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From:   Eric Dumazet <eric.dumazet@gmail.com>
To:     Ingo Molnar <mingo@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Arjun Roy <arjunroy@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Eric Dumazet <eric.dumazet@gmail.com>
Subject: [PATCH 1/3] rseq: optimize rseq_update_cpu_id()
Date:   Tue, 13 Apr 2021 00:36:55 -0700
Message-Id: <20210413073657.2308450-2-eric.dumazet@gmail.com>
In-Reply-To: <20210413073657.2308450-1-eric.dumazet@gmail.com>
References: <20210413073657.2308450-1-eric.dumazet@gmail.com>
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From: Eric Dumazet <edumazet@google.com>

Two put_user() in rseq_update_cpu_id() are replaced
by a pair of unsafe_put_user() with appropriate surroundings.

This removes one stac/clac pair on x86 in fast path.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Arjun Roy <arjunroy@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
---
 kernel/rseq.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index a4f86a9d6937cdfa2f13d1dcc9be863c1943d06f..d2689ccbb132c0fc8ec0924008771e5ee1ca855e 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -84,13 +84,20 @@
 static int rseq_update_cpu_id(struct task_struct *t)
 {
 	u32 cpu_id = raw_smp_processor_id();
+	struct rseq *r = t->rseq;
 
-	if (put_user(cpu_id, &t->rseq->cpu_id_start))
-		return -EFAULT;
-	if (put_user(cpu_id, &t->rseq->cpu_id))
-		return -EFAULT;
+	if (!user_write_access_begin(r, sizeof(*r)))
+		goto efault;
+	unsafe_put_user(cpu_id, &r->cpu_id_start, efault_end);
+	unsafe_put_user(cpu_id, &r->cpu_id, efault_end);
+	user_write_access_end();
 	trace_rseq_update(t);
 	return 0;
+
+efault_end:
+	user_write_access_end();
+efault:
+	return -EFAULT;
 }
 
 static int rseq_reset_rseq_cpu_id(struct task_struct *t)
-- 
2.31.1.295.g9ea45b61b8-goog