linux-kernel - [PATCH V5 06/11] entry: Prevent DEBUG

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Date:   Sun, 18 Sep 2022 11:52:41 -0400
From:   guoren@...nel.org
To:     arnd@...db.de, guoren@...nel.org, palmer@...osinc.com,
        tglx@...utronix.de, peterz@...radead.org, luto@...nel.org,
        conor.dooley@...rochip.com, heiko@...ech.de, jszhang@...nel.org,
        lazyparser@...il.com, falcon@...ylab.org, chenhuacai@...nel.org,
        apatel@...tanamicro.com, atishp@...shpatra.org, palmer@...belt.com,
        paul.walmsley@...ive.com, mark.rutland@....com,
        zouyipeng@...wei.com, bigeasy@...utronix.de,
        David.Laight@...lab.com
Cc:     linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, Guo Ren <guoren@...ux.alibaba.com>
Subject: [PATCH V5 06/11] entry: Prevent DEBUG_PREEMPT warning

From: Guo Ren <guoren@...ux.alibaba.com>

When DEBUG_PREEMPT=y,
	exit_to_user_mode_prepare
	->tick_nohz_user_enter_prepare
	  ->tick_nohz_full_cpu(smp_processor_id())
	    ->smp_processor_id()
	      ->debug_smp_processor_id()
		->check preempt_count() then:

[    5.717610] BUG: using smp_processor_id() in preemptible [00000000]
code: S20urandom/94
[    5.718111] caller is debug_smp_processor_id+0x24/0x38
[    5.718417] CPU: 1 PID: 94 Comm: S20urandom Not tainted
6.0.0-rc3-00010-gfd0a0d619c63-dirty #238
[    5.718886] Hardware name: riscv-virtio,qemu (DT)
[    5.719136] Call Trace:
[    5.719281] [<ffffffff800055fc>] dump_backtrace+0x2c/0x3c
[    5.719566] [<ffffffff80ae6cb0>] show_stack+0x44/0x5c
[    5.720023] [<ffffffff80aee870>] dump_stack_lvl+0x74/0xa4
[    5.720557] [<ffffffff80aee8bc>] dump_stack+0x1c/0x2c
[    5.721033] [<ffffffff80af65c0>]
check_preemption_disabled+0x104/0x108
[    5.721538] [<ffffffff80af65e8>] debug_smp_processor_id+0x24/0x38
[    5.722001] [<ffffffff800aee30>] exit_to_user_mode_prepare+0x48/0x178
[    5.722355] [<ffffffff80af5bf4>] irqentry_exit_to_user_mode+0x18/0x30
[    5.722685] [<ffffffff80af5c70>] irqentry_exit+0x64/0xa4
[    5.722953] [<ffffffff80af52f4>] do_page_fault+0x1d8/0x544
[    5.723291] [<ffffffff80003310>] ret_from_exception+0x0/0xb8

(Above is found in riscv platform with generic_entry)

The smp_processor_id() needs irqs disable or preempt_disable, so use
preempt dis/in protecting the tick_nohz_user_enter_prepare().

Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
Signed-off-by: Guo Ren <guoren@...nel.org>
---
 kernel/entry/common.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 063068a9ea9b..36e4cd50531c 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -194,8 +194,10 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
 
 	lockdep_assert_irqs_disabled();
 
+	preempt_disable();
 	/* Flush pending rcuog wakeup before the last need_resched() check */
 	tick_nohz_user_enter_prepare();
+	preempt_enable();
 
 	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
 		ti_work = exit_to_user_mode_loop(regs, ti_work);
-- 
2.36.1