linux-ext4 - Re: RFC: hold i_rwsem until aio completes

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Date:   Wed, 15 Jan 2020 07:56:14 +0100
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-ext4@...r.kernel.org, cluster-devel@...hat.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: RFC: hold i_rwsem until aio completes

On Tue, Jan 14, 2020 at 03:27:00PM -0400, Jason Gunthorpe wrote:
> I've seen similar locking patterns quite a lot, enough I've thought
> about having a dedicated locking primitive to do it. It really wants
> to be a rwsem, but as here the rwsem rules don't allow it.
> 
> The common pattern I'm looking at looks something like this:
> 
>  'try begin read'() // aka down_read_trylock()
> 
>   /* The lockdep release hackery you describe,
>      the rwsem remains read locked */
>  'exit reader'()
> 
>  .. delegate unlock to work queue, timer, irq, etc ..
> 
> in the new context:
> 
>  're_enter reader'() // Get our lockdep tracking back
> 
>  'end reader'() // aka up_read()
> 
> vs a typical write side:
> 
>  'begin write'() // aka down_write()
> 
>  /* There is no reason to unlock it before kfree of the rwsem memory.
>     Somehow the user prevents any new down_read_trylock()'s */
>  'abandon writer'() // The object will be kfree'd with a locked writer
>  kfree()
> 
> The typical goal is to provide an object destruction path that can
> serialize and fence all readers wherever they may be before proceeding
> to some synchronous destruction.
> 
> Usually this gets open coded with some atomic/kref/refcount and a
> completion or wait queue. Often implemented wrongly, lacking the write
> favoring bias in the rwsem, and lacking any lockdep tracking on the
> naked completion.
> 
> Not to discourage your patch, but to ask if we can make the solution
> more broadly applicable?

Your requirement seems a little different, and in fact in many ways
similar to the percpu_ref primitive.