linux-kernel - [PATCH AUTOSEL 5.18 38/93] rcuscale: Fix smp_processor

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Date:   Thu, 11 Aug 2022 11:41:32 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Zqiang <qiang1.zhang@...el.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Sasha Levin <sashal@...nel.org>, dave@...olabs.net,
        josh@...htriplett.org, frederic@...nel.org,
        quic_neeraju@...cinc.com, rcu@...r.kernel.org
Subject: [PATCH AUTOSEL 5.18 38/93] rcuscale: Fix smp_processor_id()-in-preemptible warnings

From: Zqiang <qiang1.zhang@...el.com>

[ Upstream commit 92366810644d5675043c792abb70eaf974a77384 ]

Systems built with CONFIG_DEBUG_PREEMPT=y can trigger the following
BUG while running the rcuscale performance test:

BUG: using smp_processor_id() in preemptible [00000000] code: rcu_scale_write/69
CPU: 0 PID: 66 Comm: rcu_scale_write Not tainted 5.18.0-rc7-next-20220517-yoctodev-standard+
caller is debug_smp_processor_id+0x17/0x20
Call Trace:
<TASK>
dump_stack_lvl+0x49/0x5e
dump_stack+0x10/0x12
check_preemption_disabled+0xdf/0xf0
debug_smp_processor_id+0x17/0x20
rcu_scale_writer+0x2b5/0x580
kthread+0x177/0x1b0
ret_from_fork+0x22/0x30
</TASK>

Reproduction method:
runqemu kvm slirp nographic qemuparams="-m 4096 -smp 8" bootparams="isolcpus=2,3
nohz_full=2,3 rcu_nocbs=2,3 rcutree.dump_tree=1 rcuscale.shutdown=false
rcuscale.gp_async=true" -d

The problem is that the rcu_scale_writer() kthreads fail to set the
PF_NO_SETAFFINITY flags, which causes is_percpu_thread() to assume
that the kthread's affinity might change at any time, thus the BUG
noted above.

This commit therefore causes rcu_scale_writer() to set PF_NO_SETAFFINITY
in its kthread's ->flags field, thus preventing this BUG.

Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/rcu/rcuscale.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
index 5e4f1f83d38e..fefaeda87de1 100644
--- a/kernel/rcu/rcuscale.c
+++ b/kernel/rcu/rcuscale.c
@@ -399,6 +399,7 @@ rcu_scale_writer(void *arg)
 	VERBOSE_SCALEOUT_STRING("rcu_scale_writer task started");
 	WARN_ON(!wdpp);
 	set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
+	current->flags |= PF_NO_SETAFFINITY;
 	sched_set_fifo_low(current);
 
 	if (holdoff)
-- 
2.35.1