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Date: Wed, 7 Nov 2018 11:27:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC PATCH v4 01/13] ktask: add documentation
Message-ID: <20181107102752.GK9781@hirez.programming.kicks-ass.net>
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To: Jason Gunthorpe <jgg@mellanox.com>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>, "rjw@rjwysocki.net" <rjw@rjwysocki.net>, "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "kvm@vger.kernel.org" <kvm@vger.kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "aarcange@redhat.com" <aarcange@redhat.com>, "aaron.lu@intel.com" <aaron.lu@intel.com>, "akpm@linux-foundation.org" <akpm@linux-foundation.org>, "alex.williamson@redhat.com" <alex.williamson@redhat.com>, "bsd@redhat.com" <bsd@redhat.com>, "darrick.wong@oracle.com" <darrick.wong@oracle.com>, "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>, "jwadams@google.com" <jwadams@google.com>, "jiangshanlai@gmail.com" <jiangshanlai@gmail.com>, "mhocko@kernel.org" <mhocko@kernel.org>, "mike.kravetz@oracle.com" <mike.kravetz@oracle.com>, "Pavel.Tatashin@microsoft.com" <Pavel.Tatashin@microsoft.com>, "prasad.singamsetty@oracle.com" <prasad.singamsetty@oracle.com>, "rdunlap@infradead.org" <rdunlap@infradead.org>, "steven.sistare@oracle.com" <steven.sistare@oracle.com>, "tim.c.chen@intel.com" <tim.c.chen@intel.com>, "tj@kernel.org" <tj@kernel.org>, "vbabka@suse.cz" <vbabka@suse.cz>

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().