linux-kernel - Re: [RFC PATCH v4 01/13] ktask: add documentation

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Message-ID: <20181107102752.GK9781@hirez.programming.kicks-ass.net>
Date:   Wed, 7 Nov 2018 11:27:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>,
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Subject: Re: [RFC PATCH v4 01/13] ktask: add documentation

On Tue, Nov 06, 2018 at 08:51:54PM +0000, Jason Gunthorpe wrote:
> On Tue, Nov 06, 2018 at 12:34:11PM -0800, Daniel Jordan wrote:
> 
> > > What isn't clear is if this calling thread is waiting or not. Only do
> > > this inheritance trick if it is actually waiting on the work. If it is
> > > not, nobody cares.
> > 
> > The calling thread waits.  Even if it didn't though, the inheritance trick
> > would still be desirable for timely completion of the job.
> 
> Can you make lockdep aware that this is synchronous?
> 
> ie if I do
> 
>   mutex_lock()
>   ktask_run()
>   mutex_lock()
> 
> Can lockdep know that all the workers are running under that lock?
> 
> I'm thinking particularly about rtnl_lock as a possible case, but
> there could also make some sense to hold the read side of the mm_sem
> or similar like the above.

Yes, the normal trick is adding a fake lock to ktask_run and holding
that over the actual job. See lock_map* in flush_workqueue() vs
process_one_work().