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time: x86: Fix race switching from vsyscall to non-vsyscall clock

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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@amacapital.net>
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
This commit is contained in:
John Stultz 2012-03-01 22:11:09 -08:00
parent a078c6d0e6
commit a939e817aa
1 changed files with 46 additions and 26 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

72
arch/x86/vdso/vclock_gettime.c
View File

@ -70,14 +70,26 @@ 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;
cycles_t cycles;
if (gtod->clock.vclock_mode == VCLOCK_TSC)
cycles = vread_tsc();
else
else if (gtod->clock.vclock_mode == VCLOCK_HPET)
cycles = vread_hpet();
else
return 0;
v = (cycles - gtod->clock.cycle_last) & gtod->clock.mask;
return (v * gtod->clock.mult) >> gtod->clock.shift;
}
@ -85,21 +97,28 @@ 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(&gtod->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(&gtod->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(&gtod->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 +135,7 @@ 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 +175,14 @@ 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 +190,33 @@ 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")));