[tip:ras/core] x86/mce: Always use 64-bit timestamps

[tip-bot for Arnd Bergmann]

Commit-ID:  bc39f010200d09dc6d4d5e613e86dfd0b22c63b3
Gitweb:     https://git.kernel.org/tip/bc39f010200d09dc6d4d5e613e86dfd0b22c63b3
Author:     Arnd Bergmann <arnd@xxxxxxxx>
AuthorDate: Fri, 22 Jun 2018 11:54:22 +0200
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Fri, 22 Jun 2018 14:37:22 +0200

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@xxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: linux-edac <linux-edac@xxxxxxxxxxxxxxx>
Cc: y2038@xxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/20180618100759.1921750-1-arnd@xxxxxxxx

---
 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 e93670d736a6..d62201e40027 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;