linux-kernel - [PATCH] smp: Fix offline cpu check in flush_smp_call_function

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Message-Id: <20220319072015.1495036-1-namit@vmware.com>
Date:   Sat, 19 Mar 2022 00:20:15 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Nadav Amit <namit@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Srivatsa S . Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: [PATCH] smp: Fix offline cpu check in flush_smp_call_function_queue

From: Nadav Amit <namit@...are.com>

The check in flush_smp_call_function_queue() for callbacks that are sent
to offline CPUs currently checks whether the queue is empty.

However, flush_smp_call_function_queue() has just deleted all the
callbacks from the queue and moved all the entries into a local list.
This checks would only be positive if some callbacks were added in the
short time after llist_del_all() was called. This does not seem to be
the intention of this check.

Change the check to look at the local list to which the entries were
moved instead of the queue from which all the callbacks were just
removed.

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
Fixes: 8d056c48e4862 ("CPU hotplug, smp: flush any pending IPI callbacks before CPU offline")
Signed-off-by: Nadav Amit <namit@...are.com>
---
 kernel/smp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index d82439bac401..eb275b69d5f3 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -582,7 +582,7 @@ static void flush_smp_call_function_queue(bool warn_cpu_offline)

 	/* There shouldn't be any pending callbacks on an offline CPU. */
 	if (unlikely(warn_cpu_offline && !cpu_online(smp_processor_id()) &&
-		     !warned && !llist_empty(head))) {
+		     !warned && entry != NULL)) {
 		warned = true;
 		WARN(1, "IPI on offline CPU %d\n", smp_processor_id());

-- 
2.25.1