linux-kernel - Possible clocksource wrapping issues w/ new vdso clock

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Message-Id: <1185213583.8850.12.camel@localhost>
Date:	Mon, 23 Jul 2007 10:59:43 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>
Subject: Possible clocksource wrapping issues w/ new vdso clock_gettime()
	code?

Hey Andi,
	I've not been able to review the new vdso code very carefully yet, but
I noticed one thing right off: the offset calculation is not masked, so
its possible w/ counters less then 64bits wide to have wrapping issues.


It seems something like the following would be needed.

diff --git a/arch/x86_64/vdso/vclock_gettime.c b/arch/x86_64/vdso/vclock_gettime.c
index 17f6a00..9570f8e 100644
--- a/arch/x86_64/vdso/vclock_gettime.c
+++ b/arch/x86_64/vdso/vclock_gettime.c
@@ -36,8 +36,8 @@ static inline long vgetns(void)
 {
 	cycles_t (*vread)(void);
 	vread = gtod->clock.vread;
-	return ((vread() - gtod->clock.cycle_last) * gtod->clock.mult) >>
-		gtod->clock.shift;
+	return (((vread() - gtod->clock.cycle_last) & gtod->clock.mask) *
+		 gtod->clock.mult) >> gtod->clock.shift;
 }
 
 static noinline int do_realtime(struct timespec *ts)



Or am I just missing something? 

thanks
-john


-
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