linux-kernel - Re: [PATCH/RFC] z3fold: use per-page read/write lock

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Message-ID: <20161110193822.GK3175@twins.programming.kicks-ass.net>
Date:   Thu, 10 Nov 2016 20:38:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vitaly Wool <vitalywool@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        Dan Streetman <ddstreet@...e.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH/RFC] z3fold: use per-page read/write lock

On Sat, Nov 05, 2016 at 02:49:46PM +0100, Vitaly Wool wrote:
> +/* Read-lock a z3fold page */
> +static void z3fold_page_rlock(struct z3fold_header *zhdr)
> +{
> +	while (!atomic_add_unless(&zhdr->page_lock, 1, Z3FOLD_PAGE_WRITE_FLAG))
> +		cpu_relax();
> +	smp_mb();
> +}
> +
> +/* Read-unlock a z3fold page */
> +static void z3fold_page_runlock(struct z3fold_header *zhdr)
> +{
> +	atomic_dec(&zhdr->page_lock);
> +	smp_mb();
> +}
> +
> +/* Write-lock a z3fold page */
> +static void z3fold_page_wlock(struct z3fold_header *zhdr)
> +{
> +	while (atomic_cmpxchg(&zhdr->page_lock, 0, Z3FOLD_PAGE_WRITE_FLAG) != 0)
> +		cpu_relax();
> +	smp_mb();
> +}
> +
> +/* Write-unlock a z3fold page */
> +static void z3fold_page_wunlock(struct z3fold_header *zhdr)
> +{
> +	atomic_sub(Z3FOLD_PAGE_WRITE_FLAG, &zhdr->page_lock);
> +	smp_mb();
> +}

This is trivially broken. What Andi said, don't roll your own locks.


The unlocks want: smp_mb__before_atomic() _before_ the atomic for
'obvious' reasons.

Also, this lock has serious starvation issues and is reader biased.