From 829a9996259e4d0b20ce7b94c49b985d6ba6b760 Mon Sep 17 00:00:00 2001 From: Hidetoshi Seto Date: Fri, 13 Jul 2007 16:21:44 -0700 Subject: [PATCH] [IA64] ar.itc access must really be after xtime_lock.sequence has been read The ".acq" semantics of the load only apply w.r.t. other data access. Reading the clock (ar.itc) isn't a data access so strange things can happen here. Specifically the read of ar.itc can be launched as soon as the read of xtime_lock.sequence is ISSUED. Since this may cache miss, and that might cause a thread switch, and there may be cache contention for the line containing xtime_lock, it may be a long time before the actual value is returned, so the ar.itc value may be very stale. Move the consumption of r28 up before the read of ar.itc to make sure that we really have got the current value of xtime_lock.sequence before look at ar.itc. Signed-off-by: Hidetoshi Seto Signed-off-by: Tony Luck --- arch/ia64/kernel/fsys.S | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/arch/ia64/kernel/fsys.S b/arch/ia64/kernel/fsys.S index 8589e84a27c6..3f926c2dc708 100644 --- a/arch/ia64/kernel/fsys.S +++ b/arch/ia64/kernel/fsys.S @@ -247,6 +247,9 @@ ENTRY(fsys_gettimeofday) .time_redo: .pred.rel.mutex p8,p9,p10 ld4.acq r28 = [r29] // xtime_lock.sequence. Must come first for locking purposes + ;; + and r28 = ~1,r28 // Make sequence even to force retry if odd + ;; (p8) mov r2 = ar.itc // CPU_TIMER. 36 clocks latency!!! add r22 = IA64_TIME_INTERPOLATOR_LAST_COUNTER_OFFSET,r20 (p9) ld8 r2 = [r30] // readq(ti->address). Could also have latency issues.. @@ -284,7 +287,6 @@ EX(.fail_efault, probe.w.fault r31, 3) // This takes 5 cycles and we have spare (p15) ld8 r17 = [r19],-IA64_TIMESPEC_TV_NSEC_OFFSET (p7) cmp.ne p7,p0 = r25,r3 // if cmpxchg not successful redo // simulate tbit.nz.or p7,p0 = r28,0 - and r28 = ~1,r28 // Make sequence even to force retry if odd getf.sig r2 = f8 mf add r8 = r8,r18 // Add time interpolator offset -- 2.39.5