linux-kernel - [PATCH] x86: mce: always use 64-bit timestamps

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Date:   Mon, 18 Jun 2018 12:06:46 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Tony Luck <tony.luck@...el.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org
Cc:     y2038@...ts.linaro.org, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Yazen Ghannam <yazen.ghannam@....com>,
        linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] x86: mce: always use 64-bit timestamps

The machine check timestamp uses get_seconds(), which returns an 'unsigned long'
number that might overflow on 32-bit architectures (in the distant future)
and is therefore deprecated.

The normal replacement would be ktime_get_real_seconds(), but that needs to
use a sequence lock that might cause a deadlock if the mce happens at just
the wrong moment. The __ktime_get_real_seconds() skips that lock and is
safer here, but has a miniscule risk of returning the wrong time when we read
it on a 32-bit architecture at the same time as updating the epoch, i.e.
from before y2106 overflow time to after, or vice versa.

This seems to be an acceptable risk in this particular case, and is the
same thing we do in kdb.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index e4cf6ff1c2e1..b887415652ed 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -123,8 +123,8 @@ void mce_setup(struct mce *m)
 {
 	memset(m, 0, sizeof(struct mce));
 	m->cpu = m->extcpu = smp_processor_id();
-	/* We hope get_seconds stays lockless */
-	m->time = get_seconds();
+	/* need the internal __ version to avoid deadlocks */
+	m->time = __ktime_get_real_seconds();
 	m->cpuvendor = boot_cpu_data.x86_vendor;
 	m->cpuid = cpuid_eax(1);
 	m->socketid = cpu_data(m->extcpu).phys_proc_id;
-- 
2.9.0