linux-kernel - Re: [PATCH 1/2] seqlock: Add a new blocking reader type

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Date:	Wed, 11 Sep 2013 19:52:48 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Waiman Long <waiman.long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH 1/2] seqlock: Add a new blocking reader type

On Wed, Sep 11, 2013 at 02:40:38PM -0400, J. Bruce Fields wrote:
> On Wed, Sep 11, 2013 at 06:26:25PM +0100, Al Viro wrote:
> > On Wed, Sep 11, 2013 at 12:33:35PM -0400, Waiman Long wrote:
> > 
> > > >Folks, any suggestions on better names?  The semantics we are getting is
> > > 
> > > I will welcome any better name suggestion and will incorporate that
> > > in the patch.
> > 
> > FWIW, the suggestions I've seen so far had been
> > 
> > seq_exreadlock() [ex for exclusive]
> > seq_exclreadlock() [ditto, and IMO fails the "easily read over the phone"
> > test - /sekv-excre...ARRGH/]
> > seq_prot_readlock() [prot for protected, as in DLM protected read]
> 
> Though the DLM protected read doesn't self-conflict either so that's a
> poor analogy, my bad.
> 
> (Do the users really require that the read be exclusive?)

We want to exclude writers and since the writer is
lock:
	spin_lock(&sl->lock), bump sl->sequence by 1, smp_wmb()
unlock:
	smp_wmb(), bump sl->sequence by 1, spin_unlock(&sl->lock)
the obvious implementation for this new primitive is simply spin_{lock,unlock}
on the same spinlock...
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