linux-kernel - Re: [RFC v1 0/5] fs/locks: Use plain percpu spinlocks instead of lglock to protect file

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Message-ID: <54F710B0.7040507@bmw-carit.de>
Date:	Wed, 4 Mar 2015 15:03:28 +0100
From:	Daniel Wagner <daniel.wagner@...-carit.de>
To:	Jeff Layton <jlayton@...chiereds.net>
CC:	Andi Kleen <andi@...stfloor.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, John Kacur <jkacur@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"J. Bruce Fields" <bfields@...ldses.org>
Subject: Re: [RFC v1 0/5] fs/locks: Use plain percpu spinlocks instead of
 lglock to protect file_lock

On 03/03/2015 01:29 AM, Jeff Layton wrote:
>> Hmm, are you sure about that? I read the code this way that when a lock
>> is added to flock_list it stays on that CPU. The locks are not moved
>> from one flock_list to another during their existent.
>>
> 
> Yes, I'm sure. When a file lock is acquired, we assign the fl_link_cpu
> to the current CPU and add it to the current CPU's global list. When
> the lock is released, any blocked lock that might have been blocking on
> it could acquire it at that point, and that doesn't necessarily occur
> on the same CPU as where the lock was originally held.
> 
> So, it's entirely possible that between when you drop the spinlock on
> one CPU and pick it up on another, the lock could have been released
> and then reacquired on a different CPU.

D'oh. I am an idiot. I didn't really understand it the first time. Yes,
you are right.

cheers,
daniel
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