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

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Message-ID: <849239ff-d2d1-4048-da58-b4347e0aa2bd@redhat.com>
Date:   Wed, 15 Jan 2020 14:03:22 -0500
From:   Waiman Long <longman@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.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 1/15/20 9:49 AM, Jason Gunthorpe wrote:
> On Wed, Jan 15, 2020 at 03:33:47PM +0100, Peter Zijlstra wrote:
>> On Wed, Jan 15, 2020 at 09:24:28AM -0400, Jason Gunthorpe wrote:
>>
>>> I was interested because you are talking about allowing the read/write side
>>> of a rw sem to be held across a return to user space/etc, which is the
>>> same basic problem.
>> No it is not; allowing the lock to be held across userspace doesn't
>> change the owner. This is a crucial difference, PI depends on there
>> being a distinct owner. That said, allowing the lock to be held across
>> userspace still breaks PI in that it completely wrecks the ability to
>> analyze the critical section.
> I'm not sure what you are contrasting?
>
> I was remarking that I see many places open code a rwsem using an
> atomic and a completion specifically because they need to do the
> things Christoph identified:
>
>> (1) no unlocking by another process than the one that acquired it
>> (2) no return to userspace with locks held
> As an example flow: obtain the read side lock, schedual a work queue,
> return to user space, and unlock the read side from the work queue.

We currently have down_read_non_owner() and up_read_non_owner() that
perform the lock and unlock without lockdep tracking. Of course, that is
a hack and their use must be carefully scrutinized to make sure that
there is no deadlock or other potentially locking issues.

Cheers,
Longman