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From:   Joel Savitz <jsavitz@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     Joel Savitz <jsavitz@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Yuan ZhaoXiong <yuanzhaoxiong@baidu.com>,
        Baokun Li <libaokun1@huawei.com>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        YueHaibing <yuehaibing@huawei.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        David Hildenbrand <dhildenb@redhat.com>
Subject: [RFC PATCH] kernel/cpu: restart cpu_up when hotplug is disabled
Date:   Mon, 18 Apr 2022 15:54:02 -0400
Message-Id: <20220418195402.2986573-1-jsavitz@redhat.com>
MIME-Version: 1.0
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The cpu hotplug path may be utilized while hotplug is disabled for a
brief moment leading to failures. As an example, attempts to perform
cpu hotplug by userspace soon after boot may race with pci_device_probe
leading to inconsistent results.

Proposed idea:
Call restart_syscall instead of returning -EBUSY since
cpu_hotplug_disabled seems to only have a positive value
for short, temporary amounts of time.

Does anyone see any serious problems with this?

Signed-off-by: Joel Savitz <jsavitz@redhat.com>
---
 kernel/cpu.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5797c2a7a93f..2992c7d1d24e 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -35,6 +35,7 @@
 #include <linux/percpu-rwsem.h>
 #include <linux/cpuset.h>
 #include <linux/random.h>
+#include <linux/delay.h>
 
 #include <trace/events/power.h>
 #define CREATE_TRACE_POINTS
@@ -1401,7 +1402,9 @@ static int cpu_up(unsigned int cpu, enum cpuhp_state target)
 	cpu_maps_update_begin();
 
 	if (cpu_hotplug_disabled) {
-		err = -EBUSY;
+		/* avoid busy looping (5ms of sleep should be enough) */
+		msleep(5);
+		err = restart_syscall();
 		goto out;
 	}
 	if (!cpu_smt_allowed(cpu)) {
-- 
2.27.0