Hi Kirill,
On 06/10, Kirill Tkhai wrote:
>
> В Вт, 09/06/2015 в 23:33 +0200, Oleg Nesterov пишет:
> >
> > hrtimer_active(timer)
> > {
> >
> > do {
> > base = READ_ONCE(timer->base->cpu_base);
> > seq = read_seqcount_begin(&cpu_base->seq);
> >
> > if (timer->state & ENQUEUED ||
> > base->running == timer)
> > return true;
> >
> > } while (read_seqcount_retry(&cpu_base->seq, seq) ||
> > base != READ_ONCE(timer->base->cpu_base));
> >
> > return false;
> > }
> >
> > And we need to avoid the races with 2 transitions in __run_hrtimer().
> >
> > The first race is trivial, we change __run_hrtimer() to do
> >
> > write_seqcount_begin(cpu_base->seq);
> > cpu_base->running = timer;
> > __remove_hrtimer(timer); // clears ENQUEUED
> > write_seqcount_end(cpu_base->seq);
>
> We use seqcount, because we are afraid that hrtimer_active() may miss
> timer->state or cpu_base->running, when we are clearing it.
Yes,
> If we use two pairs of write_seqcount_{begin,end} in __run_hrtimer(),
> we may protect only the places where we do that:
>
> cpu_base->running = timer;
> write_seqcount_begin(cpu_base->seq);
> __remove_hrtimer(timer); // clears ENQUEUED
> write_seqcount_end(cpu_base->seq);
>
> ....
>
> timer->state |= HRTIMER_STATE_ENQUEUED;
> write_seqcount_begin(cpu_base->seq);
> base->running = NULL;
> write_seqcount_end(cpu_base->seq);
Afaics, no. Afaics, the following code is correct:
seqcount_t LOCK;
bool X = true, Y = false;
void read(void)
{
bool x, y;
do {
seq = read_seqcount_begin(&LOCK);
x = X; y = Y;
} while (read_seqcount_retry(&LOCK, seq));
BUG_ON(!x && !y);
}
void write(void)
{
Y = true;
write_seqcount_begin(LOCK);
write_seqcount_end(LOCK);
X = false;
}
If we rely on the "locking" semantics of seqcount_t, this doesn't really
differ from
spinlock_t LOCK;
bool X = true, Y = false;
void read(void)
{
bool x, y;
spin_lock(LOCK);
x = X; y = Y;
spin_unlock(LOCK);
BUG_ON(!x && !y);
}
void write(void)
{
Y = true;
spin_lock(LOCK);
spin_unlock(LOCK);
X = false;
}
If "read" takes the lock before "write", it must see X == true.
Otherwise "read" should see all memory changes done before or
inside the "write" critical section, so it must see Y == true.
No?
Oleg.
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