x86: move rdtsc_barrier() into the TSC vread method
Petr Tesarik [Mon, 25 May 2009 09:02:02 +0000 (11:02 +0200)]
The *fence instructions were moved to vsyscall_64.c by commit
cb9e35dce94a1b9c59d46224e8a94377d673e204.  But this breaks the
vDSO, because vread methods are also called from there.

Besides, the synchronization might be unnecessary for other
time sources than TSC.

[ Impact: fix potential time warp in VDSO ]

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
LKML-Reference: <9d0ea9ea0f866bdc1f4d76831221ae117f11ea67.1243241859.git.ptesarik@suse.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@kernel.org>

arch/x86/kernel/tsc.c
arch/x86/kernel/vsyscall_64.c

index d57de05..cf8611d 100644 (file)
@@ -710,7 +710,16 @@ static cycle_t read_tsc(struct clocksource *cs)
 #ifdef CONFIG_X86_64
 static cycle_t __vsyscall_fn vread_tsc(void)
 {
-       cycle_t ret = (cycle_t)vget_cycles();
+       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:
+        */
+       rdtsc_barrier();
+       ret = (cycle_t)vget_cycles();
+       rdtsc_barrier();
 
        return ret >= __vsyscall_gtod_data.clock.cycle_last ?
                ret : __vsyscall_gtod_data.clock.cycle_last;
index 44153af..25ee06a 100644 (file)
@@ -132,15 +132,7 @@ static __always_inline void do_vgettimeofday(struct timeval * tv)
                        return;
                }
 
-               /*
-                * Surround the RDTSC by barriers, to make sure it's not
-                * speculated to outside the seqlock critical section and
-                * does not cause time warps:
-                */
-               rdtsc_barrier();
                now = vread();
-               rdtsc_barrier();
-
                base = __vsyscall_gtod_data.clock.cycle_last;
                mask = __vsyscall_gtod_data.clock.mask;
                mult = __vsyscall_gtod_data.clock.mult;