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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