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

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Date:   Sat, 17 Feb 2018 00:49:35 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] fs/dcache: Avoid the try_lock loops in dentry_kill()

On 2018-02-17, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> dentry_kill() calls both dentry_lock_inode() and lock_parent() in the
>> common case. So by changing the semantics of lock_parent(), I am
>> removing two "recheck in case I dropped" in the common case rather
>> than just the one you pointed out.
>
> Ok, that would be lovely, but doesn't that end up being a nasty patch?

After reading your initial feedback my idea was to change both
lock_parent() and dentry_lock_inode() to not only communicate _if_ the
lock was successful, but also if d_lock was dropped in the process. (For
example, with a tristate rather than boolean return value.) Then callers
would know if they needed to recheck the dentry contents.

> So it may be that my dislike of the "re-check after possibly dropping
> the lock" is not really about the re-checking, but about just how it
> made that function look much more complicated.

I understand what you are saying and I appreciate the comments. I will
code up some variations for myself and try to pick the one that is the
least complicated for my v2.

John Ogness