Re: [RFC][PATCH 5/5] percpu-rwsem: Optimize readers and reduce global impact

[Oleg Nesterov]

Sorry for delay, finally I found the time to read this series...
The code matches our previous discussion and I believe it is correct.

Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>


Just one nit below,

On 05/26, Peter Zijlstra wrote:
>
>  struct percpu_rw_semaphore {
> -	unsigned int __percpu	*fast_read_ctr;
> -	atomic_t		write_ctr;
> +	unsigned int __percpu	*refcount;
> +	int			state;

....

> +enum { readers_slow, readers_block };

Now that we rely on rss_sync() and thus we do not have "readers_fast",
I think that "bool reader_should_block" will look better.

> +void percpu_down_write(struct percpu_rw_semaphore *sem)
>  {
...

so it does

	rcu_sync_enter(&sem->rss);

	state = BLOCK;

	mb();

	wait_event(sem->writer, readers_active_check(sem));

and this looks correct.

The only nontrivial thing we need to ensure is that
per_cpu_sum(*sem->refcount) == 0 can't be false positive. False
negative is fine.

And this means that if we see the result of this_cpu_dec() we must
not miss the result of the previous this_cpu_inc() on another CPU.
same or _another_ CPU.

And this is true because if the reader does dec() on another CPU
it does up_read() and this is only possible if down_read() didn't
see state == BLOCK.

But if it didn't see state == BLOCK then the writer must see the
result of the previous down_read()->inc().

IOW, we just rely on STORE-MB-LOAD, just the writer does LOAD
multiple times in per_cpu_sum():

DOWN_WRITE:			DOWN_READ on CPU X:

state = BLOCK;			refcount[X]++;

mb();				mb();

sum = 0;			if (state != BLOCK)
sum += refcount[0];			return;	 /* success* /
sum += refcount[1];		
...				refcount[X]--;
sum += refcount[NR_CPUS];


If the reader wins and takes the lock, then its addition to
refcount[X] must be accounted by the writer.

The writer can obviously miss dec() from the reader, but we rely
on wake_up/wait_event in this case.

Oleg.

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