[PATCH v3 15/18] x86/tsc: Use rdtsc_ordered() in check_tsc_warp() and drop extra barriers

Andy Lutomirski
2015-06-17T00:37:30Z

Using get_cycles was unnecessary: check_tsc_warp() is not called on
TSC-less systems. Replace rdtsc_barrier(); get_cycles() with
rdtsc_ordered().

While we're at it, make the somewhat more dangerous change of
removing barrier_before_rdtsc after RDTSC in the TSC warp check
code. This should be okay, though -- the vDSO TSC code doesn't have
that barrier, so, if removing the barrier from the warp check would
cause us to detect a warp that we otherwise wouldn't detect, then we
have a genuine bug.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/kernel/tsc_sync.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
index dd8d0791dfb5..78083bf23ed1 100644
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -39,16 +39,15 @@ static cycles_t max_warp;
static int nr_warps;

/*
- * TSC-warp measurement loop running on both CPUs:
+ * TSC-warp measurement loop running on both CPUs. This is not called
+ * if there is no TSC.
*/
static void check_tsc_warp(unsigned int timeout)
{
cycles_t start, now, prev, end;
int i;

- rdtsc_barrier();
- start = get_cycles();
- rdtsc_barrier();
+ start = rdtsc_ordered();
/*
* The measurement runs for 'timeout' msecs:
*/
@@ -63,9 +62,7 @@ static void check_tsc_warp(unsigned int timeout)
*/
arch_spin_lock(&sync_lock);
prev = last_tsc;
- rdtsc_barrier();
- now = get_cycles();
- rdtsc_barrier();
+ now = rdtsc_ordered();
last_tsc = now;
arch_spin_unlock(&sync_lock);

@@ -126,7 +123,7 @@ void check_tsc_sync_source(int cpu)

/*
* No need to check if we already know that the TSC is not
- * synchronized:
+ * synchronized or if we have no TSC.
*/
if (unsynchronized_tsc())
return;
@@ -190,6 +187,7 @@ void check_tsc_sync_target(void)
{
int cpus = 2;

+ /* Also aborts if there is no TSC. */
if (unsynchronized_tsc() || tsc_clocksource_reliable)
return;

--
2.4.2