Commit 31467b23 authored by Sayali Patil's avatar Sayali Patil Committed by Madhavan Srinivasan
Browse files

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 <SF,ME,RI,LE>  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 0fe1ac48 ("powerpc/perf_event: Fix
oops due to perf_event_do_pending call") to avoid races while raising
irq work from exception context.

Later, commit 471ba0e6 ("irq_work: Do not raise an IPI when
queueing work on the local CPU") added preemption protection in
irq_work_queue() path, while commit 20b87691 ("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: cc15ff32 ("powerpc/mce: Avoid using irq_work_queue() in realmode")
Suggested-by: default avatarMahesh Salgaonkar <mahesh@linux.ibm.com>
Acked-by: default avatarShrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: default avatarRitesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: default avatarSayali Patil <sayalip@linux.ibm.com>
[Maddy: Fixed the commit title]
Signed-off-by: default avatarMadhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260513081413.222490-1-sayalip@linux.ibm.com
parent dbc30a57

arch/powerpc/kernel/time.c

+4 −2
Original line number Diff line number Diff line
@@ -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)