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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
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