Re: [PATCH 1/2] scalable rw_mutex

[Oleg Nesterov <oleg@tv-sign.ru>]

On 05/12, Peter Zijlstra wrote:
>
> +static inline void rw_mutex_readers_dec(struct rw_mutex *rw_mutex)
> +{
> +	percpu_counter_dec(&rw_mutex->readers);
> +	smp_wmb();
> +}
>
> +void rw_mutex_read_unlock(struct rw_mutex *rw_mutex)
> +{
> +	rw_mutex_readers_dec(rw_mutex);
> +	/*
> +	 * on the slow path;
> +	 * nudge the writer waiting for the last reader to go away
> +	 */
> +	if (unlikely(rw_mutex_reader_slow(rw_mutex)))
> +		rw_mutex_writer_wake(rw_mutex);
> +}
>
> +void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass)
> +{
> +	mutex_lock_nested(&rw_mutex->write_mutex, subclass);
> +
> +	/*
> +	 * block new readers
> +	 */
> +	mutex_lock_nested(&rw_mutex->read_mutex, subclass);
> +	rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW);
> +	/*
> +	 * and wait for all current readers to go away
> +	 */
> +	rw_mutex_writer_wait(rw_mutex, (rw_mutex_readers(rw_mutex) == 0));
> +}

I think this is still not right, but when it comes to barriers we
need a true expert (Paul cc-ed).

this code roughly does (the only reader does unlock)

	READER			WRITER

	readers = 0;		state = 1;
	wmb();			wmb();
	CHECK(state != 0)	CHECK(readers == 0)

We need to ensure that we can't miss both CHECKs. Either reader
should see RW_MUTEX_READER_SLOW, o writer sees "readers == 0"
and does not sleep.

In that case both barriers should be converted to smp_mb(). There
was a _long_ discussion about STORE-MB-LOAD behaviour, and experts
seem to believe everething is ok.

Another question. Isn't it possible to kill rw_mutex->status ?

I have a vague feeling you can change the code so that

	rw_mutex_reader_slow() <=> "->waiter != NULL"

, but I am not sure.

Oleg.

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