From 3fb95e3861ce3a817f495cf2269667ece0991aea Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 23 May 2011 09:31:25 -0400 Subject: [PATCH] --- yaml --- r: 251979 b: refs/heads/master c: 057e6a8c660e95c3f4e7162e00e2fee1fc90c50d h: refs/heads/master i: 251977: b21c0688be5c703f8b5c0de6df2e8c47dea05777 251975: d2872de74aca0761c23c9bb6b9ed186900807691 v: v3 --- [refs] | 2 +- trunk/arch/x86/kernel/tsc.c | 9 +++++---- 2 files changed, 6 insertions(+), 5 deletions(-) diff --git a/[refs] b/[refs] index f9d8674015df..0d1d2475a51f 100644 --- a/[refs] +++ b/[refs] @@ -1,2 +1,2 @@ --- -refs/heads/master: 8c49d9a74bac5ea3f18480307057241b808fcc0c +refs/heads/master: 057e6a8c660e95c3f4e7162e00e2fee1fc90c50d diff --git a/trunk/arch/x86/kernel/tsc.c b/trunk/arch/x86/kernel/tsc.c index db697b81b8b1..1e6244202612 100644 --- a/trunk/arch/x86/kernel/tsc.c +++ b/trunk/arch/x86/kernel/tsc.c @@ -769,13 +769,14 @@ static cycle_t __vsyscall_fn vread_tsc(void) cycle_t ret; /* - * Surround the RDTSC by barriers, to make sure it's not - * speculated to outside the seqlock critical section and - * does not cause time warps: + * Empirically, a fence (of type that depends on the CPU) + * before rdtsc is enough to ensure that rdtsc is ordered + * with respect to loads. The various CPU manuals are unclear + * as to whether rdtsc can be reordered with later loads, + * but no one has ever seen it happen. */ rdtsc_barrier(); ret = (cycle_t)vget_cycles(); - rdtsc_barrier(); return ret >= VVAR(vsyscall_gtod_data).clock.cycle_last ? ret : VVAR(vsyscall_gtod_data).clock.cycle_last;