:01:00 +0200

locking/atomics, net/ipv4/tcp_input.c: Convert ACCESS_ONCE() to READ_ONCE()/WRITE_ONCE()

For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
preference to ACCESS_ONCE(), and new code is expected to use one of the
former. So far, there's been no reason to change most existing uses of
ACCESS_ONCE(), as these aren't currently harmful.

However, for some features it is necessary to instrument reads and
writes separately, which is not possible with ACCESS_ONCE(). This
distinction is critical to correct operation.

It's possible to transform the bulk of kernel code using the Coccinelle
script below. However, this doesn't handle comments, leaving references
to ACCESS_ONCE() instances which have been removed. As a preparatory
step, this patch converts the IPv4 TCP input code and comments to use
{READ,WRITE}_ONCE() consistently.

----
virtual patch

@ depends on patch @
expression E1, E2;
@@

- ACCESS_ONCE(E1) = E2
+ WRITE_ONCE(E1, E2)

@ depends on patch @
expression E;
@@

- ACCESS_ONCE(E)
+ READ_ONCE(E)
----

Signed-off-by: Mark Rutland <mark.rutland@....com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: David S. Miller <davem@...emloft.net>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-arch@...r.kernel.org
Cc: mpe@...erman.id.au
Cc: shuah@...nel.org
Cc: snitzer@...hat.com
Cc: thor.thayer@...ux.intel.com
Cc: tj@...nel.org
Cc: viro@...iv.linux.org.uk
Cc: will.deacon@....com
Link: http://lkml.kernel.org/r/1508792849-3115-8-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 net/ipv4/tcp_input.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 7eec338..74480e0 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -815,12 +815,12 @@ static void tcp_update_pacing_rate(struct sock *sk)
 	if (likely(tp->srtt_us))
 		do_div(rate, tp->srtt_us);
 
-	/* ACCESS_ONCE() is needed because sch_fq fetches sk_pacing_rate
+	/* WRITE_ONCE() is needed because sch_fq fetches sk_pacing_rate
 	 * without any lock. We want to make sure compiler wont store
 	 * intermediate values in this location.
 	 */
-	ACCESS_ONCE(sk->sk_pacing_rate) = min_t(u64, rate,
-						sk->sk_max_pacing_rate);
+	WRITE_ONCE(sk->sk_pacing_rate, min_t(u64, rate,
+					     sk->sk_max_pacing_rate));
 }
 
 /* Calculate rto without backoff.  This is the second half of Van Jacobson's