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

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Date:	Mon, 04 Aug 2014 22:30:32 -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 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));
}

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