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

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Message-ID: <20200115143347.GL2827@hirez.programming.kicks-ass.net>
Date:   Wed, 15 Jan 2020 15:33:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
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>,
        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 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.