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Date: Wed, 14 Mar 2012 17:42:33 -0700
From: John Stultz <john.stultz@...aro.org>
To: Thomas Gleixner <tglx@...utronix.de>
CC: linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 1/2] time: x86: Fix race switching from vsyscall to non-vsyscall
clock
On 03/14/2012 05:34 PM, Thomas Gleixner wrote:
> On Wed, 14 Mar 2012, John Stultz wrote:
>> notrace static noinline int do_realtime(struct timespec *ts)
>> {
>> unsigned long seq, ns;
>> + int mode;
> Please keep a newline between declarations and code.
Fixed below. Thanks!
(Let me know if you see whitespace damage, I switched mail clients today
and am learning the quirks here.)
-john
When switching from a vsyscall capable to a non-vsyscall capable
clocksource, there was a small race, where the last vsyscall
gettimeofday before the switch might return a invalid time value
using the new non-vsyscall enabled clocksource values after the
switch is complete.
This is due to the vsyscall code checking the vclock_mode once
outside of the seqcount protected section. After it reads the
vclock mode, it doesn't re-check that the sampled clock data
that is obtained in the seqcount critical section still matches.
The fix is to sample vclock_mode inside the protected section,
and as long as it isn't VCLOCK_NONE, return the calculated
value. If it has changed and is now VCLOCK_NONE, fall back
to the syscall gettime calculation.
v2:
* Cleanup checks as suggested by tglx
* Also fix same issue present in gettimeofday path
CC: Andy Lutomirski<luto@...capital.net>
CC: Thomas Gleixner<tglx@...utronix.de>
Signed-off-by: John Stultz<john.stultz@...aro.org>
---
arch/x86/vdso/vclock_gettime.c | 68 +++++++++++++++++++++++++--------------
1 files changed, 43 insertions(+), 25 deletions(-)
diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 6bc0e72..e5ba922 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -70,6 +70,16 @@ notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
return ret;
}
+notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
+{
+ long ret;
+
+ asm("syscall" : "=a" (ret) :
+ "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
+ return ret;
+}
+
+
notrace static inline long vgetns(void)
{
long v;
@@ -85,21 +95,28 @@ notrace static inline long vgetns(void)
notrace static noinline int do_realtime(struct timespec *ts)
{
unsigned long seq, ns;
+ int mode;
+
do {
seq = read_seqbegin(>od->lock);
+ mode = gtod->clock.vclock_mode;
ts->tv_sec = gtod->wall_time_sec;
ts->tv_nsec = gtod->wall_time_nsec;
ns = vgetns();
} while (unlikely(read_seqretry(>od->lock, seq)));
+
timespec_add_ns(ts, ns);
- return 0;
+ return mode;
}
notrace static noinline int do_monotonic(struct timespec *ts)
{
unsigned long seq, ns, secs;
+ int mode;
+
do {
seq = read_seqbegin(>od->lock);
+ mode = gtod->clock.vclock_mode;
secs = gtod->wall_time_sec;
ns = gtod->wall_time_nsec + vgetns();
secs += gtod->wall_to_monotonic.tv_sec;
@@ -116,7 +133,7 @@ notrace static noinline int do_monotonic(struct timespec *ts)
ts->tv_sec = secs;
ts->tv_nsec = ns;
- return 0;
+ return mode;
}
notrace static noinline int do_realtime_coarse(struct timespec *ts)
@@ -156,14 +173,14 @@ notrace static noinline int do_monotonic_coarse(struct timespec *ts)
notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
{
+ int ret = VCLOCK_NONE;
+
switch (clock) {
case CLOCK_REALTIME:
- if (likely(gtod->clock.vclock_mode != VCLOCK_NONE))
- return do_realtime(ts);
+ ret = do_realtime(ts);
break;
case CLOCK_MONOTONIC:
- if (likely(gtod->clock.vclock_mode != VCLOCK_NONE))
- return do_monotonic(ts);
+ ret = do_monotonic(ts);
break;
case CLOCK_REALTIME_COARSE:
return do_realtime_coarse(ts);
@@ -171,32 +188,33 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
return do_monotonic_coarse(ts);
}
- return vdso_fallback_gettime(clock, ts);
+ if (ret == VCLOCK_NONE)
+ return vdso_fallback_gettime(clock, ts);
+ return 0;
}
int clock_gettime(clockid_t, struct timespec *)
__attribute__((weak, alias("__vdso_clock_gettime")));
notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
{
- long ret;
- if (likely(gtod->clock.vclock_mode != VCLOCK_NONE)) {
- if (likely(tv != NULL)) {
- BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
- offsetof(struct timespec, tv_nsec) ||
- sizeof(*tv) != sizeof(struct timespec));
- do_realtime((struct timespec *)tv);
- tv->tv_usec /= 1000;
- }
- if (unlikely(tz != NULL)) {
- /* Avoid memcpy. Some old compilers fail to inline it */
- tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
- tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
- }
- return 0;
+ long ret = VCLOCK_NONE;
+
+ if (likely(tv != NULL)) {
+ BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
+ offsetof(struct timespec, tv_nsec) ||
+ sizeof(*tv) != sizeof(struct timespec));
+ ret = do_realtime((struct timespec *)tv);
+ tv->tv_usec /= 1000;
}
- asm("syscall" : "=a" (ret) :
- "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
- return ret;
+ if (unlikely(tz != NULL)) {
+ /* Avoid memcpy. Some old compilers fail to inline it */
+ tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
+ tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
+ }
+
+ if (ret == VCLOCK_NONE)
+ return vdso_fallback_gtod(tv, tz);
+ return 0;
}
int gettimeofday(struct timeval *, struct timezone *)
__attribute__((weak, alias("__vdso_gettimeofday")));
--
1.7.3.2.146.gca209
--
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