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From:   Rik van Riel <riel@surriel.com>
To:     linux-kernel@vger.kernel.org
Cc:     kernel-team@fb.com, peterz@infradead.org, luto@kernel.org,
        x86@kernel.org, vkuznets@redhat.com, mingo@kernel.org,
        efault@gmx.de, dave.hansen@intel.com, will.daecon@arm.com,
        catalin.marinas@arm.com, benh@kernel.crashing.org,
        Rik van Riel <riel@surriel.com>
Subject: [PATCH 01/10] x86,tlb: clarify memory barrier in switch_mm_irqs_off
Date:   Sat, 28 Jul 2018 17:53:48 -0400
Message-Id: <20180728215357.3249-2-riel@surriel.com>
In-Reply-To: <20180728215357.3249-1-riel@surriel.com>
References: <20180728215357.3249-1-riel@surriel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Clarify exactly what the memory barrier synchronizes with.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Rik van Riel <riel@surriel.com>
---
 arch/x86/mm/tlb.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 752dbf4e0e50..5321e02c4e09 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -263,8 +263,11 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		/*
 		 * Read the tlb_gen to check whether a flush is needed.
 		 * If the TLB is up to date, just use it.
-		 * The barrier synchronizes with the tlb_gen increment in
-		 * the TLB shootdown code.
+		 * The TLB shootdown code first increments tlb_gen, and then
+		 * sends IPIs to CPUs that have this CPU loaded and are not
+		 * in lazy TLB mode. The barrier ensures we handle
+		 * cpu_tlbstate.is_lazy before tlb_gen, keeping this code
+		 * synchronized with the TLB flush code.
 		 */
 		smp_mb();
 		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
-- 
2.14.4