[PATCH] x86/hpet: Read HPET directly if panic in progress

[Tony W Wang-oc]

When the clocksource of the system is HPET，a CPU executing read_hpet
might be interrupted by exceptions to executing the panic，this may
lead to read_hpet dead loops:

CPU x				    CPU x
----                                ----
read_hpet()
  arch_spin_trylock(&hpet.lock)
  [CPU x got the hpet.lock]         #MCE happened
				    do_machine_check()
				      mce_panic()
				        panic()
					  kmsg_dump()
					    pstore_dump()
					      pstore_record_init()
						ktime_get_real_fast_ns()
						  read_hpet()
						  [dead loops]

To avoid this dead loops, read HPET directly if panic in progress.

Signed-off-by: Tony W Wang-oc <TonyWWang-oc@xxxxxxxxxxx>
---
 arch/x86/kernel/hpet.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index c96ae8fee95e..ecadd0698d6a 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -804,6 +804,12 @@ static u64 read_hpet(struct clocksource *cs)
 	if (in_nmi())
 		return (u64)hpet_readl(HPET_COUNTER);
 
+	/*
+	 * Read HPET directly if panic in progress.
+	 */
+	if (unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID))
+		return (u64)hpet_readl(HPET_COUNTER);
+
 	/*
 	 * Read the current state of the lock and HPET value atomically.
 	 */
-- 
2.25.1