commit 0f51f2852ccf0fe38a02d340d0ba625e8e32a863
author Andy Lutomirski <luto@MIT.EDU> Mon May 23 09:31:27 2011 -0400
committer Thomas Gleixner <tglx@linutronix.de> Tue May 24 14:51:28 2011 +0200
tree 075e1f7c85c4deec2e0ab7509dcafa859842abd9
parent 3729db5ca2b2000c660e5a5d0eb68b1053212cab

x86-64: Vclock_gettime(CLOCK_MONOTONIC) can't ever see nsec < 0

vclock_gettime's do_monotonic helper can't ever generate a negative
nsec value, so it doesn't need to check whether it's negative.  In
the CLOCK_MONOTONIC_COARSE case, ns can't ever exceed 2e9-1, so we
can avoid the loop entirely.  This saves a single easily-predicted
branch.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Borislav Petkov <bp@amd64.org>
Link: http://lkml.kernel.org/r/%3Cd6d528d32c7a21618057cfc9005942a0fe5cb54a.1306156808.git.luto%40mit.edu%3E
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/vdso/vclock_gettime.c

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 0b873d4..28b2c00 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -55,22 +55,6 @@
 	return 0;
 }
 
-/* Copy of the version in kernel/time.c which we cannot directly access */
-notrace static void
-vset_normalized_timespec(struct timespec *ts, long sec, long nsec)
-{
-	while (nsec >= NSEC_PER_SEC) {
-		nsec -= NSEC_PER_SEC;
-		++sec;
-	}
-	while (nsec < 0) {
-		nsec += NSEC_PER_SEC;
-		--sec;
-	}
-	ts->tv_sec = sec;
-	ts->tv_nsec = nsec;
-}
-
 notrace static noinline int do_monotonic(struct timespec *ts)
 {
 	unsigned long seq, ns, secs;
@@ -81,7 +65,17 @@
 		secs += gtod->wall_to_monotonic.tv_sec;
 		ns += gtod->wall_to_monotonic.tv_nsec;
 	} while (unlikely(read_seqretry(&gtod->lock, seq)));
-	vset_normalized_timespec(ts, secs, ns);
+
+	/* wall_time_nsec, vgetns(), and wall_to_monotonic.tv_nsec
+	 * are all guaranteed to be nonnegative.
+	 */
+	while (ns >= NSEC_PER_SEC) {
+		ns -= NSEC_PER_SEC;
+		++secs;
+	}
+	ts->tv_sec = secs;
+	ts->tv_nsec = ns;
+
 	return 0;
 }
 
@@ -106,7 +100,17 @@
 		secs += gtod->wall_to_monotonic.tv_sec;
 		ns += gtod->wall_to_monotonic.tv_nsec;
 	} while (unlikely(read_seqretry(&gtod->lock, seq)));
-	vset_normalized_timespec(ts, secs, ns);
+
+	/* wall_time_nsec and wall_to_monotonic.tv_nsec are
+	 * guaranteed to be between 0 and NSEC_PER_SEC.
+	 */
+	if (ns >= NSEC_PER_SEC) {
+		ns -= NSEC_PER_SEC;
+		++secs;
+	}
+	ts->tv_sec = secs;
+	ts->tv_nsec = ns;
+
 	return 0;
 }