linux-kernel - Re: [PATCH 4/7] locking/rwsem: threshold limited spinning for active readers

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Date:	Mon, 04 Aug 2014 22:41:41 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-doc@...r.kernel.org, Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 4/7] locking/rwsem: threshold limited spinning for
 active readers

On Mon, 2014-08-04 at 22:30 -0700, Davidlohr Bueso wrote:
> On Mon, 2014-08-04 at 21:54 -0700, Davidlohr Bueso wrote:
> > >  #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
> > > +/*
> > > + * The owner field is set to RWSEM_READ_OWNED if the last owner(s) are
> > > + * readers. It is not reset until a writer takes over and set it to its
> > > + * task structure pointer or NULL when it frees the lock. So a value
> > > + * of RWSEM_READ_OWNED doesn't mean it currently has active readers.
> > > + */
> > > +#define RWSEM_READ_OWNED	((struct task_struct *)-1)
> > 
> > Looks rather weird...
> 
> Instead of populating owner when taking the reader lock, why not just
> leave it NULL. Then, we can differentiate between the owner being NULL
> either because it is taken by reader(s) or simply because it is not
> taken. So something like this:
> 
> static inline bool rwsem_owner_is_reader(struct rw_semaphore *sem)
> {
> 	return !sem->owner && rwsem_is_locked(sem));
> }

Although we could race between both checks, specially when going into
slowpaths, too bad. Probably a read barrier before calling is_locked
too. Anyway, you see, things start getting funky.

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