x86: move rdtsc_barrier() into the TSC vread method

commit 7d96fd41cadc55f4e00231c8c71b8e25c779f122 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index d57de05dc43093e0831ebb544a2baddf3d72f184..cf8611d991e089b4f3951959314ee7e5be82ec6a 100644 (file)
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -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;

diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 44153afc9067558cef387ba3237ffcf439ca3800..25ee06a80aad3cd116292227826f9a32fa4b3f7a 100644 (file)
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -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;