From 31467b23823ffec1f6fff407f8e3ca9af8b7491a Mon Sep 17 00:00:00 2001 From: Sayali Patil Date: Wed, 13 May 2026 13:44:13 +0530 Subject: [PATCH] powerpc/time: Remove redundant preempt_disable|enable() calls from arch_irq_work_raise() A kernel panic is observed when handling machine check exceptions from real mode. BUG: Unable to handle kernel data access on read at 0xc00000006be21300 Oops: Kernel access of bad area, sig: 11 [#1] MSR: 8000000000001003 CR: 88222248 XER: 00000005 CFAR: c00000000003ffc4 DAR: c00000006be21300 DSISR: 40000000 IRQMASK: 0 NIP [c000000000029e40] arch_irq_work_raise+0x10/0x70 LR [c00000000003ffc8] machine_check_queue_event+0xa8/0x150 Call Trace: [c0000000179d3c70] [c00000000003ff64] machine_check_queue_event+0x44/0x150 [c0000000179d3d30] [c0000000000084e0] machine_check_early_common+0x1f0/0x2c0 The crash occurs because arch_irq_work_raise() calls preempt_disable() from machine check exception (MCE) handlers running in real mode. In this context, accessing the preempt_count can fault, leading to the panic. The preempt_disable()/preempt_enable() pair in arch_irq_work_raise() was originally added by commit 0fe1ac48bef0 ("powerpc/perf_event: Fix oops due to perf_event_do_pending call") to avoid races while raising irq work from exception context. Later, commit 471ba0e686cb ("irq_work: Do not raise an IPI when queueing work on the local CPU") added preemption protection in irq_work_queue() path, while commit 20b876918c06 ("irq_work: Use per cpu atomics instead of regular atomics") added equivalent protection in irq_work_queue_on() before reaching arch_irq_work_raise(): irq_work_queue() / irq_work_queue_on() -> preempt_disable() -> __irq_work_queue_local() -> irq_work_raise() -> arch_irq_work_raise() As a result, callers other than mce_irq_work_raise() already execute with preemption disabled, making the additional preempt_disable()/preempt_enable() pair in arch_irq_work_raise() redundant. The arch_irq_work_raise() function executes in NMI context when called from MCE handler. Hence we will not be preempted or scheduled out since we are in NMI context with MSR[EE]=0. Therefore, it is safe to remove the preempt_disable()/preempt_enable() calls from here. Remove it to avoid accessing preempt_count from real mode context. Fixes: cc15ff327569 ("powerpc/mce: Avoid using irq_work_queue() in realmode") Suggested-by: Mahesh Salgaonkar Acked-by: Shrikanth Hegde Reviewed-by: Ritesh Harjani (IBM) Signed-off-by: Sayali Patil [Maddy: Fixed the commit title] Signed-off-by: Madhavan Srinivasan Link: https://patch.msgid.link/20260513081413.222490-1-sayalip@linux.ibm.com --- arch/powerpc/kernel/time.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c index 4bbeb8644d3d..b4472288e0d4 100644 --- a/arch/powerpc/kernel/time.c +++ b/arch/powerpc/kernel/time.c @@ -458,6 +458,10 @@ DEFINE_PER_CPU(u8, irq_work_pending); #endif /* 32 vs 64 bit */ +/* + * Must be called with preemption disabled since it updates + * per-CPU irq_work state and programs the local CPU decrementer. + */ void arch_irq_work_raise(void) { /* @@ -471,10 +475,8 @@ void arch_irq_work_raise(void) * which could get tangled up if we're messing with the same state * here. */ - preempt_disable(); set_irq_work_pending_flag(); set_dec(1); - preempt_enable(); } static void set_dec_or_work(u64 val)