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timekeeping: Don't use seqcount loop in ktime_mono_to_any() on 64-bit systems

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ktime_mono_to_any() only fetches the offset inside the loop. This is a
single word on 64-bit CPUs, and seqcount_read_begin() implies a full SMP
barrier.

Use READ_ONCE() to fetch the offset instead of doing a seqcount loop on
64-bit and add the matching WRITE_ONCE()'s to update the offsets in
tk_set_wall_to_mono() and tk_update_sleep_time().

[ tglx: Get rid of the #ifdeffery ]

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20240910-mgtime-v3-1-84406ed53fad@kernel.org
This commit is contained in:
Jeff Layton 2024-09-10 13:43:34 -04:00 • committed by Thomas Gleixner
parent b98b276873
commit 8c111f1b96
1 changed files with 13 additions and 3 deletions
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16
kernel/time/timekeeping.c
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@ -161,13 +161,15 @@ static void tk_set_wall_to_mono(struct timekeeper *tk, struct timespec64 wtm)
WARN_ON_ONCE(tk->offs_real != timespec64_to_ktime(tmp));
tk->wall_to_monotonic = wtm;
set_normalized_timespec64(&tmp, -wtm.tv_sec, -wtm.tv_nsec);
tk->offs_real = timespec64_to_ktime(tmp);
tk->offs_tai = ktime_add(tk->offs_real, ktime_set(tk->tai_offset, 0));
/* Paired with READ_ONCE() in ktime_mono_to_any() */
WRITE_ONCE(tk->offs_real, timespec64_to_ktime(tmp));
WRITE_ONCE(tk->offs_tai, ktime_add(tk->offs_real, ktime_set(tk->tai_offset, 0)));
}
static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta)
{
tk->offs_boot = ktime_add(tk->offs_boot, delta);
/* Paired with READ_ONCE() in ktime_mono_to_any() */
WRITE_ONCE(tk->offs_boot, ktime_add(tk->offs_boot, delta));
/*
* Timespec representation for VDSO update to avoid 64bit division
* on every update.
@ -930,6 +932,14 @@ ktime_t ktime_mono_to_any(ktime_t tmono, enum tk_offsets offs)
unsigned int seq;
ktime_t tconv;
if (IS_ENABLED(CONFIG_64BIT)) {
/*
* Paired with WRITE_ONCE()s in tk_set_wall_to_mono() and
* tk_update_sleep_time().
*/
return ktime_add(tmono, READ_ONCE(*offset));
}
do {
seq = read_seqcount_begin(&tk_core.seq);
tconv = ktime_add(tmono, *offset);