[PATCH 1/3] powerpc/64s/interrupt: Fix decrementer storm

From: Nicholas Piggin <npiggin@gmail.com>
To: linuxppc-dev@lists.ozlabs.org
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>,
	Nicholas Piggin <npiggin@gmail.com>
Subject: [PATCH 1/3] powerpc/64s/interrupt: Fix decrementer storm
Date: Tue, 25 Jan 2022 00:39:28 +1000	[thread overview]
Message-ID: <20220124143930.3923442-1-npiggin@gmail.com> (raw)

The decrementer exception can fail to be cleared when the interrupt
returns in the case where the decrementer wraps with the next timer
still beyond decrementer_max. This results in a decrementer interrupt
storm. This is triggerable with small decrementer system with hard
and soft watchdogs disabled.

Fix this by always programming the decrementer if there was no timer.

Fixes: 0faf20a1ad16 ("powerpc/64s/interrupt: Don't enable MSR[EE] in irq handlers unless perf is in use")
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kernel/time.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 62361cc7281c..cd0b8b71ecdd 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -649,8 +649,9 @@ DEFINE_INTERRUPT_HANDLER_ASYNC(timer_interrupt)
 		__this_cpu_inc(irq_stat.timer_irqs_event);
 	} else {
 		now = *next_tb - now;
-		if (now <= decrementer_max)
-			set_dec_or_work(now);
+		if (now > decrementer_max)
+			now = decrementer_max;
+		set_dec_or_work(now);
 		__this_cpu_inc(irq_stat.timer_irqs_others);
 	}
 
-- 
2.23.0

