Re: [PATCH 1/2] scalable rw_mutex

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2007-05-11 at 09:31 -0700, Andrew Morton wrote:
> On Fri, 11 May 2007 15:15:42 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > Scalable reader/writer lock.
> > 
> > Its scalable in that the read count is a percpu counter and the reader fast
> > path does not write to a shared cache-line.
> > 
> > Its not FIFO fair, but starvation proof by alternating readers and writers.
> 
> It looks .... surprisingly sane, given the history of these things ;)

Thanks!

> > ---
> >  include/linux/rwmutex.h |  103 +++++++++++++++++++++++++++++++++++++
> >  kernel/Makefile         |    3 -
> >  kernel/rwmutex.c        |  132 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 237 insertions(+), 1 deletion(-)
> > 
> > Index: linux-2.6/include/linux/rwmutex.h
> > ===================================================================
> > --- /dev/null	1970-01-01 00:00:00.000000000 +0000
> > +++ linux-2.6/include/linux/rwmutex.h	2007-05-11 14:59:09.000000000 +0200
> > @@ -0,0 +1,103 @@
> > +/*
> > + * Scalable reader/writer lock.
> > + *
> > + *  Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
> > + *
> > + * This file contains the public data structure and API definitions.
> > + */
> > +#ifndef _LINUX_RWMUTEX_H
> > +#define _LINUX_RWMUTEX_H
> > +
> > +#include <linux/preempt.h>
> > +#include <linux/wait.h>
> > +#include <linux/percpu_counter.h>
> > +#include <linux/lockdep.h>
> > +#include <linux/mutex.h>
> > +#include <asm/atomic.h>
> > +
> > +struct rw_mutex {
> > +	/* Read mostly global */
> > +	struct percpu_counter	readers;
> > +	unsigned int		status;
> > +
> > +	/* The following variables are only for the slowpath */
> > +	struct mutex		read_mutex;	/* r -> w waiting */
> > +	struct mutex		write_mutex;	/* w -> w waiting */
> > +	wait_queue_head_t	wait_queue;	/* w -> r waiting */
> > +	atomic_t		read_waiters;
> > +
> > +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> > +	struct lockdep_map dep_map;
> > +#endif
> > +};
> >
> 
> A nice comment describing the overall design and the runtime dynamics and
> the lock's characteristics would be useful.  It should include a prominent
> description of the lock's storage requirements, which are considerable.

Yes, storage-wise it is a tad heavy.

I'll try to write up a coherent description.

> > +extern void __rw_mutex_init(struct rw_mutex *rw_mutex, const char * name,
> > +		struct lock_class_key *key);
> > +extern void rw_mutex_destroy(struct rw_mutex *rw_mutex);
> 
> Sometimes you use `extern'.

/me does 's/extern //'

> > +#define rw_mutex_init(rw_mutex)					\
> > +	do {							\
> > +		static struct lock_class_key __key;		\
> > +		__rw_mutex_init((rw_mutex), #rw_mutex, &__key);	\
> > +	} while (0)
> > +
> > +extern void __rw_mutex_read_lock(struct rw_mutex *rw_mutex);
> > +
> > +extern void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass);
> > +extern void rw_mutex_write_unlock(struct rw_mutex *rw_mutex);
> > +
> > +static inline unsigned int __rw_mutex_reader_slow(struct rw_mutex *rw_mutex)
> > +{
> > +	unsigned int ret;
> > +
> > +	smp_rmb();
> > +	ret = rw_mutex->status;
> > +
> > +	return ret;
> > +}
> 
> An undocumented barrier!

/me adds documentation pointing to the smp_wmb() in
__rw_mutex_status_set() and expands the comment there.

> > +static inline int __rw_mutex_read_trylock(struct rw_mutex *rw_mutex)
> > +{
> > +	preempt_disable();
> > +	if (likely(!__rw_mutex_reader_slow(rw_mutex))) {
> > +		percpu_counter_mod(&rw_mutex->readers, 1);
> > +		preempt_enable();
> > +		return 1;
> > +	}
> > +	preempt_enable();
> > +	return 0;
> > +}
> 
> What does the preempt_disable() do?

Good question; and while writing up the answer I had for myself I found
it wrong. So this might very well be a race where a read lock succeeds
concurrently with a writer - bad!

I seem to need some rest to untangle my brains here. :-(

> > +EXPORT_SYMBOL_GPL(__rw_mutex_init);
> 
> down_foo(mmap_sem) was previously accessible to non-gpl modules, so the GPL
> export might be a problem.

Right, always breaks my heart to remove _GPL. But I guess 

> > +void rw_mutex_destroy(struct rw_mutex *rw_mutex)
> > +{
> > +	percpu_counter_destroy(&rw_mutex->readers);
> > +	mutex_destroy(&rw_mutex->read_mutex);
> > +	mutex_destroy(&rw_mutex->write_mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(rw_mutex_destroy);
> > +
> > +void __rw_mutex_read_lock(struct rw_mutex *rw_mutex)
> > +{
> > +	/*
> > +	 * read lock slow path;
> > +	 * count the number of readers waiting on the read_mutex
> > +	 */
> > +	atomic_inc(&rw_mutex->read_waiters);
> > +	mutex_lock(&rw_mutex->read_mutex);
> > +	/*
> > +	 * rw_mutex->state is only set while the read_mutex is held
> > +	 * so by serialising on this lock, we're sure its free.
> > +	 */
> > +	BUG_ON(rw_mutex->status);
> > +	/*
> > +	 * take the read reference, and drop the read_waiters count
> > +	 * and nudge all those waiting on the read_waiters count.
> > +	 */
> > +	percpu_counter_mod(&rw_mutex->readers, 1);
> > +	atomic_dec(&rw_mutex->read_waiters);
> > +	wake_up_all(&rw_mutex->wait_queue);
> > +	mutex_unlock(&rw_mutex->read_mutex);
> > +}
> > +EXPORT_SYMBOL_GPL(__rw_mutex_read_lock);
> 
> hm, I'm surprised that any foo_lock() would ever wake anyone up.

Yeah, this could use some more documentation; it wakes _write_unlock()
which can wait holding off new writers until at least a single reader
has had a chance.

> > +void rw_mutex_read_unlock(struct rw_mutex *rw_mutex)
> > +{
> > +	rwsem_release(&rw_mutex->dep_map, 1, _RET_IP_);
> > +
> > +	percpu_counter_mod(&rw_mutex->readers, -1);
> 
> percpu_counter_dec()?

Where was my brain... :-)

> > +	if (unlikely(__rw_mutex_reader_slow(rw_mutex)) &&
> > +			percpu_counter_sum(&rw_mutex->readers) == 0)
> > +		wake_up_all(&rw_mutex->wait_queue);
> > +}
> > +EXPORT_SYMBOL_GPL(rw_mutex_read_unlock);
> 
> yipes.  percpu_counter_sum() is expensive.

Right, and this instance is not strictly needed for correctness.

It might be possible to remove the other from the wait_event() loop, if
that makes any difference. If we fold the counter when switching to the
slow path, and use the shared counter there so it doesn't diverge again.

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