[PATCH v2] powerpc/time: Remove redundant preempt_disable|enable() calls from arch_irq_work_raise() - part1

[Sayali Patil <sayalip@linux.ibm.com>]

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 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 <mahesh@linux.ibm.com>
Acked-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>

---

v1->v2
 - Split the patch series into two parts; v2 part 2 of remaining two patches
   will be sent later due to changes required in 
   "lkdtm/powerpc: add PPC_RADIX_TLBIEL test for radix MCE validation".
 - Updated commit message as per review feedback.
 - Added comment for arch_irq_work_raise() to indicate it must be called
   with preemption disabled, as per review feedback.
 - Added Acked-By from Shrikanth and Reviewed-by tag from Ritesh.

v1: https://lore.kernel.org/all/cover.1778057685.git.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)
-- 
2.52.0