linux-kernel - Re: [PATCH 4/4] fs/dcache: Avoid the try_lock loops in dentry

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Message-ID: <20180220083937.GX25201@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Feb 2018 09:39:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        hjl.tools@...il.com
Subject: Re: [PATCH 4/4] fs/dcache: Avoid the try_lock loops in dentry_kill()

On Tue, Feb 20, 2018 at 12:34:57AM +0100, John Ogness wrote:
> Implementation 2: Using switch on a dentry_lock_inode() that returns a
> tristate value. Does not support branch prediction. This approach is
> probably easiest to understand.
> 
>  		/*
>  		 * Lock the inode. Might drop dentry->d_lock temporarily
>  		 * which allows inode to change. Start over if that happens.
>  		 */
>                 switch (dentry_lock_inode(dentry)) {
>                 case LOCK_FAST:

Bah, I just checked, you cannot use GCC label attributes on statements
:/ Otherwise you could've done:

		case LOCK_FAST: __attribute__((hot));

>                         break;
>                 case LOCK_SLOW:
>                         /*
>                          * Recheck refcount as it might have been
>                          * incremented while d_lock was dropped.
>                          */
>                         if (unlikely(dentry->d_lockref.count != 1))
>                                 goto drop_ref;
>                         break;
>                 case LOCK_FAILED:
>                         goto again;
>                 }
>