linux-kernel - [PATCH RFC tip/core/rcu 09/15] uprobes: Remove now-redundant smp_read_barrier

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Date:   Mon,  9 Oct 2017 17:22:43 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, torvalds@...ux-foundation.org,
        mark.rutland@....com, dhowells@...hat.com,
        linux-arch@...r.kernel.org, peterz@...radead.org,
        will.deacon@....com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: [PATCH RFC tip/core/rcu 09/15] uprobes: Remove now-redundant smp_read_barrier_depends()

Now that READ_ONCE() implies smp_read_barrier_depends(), the
get_xol_area() and get_trampoline_vaddr() no longer need their
smp_read_barrier_depends() calls, which this commit removes.
While we are here, convert the corresponding smp_wmb() to an
smp_store_release().

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
---
 kernel/events/uprobes.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 267f6ef91d97..ce6848e46e94 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1167,8 +1167,8 @@ static int xol_add_vma(struct mm_struct *mm, struct xol_area *area)
 	}
 
 	ret = 0;
-	smp_wmb();	/* pairs with get_xol_area() */
-	mm->uprobes_state.xol_area = area;
+	/* pairs with get_xol_area() */
+	smp_store_release(&mm->uprobes_state.xol_area, area); /* ^^^ */
  fail:
 	up_write(&mm->mmap_sem);
 
@@ -1230,8 +1230,8 @@ static struct xol_area *get_xol_area(void)
 	if (!mm->uprobes_state.xol_area)
 		__create_xol_area(0);
 
-	area = mm->uprobes_state.xol_area;
-	smp_read_barrier_depends();	/* pairs with wmb in xol_add_vma() */
+	/* Pairs with xol_add_vma() smp_store_release() */
+	area = READ_ONCE(mm->uprobes_state.xol_area); /* ^^^ */
 	return area;
 }
 
@@ -1528,8 +1528,8 @@ static unsigned long get_trampoline_vaddr(void)
 	struct xol_area *area;
 	unsigned long trampoline_vaddr = -1;
 
-	area = current->mm->uprobes_state.xol_area;
-	smp_read_barrier_depends();
+	/* Pairs with xol_add_vma() smp_store_release() */
+	area = READ_ONCE(current->mm->uprobes_state.xol_area); /* ^^^ */
 	if (area)
 		trampoline_vaddr = area->vaddr;
 
-- 
2.5.2