linux-kernel - Re: [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock holder auto-demotion

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Message-ID: <43cf01f7-1fb5-26a9-bcda-5c9748c6f32e@redhat.com>
Date:   Mon, 23 Aug 2021 11:36:10 -0500
From:   Bob Peterson <rpeterso@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Steven Whitehouse <swhiteho@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        ocfs2-devel@....oracle.com
Subject: Re: [Cluster-devel] [PATCH v6 10/19] gfs2: Introduce flag for glock
 holder auto-demotion

On 8/23/21 11:05 AM, Matthew Wilcox wrote:
> On Mon, Aug 23, 2021 at 05:18:12PM +0200, Andreas Gruenbacher wrote:
>> On Mon, Aug 23, 2021 at 10:14 AM Steven Whitehouse <swhiteho@...hat.com> wrote:
>>> If the goal here is just to allow the glock to be held for a longer
>>> period of time, but with occasional interruptions to prevent
>>> starvation, then we have a potential model for this. There is
>>> cond_resched_lock() which does this for spin locks.
>>
>> This isn't an appropriate model for what I'm trying to achieve here.
>> In the cond_resched case, we know at the time of the cond_resched call
>> whether or not we want to schedule. If we do, we want to drop the spin
>> lock, schedule, and then re-acquire the spin lock. In the case we're
>> looking at here, we want to fault in user pages. There is no way of
>> knowing beforehand if the glock we're currently holding will have to
>> be dropped to achieve that. In fact, it will almost never have to be
>> dropped. But if it does, we need to drop it straight away to allow the
>> conflicting locking request to succeed.
> 
> It occurs to me that this is similar to the wound/wait mutexes
> (include/linux/ww_mutex.h & Documentation/locking/ww-mutex-design.rst).
> You want to mark the glock as woundable before faulting, and then discover
> if it was wounded after faulting.  Maybe sharing this terminology will
> aid in understanding?
> 
Hmm. Woundable. I like it.
Andreas and I argued about the terminology but we never found a 
middle-ground. Perhaps this is it. Thanks, Matthew.

Regards,

Bob Peterson