linux-kernel - [tip: smp/core] smp: reword smp call IPI comment

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Date:   Tue, 28 Mar 2023 08:34:45 -0000
From:   "tip-bot2 for Valentin Schneider" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Valentin Schneider <vschneid@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: smp/core] smp: reword smp call IPI comment

The following commit has been merged into the smp/core branch of tip:

Commit-ID:     253a0fb4c62827cdcaf43afcea5d675507eaf7a3
Gitweb:        https://git.kernel.org/tip/253a0fb4c62827cdcaf43afcea5d675507eaf7a3
Author:        Valentin Schneider <vschneid@...hat.com>
AuthorDate:    Tue, 07 Mar 2023 14:35:57 
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 24 Mar 2023 11:01:28 +01:00

smp: reword smp call IPI comment

Accessing the call_single_queue hasn't involved a spinlock since 2014:

  6897fc22ea01 ("kernel: use lockless list for smp_call_function_single")

The llist operations (namely cmpxchg() and xchg()) provide similar ordering
guarantees, update the comment to lessen confusion.

Signed-off-by: Valentin Schneider <vschneid@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lore.kernel.org/r/20230307143558.294354-7-vschneid@redhat.com
---
 kernel/smp.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 03e6d57..6bbfabb 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -312,9 +312,10 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
 void __smp_call_single_queue(int cpu, struct llist_node *node)
 {
 	/*
-	 * The list addition should be visible before sending the IPI
-	 * handler locks the list to pull the entry off it because of
-	 * normal cache coherency rules implied by spinlocks.
+	 * The list addition should be visible to the target CPU when it pops
+	 * the head of the list to pull the entry off it in the IPI handler
+	 * because of normal cache coherency rules implied by the underlying
+	 * llist ops.
 	 *
 	 * If IPIs can go out of order to the cache coherency protocol
 	 * in an architecture, sufficient synchronisation should be added