On 5/13/26 10:00 AM, Ritesh Harjani (IBM) wrote:
Sayali Patil <[email protected]> writes:
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]
NIP [c000000000029e40] arch_irq_work_raise+0x10/0x70
LR [c00000000003ffc8] machine_check_queue_event+0xa8/0x150
[14626.841925] MSR: 8000000000001003 <SF,ME,RI,LE> CR: 88222248 XER: 00000005
[14626.841939] CFAR: c00000000003ffc4 DAR: c00000006be21300 DSISR: 40000000
IRQMASK: 0
Let's also add the above MSR state along with the call stack showing
MSR[EE] was 0 when this triggered. This also shows the DAR as 0xc....
while MSR[IR|DR] = 0.
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.
Remove it to avoid accessing preempt_count from real mode context.
Fixes: cc15ff327569 ("powerpc/mce: Avoid using irq_work_queue() in realmode")
Agree with the Fixes tag. This patch actually moved mce to use
arch_irq_work_raise(). It was ok until the CONFIG_PREEMPTION was
disabled on powerpc since macros like preempt_enable|disable() were
mostly a no-op. However, after lazy preemption got enabled, access to
Both full/lazy preemption. With upstream now, one can choose full or lazy only.
Leading to issue being discovered.
preempt_count while in real mode can cause the issue you described.
One more thing which we should add to the commit msg is:
The arch_irq_work_raise() function executes in NMI context when called
from MCE handler, hence we won't be preempted or scheduled out since we
are in NMI context with MSR[EE]=0, hence it is safe to remove
preempt_disable|enable() call from here.
And let's change the commit subject to:
powerpc/time: Remove redundant preempt_disable|enable() calls from
arch_irq_work_raise()
BTW, thanks for adding a nice commit msg with the sequence of events.
With the above changes - pease feel free to add:
Reviewed-by: Ritesh Harjani (IBM) <[email protected]>
Suggested-by: Mahesh Salgaonkar <[email protected]>
Signed-off-by: Sayali Patil <[email protected]>
---
arch/powerpc/kernel/time.c | 2 --
1 file changed, 2 deletions(-)
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 4bbeb8644d3d..a99eb43f6ce9 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -471,10 +471,8 @@ 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)
--
2.52.0
