linux-kernel - Re: [PATCH] zram: remove global tb

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Date:	Mon, 5 May 2014 13:46:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Seth Jennings <sjennings@...iantweb.net>,
	Weijie Yang <weijie.yang@...sung.com>,
	"'Minchan Kim'" <minchan@...nel.org>,
	"'Nitin Gupta'" <ngupta@...are.org>,
	"'Sergey Senozhatsky'" <sergey.senozhatsky@...il.com>,
	"'Bob Liu'" <bob.liu@...cle.com>,
	"'Dan Streetman'" <ddstreet@...e.org>, weijie.yang.kh@...il.com,
	heesub.shin@...sung.com,
	"'linux-kernel'" <linux-kernel@...r.kernel.org>,
	"'Linux-MM'" <linux-mm@...ck.org>
Subject: Re: [PATCH] zram: remove global tb_lock by using lock-free CAS

On Mon, 05 May 2014 11:00:44 -0700 Davidlohr Bueso <davidlohr@...com> wrote:

> > > @@ -339,12 +338,14 @@ static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> > >  	unsigned long handle;
> > >  	u16 size;
> > >  
> > > -	read_lock(&meta->tb_lock);
> > > +	while(atomic_cmpxchg(&meta->table[index].state, IDLE, ACCESS) != IDLE)
> > > +		cpu_relax();
> > > +
> > 
> > So... this might be dumb question, but this looks like a spinlock
> > implementation.
> > 
> > What advantage does this have over a standard spinlock?
> 
> I was wondering the same thing. Furthermore by doing this you'll loose
> the benefits of sharing the lock... your numbers do indicate that it is
> for the better. Also, note that hopefully rwlock_t will soon be updated
> to be fair and perform up to par with spinlocks, something which is long
> overdue. So you could reduce the critical region by implementing the
> same granularity, just don't implement your own locking schemes, like
> this.

It sounds like seqlocks will match this access pattern pretty well?
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