From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v8 6/8] cgroup/cpuset: Change cpuset_rwsem and hotplug
 lock order
Date: Mon, 1 Jul 2019 10:27:31 +0200
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To: Juri Lelli <juri.lelli@redhat.com>
Cc: mingo@redhat.com, rostedt@goodmis.org, tj@kernel.org, linux-kernel@vger.kernel.org, luca.abeni@santannapisa.it, claudio@evidence.eu.com, tommaso.cucinotta@santannapisa.it, bristot@redhat.com, mathieu.poirier@linaro.org, lizefan@huawei.com, cgroups@vger.kernel.org, Prateek Sood <prsood@codeaurora.org>

On Mon, Jul 01, 2019 at 08:52:33AM +0200, Juri Lelli wrote:
> Hi,
> 
> On 28/06/19 15:03, Peter Zijlstra wrote:
> > On Fri, Jun 28, 2019 at 10:06:16AM +0200, Juri Lelli wrote:
> > > cpuset_rwsem is going to be acquired from sched_setscheduler() with a
> > > following patch. There are however paths (e.g., spawn_ksoftirqd) in
> > > which sched_scheduler() is eventually called while holding hotplug lock;
> > > this creates a dependecy between hotplug lock (to be always acquired
> > > first) and cpuset_rwsem (to be always acquired after hotplug lock).
> > > 
> > > Fix paths which currently take the two locks in the wrong order (after
> > > a following patch is applied).
> > > Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> > 
> > This all reminds me of this:
> > 
> >   https://lkml.kernel.org/r/1510755615-25906-1-git-send-email-prsood@codeaurora.org
> > 
> > Which sadly got reverted again. If we do this now (I've always been a
> > proponent), then we can make that rebuild synchronous again, which
> > should also help here IIRC.
> 
> Why was that reverted? Perf regression of some type?

IIRC TJ figured it wasn't strictly required to fix the lock invertion at
that time and they sorted it differently. If I (re)read the thread
correctly the other day, he didn't have fundamental objections against
it, but wanted the simpler fix.