On 10/2/26 1:54 PM, Michal Suchánek wrote:
On Tue, Sep 29, 2026 at 12:46:00PM +0530, Mukesh Kumar Chaurasiya (IBM) wrote:
When running with CONFIG_PPC_IRQ_SOFT_MASK_DEBUG=y and LOCKDEP=y, an
Oops is triggered during NMI exit:
[ 202.258332] Oops: Exception in kernel mode, sig: 4 [#1]
NIP [000000000003e178] 0x3e178
LR [c00000000003e174] arch_local_irq_restore+0x2a4/0x328
TRAP: 0700 MSR: 0000000000081000 <ME>
The faulting instruction at NIP is a 'trap' (0fe00000) inside the
rfi_flush_fallback trampoline at a userspace-looking low address, reached
after a bad rfid caused by an incorrect SRR0 value.
Root cause
----------
DEFINE_INTERRUPT_HANDLER_NMI used to call arch_interrupt_nmi_exit_prepare()
*before* irqentry_nmi_exit(), giving this sequence:
arch_interrupt_nmi_enter_prepare() // saves irq_soft_mask, irq_happened
irqentry_nmi_enter() // __nmi_enter() -> nmi_count++,
in_nmi()=true
____func(regs) // NMI handler body
arch_interrupt_nmi_exit_prepare() // RESTORES irq_soft_mask=0
(IRQS_ENABLED)
// RESTORES irq_happened with pending
bits
irqentry_nmi_exit() // instrumentation block runs here...
trace_hardirqs_on_prepare() // ...with in_nmi()==true still
lockdep_hardirqs_on_prepare() // ...and irq_soft_mask already
restored to 0
-> arch_local_irq_restore(0) // mask=0 -> enable path entered
WARN_ON_ONCE(in_nmi()) // FIRES: __nmi_exit() hasn't run yet
__nmi_exit() // nmi_count-- (too late)
arch_interrupt_nmi_exit_prepare() restores irq_soft_mask to whatever
the interrupted thread had (IRQS_ENABLED / 0) and restores irq_happened
with any pending deferred-interrupt bits that were present when the NMI
fired. This is correct and necessary before rfid back to the interrupted
context, but it must not happen until irqentry_nmi_exit() has fully
completed.
With irq_soft_mask=0 and pending bits in irq_happened, the
trace_hardirqs_on_prepare() call inside irqentry_nmi_exit()'s
instrumentation block reaches lockdep_hardirqs_on_prepare() which
calls __trace_hardirqs_on_caller(), which eventually calls
arch_local_irq_restore(0) (the IRQS_ENABLED / enable path).
arch_local_irq_restore() with mask=0 hits the CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
guards at irq_64.c:217:
WARN_ON_ONCE(in_nmi()); // TRUE: __nmi_exit() not yet called
The WARN_ON_ONCE fires a 'trap' instruction (tw 31,0,0 / 0fe00000) in
kernel mode with MSR_PR=0, MSR_IR=0 (real-mode). The program check
handler (TRAP 0700) takes the sig:4 (SIGILL) path via do_program_check()
and calls die(), producing the Oops.
The RESTART_TABLE mechanism in arch_local_irq_restore then causes the
bad rfid that lands at the userspace-looking address visible in the NIP.
The bug is invisible with LOCKDEP=n because trace_hardirqs_on_prepare()
is a no-op when CONFIG_TRACE_IRQFLAGS is not set (TRACE_IRQFLAGS is
selected by PROVE_LOCKING which selects LOCKDEP).
Fix
---
Hello,
please avoid excessive formatting in commit messages.
When git am is used to apply this patch it strips everything after this
line
Yes. Good catch. Please run ./scripts/checkpatch.pl --strict always.
I am seeing this pattern quite often these days.
Maybe it is worth updating checkpatch.pl to catch such things.
ERROR: Invalid commit separator - some tools may have problems applying this
#18:
----------
It flags this one, but if i remove that line it doesn't flag subsequent the ---
tag.
What I would want checkpatch to see if there are multiple --- in the commit
message,
Flag it and show it as error.
+cc Andy, Joe. Do you think that's good to fix it?
Move arch_interrupt_nmi_exit_prepare() to after the irqentry_nmi_exit()
block, so that:
irqentry_nmi_exit() // __nmi_exit() -> nmi_count--,
in_nmi()=false
arch_interrupt_nmi_exit_prepare() // now safe to restore soft mask state
The instrumentation inside irqentry_nmi_exit() now executes while
in_nmi() is still true (nmi_count not yet decremented), but
irq_soft_mask is still IRQS_ALL_DISABLED (NMI-entry value), so
arch_local_irq_restore() is never called with mask=0 from that path.
After __nmi_exit() returns, arch_interrupt_nmi_exit_prepare() restores
the pre-NMI soft mask and irq_happened, which is the correct state for
the rfid back to the interrupted context.
Reported-by: Venkat Rao Bagalkote <[email protected]>
Closes:
https://lore.kernel.org/all/[email protected]/
Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <[email protected]>
---
because it's the same as this line that marks the end of the commit
message.
Thanks
Michal
arch/powerpc/include/asm/interrupt.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/powerpc/include/asm/interrupt.h
b/arch/powerpc/include/asm/interrupt.h
index 1b45a49e9bed..56f855c6f07e 100644
--- a/arch/powerpc/include/asm/interrupt.h
+++ b/arch/powerpc/include/asm/interrupt.h
@@ -301,7 +301,6 @@ interrupt_handler long func(struct pt_regs *regs)
\
state = irqentry_nmi_enter(regs); \
} \
ret = ____##func (regs); \
- arch_interrupt_nmi_exit_prepare(regs, &nmi_state); \
if (mfmsr() & MSR_DR) { \
/* nmi_exit if relocations are on */ \
irqentry_nmi_exit(regs, state); \
@@ -317,6 +316,7 @@ interrupt_handler long func(struct pt_regs *regs)
\
} else { \
irqentry_nmi_exit(regs, state); \
} \
+ arch_interrupt_nmi_exit_prepare(regs, &nmi_state); \
\
return ret; \
} \
--
2.55.0
