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Date: Tue, 20 Feb 2018 09:43:54 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: John Ogness <john.ogness@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Christoph Hellwig <hch@lst.de>,
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        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        hjl.tools@gmail.com
Subject: Re: [PATCH 4/4] fs/dcache: Avoid the try_lock loops in dentry_kill()
Message-ID: <20180220084354.GO25235@hirez.programming.kicks-ass.net>
References: <20180216150933.971-5-john.ogness@linutronix.de>
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On Tue, Feb 20, 2018 at 09:39:37AM +0100, Peter Zijlstra wrote:
> 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));

Oooh shiny, you can actually write:

		switch(__builtin_expect(dentry_lock_inode(dentry), LOCK_FAST)) {

and have that work, see:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59521

> >                         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;
> >                 }
> >