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timekeeping: Split jiffies seqlock

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seqlock consists of a sequence counter and a spinlock_t which is used to
serialize the writers. spinlock_t is substituted by a "sleeping" spinlock
on PREEMPT_RT enabled kernels which breaks the usage in the timekeeping
code as the writers are executed in hard interrupt and therefore
non-preemptible context even on PREEMPT_RT.

The spinlock in seqlock cannot be unconditionally replaced by a
raw_spinlock_t as many seqlock users have nesting spinlock sections or
other code which is not suitable to run in truly atomic context on RT.

Instead of providing a raw_seqlock API for a single use case, open code the
seqlock for the jiffies use case and implement it with a raw_spinlock_t and
a sequence counter.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200321113242.120587764@linutronix.de
This commit is contained in:
Thomas Gleixner
2020-03-21 12:25:58 +01:00
committed by Peter Zijlstra
parent 919e9e6395
commit e5d4d1756b
5 changed files with 28 additions and 17 deletions
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10
kernel/time/tick-common.c
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@@ -84,13 +84,15 @@ int tick_is_oneshot_available(void)
static void tick_periodic(int cpu)
{
if (tick_do_timer_cpu == cpu) {
write_seqlock(&jiffies_lock);
raw_spin_lock(&jiffies_lock);
write_seqcount_begin(&jiffies_seq);
/* Keep track of the next tick event */
tick_next_period = ktime_add(tick_next_period, tick_period);
do_timer(1);
write_sequnlock(&jiffies_lock);
write_seqcount_end(&jiffies_seq);
raw_spin_unlock(&jiffies_lock);
update_wall_time();
}
@@ -162,9 +164,9 @@ void tick_setup_periodic(struct clock_event_device *dev, int broadcast)
ktime_t next;
do {
seq = read_seqbegin(&jiffies_lock);
seq = read_seqcount_begin(&jiffies_seq);
next = tick_next_period;
} while (read_seqretry(&jiffies_lock, seq));
} while (read_seqcount_retry(&jiffies_seq, seq));
clockevents_switch_state(dev, CLOCK_EVT_STATE_ONESHOT);