linux-kernel - Re: [PATCH] fs/dcache: optimize start_dir

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Message-ID: <20201026215923.GA306023@dread.disaster.area>
Date:   Tue, 27 Oct 2020 08:59:23 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        peterz@...radead.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH] fs/dcache: optimize start_dir_add()

On Thu, Oct 22, 2020 at 02:16:50PM -0700, Davidlohr Bueso wrote:
> Considering both end_dir_add() and d_alloc_parallel(), the
> dir->i_dir_seq wants acquire/release semantics, therefore
> micro-optimize for ll/sc archs and use finer grained barriers
> to provide (load)-ACQUIRE ordering (L->S + L->L). This comes
> at no additional cost for most of x86, as sane tso models will
> have a nop for smp_rmb/smp_acquire__after_ctrl_dep.
> 
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> ---
> Alternatively I guess we could just use cmpxchg_acquire().

Please us cmpxchg_acquire() so that people who have no clue what the
hell smp_acquire__after_ctrl_dep() means or does have some hope of
understanding of what objects the ordering semantics in the function
actually apply to....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com