[PATCH 1/3] rseq: optimize rseq_update_cpu_id()

[Eric Dumazet]

From: Eric Dumazet <eduma...@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 <eduma...@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: "Paul E. McKenney" <paul...@kernel.org>
Cc: Boqun Feng <boqun.f...@gmail.com>
Cc: Arjun Roy <arjun...@google.com>
Cc: Ingo Molnar <mi...@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