linux-kernel - Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace signal_struct->group_rwsem with a global percpu

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Date:	Tue, 15 Sep 2015 18:42:19 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org >> Linux Kernel Mailing List" 
	<linux-kernel@...r.kernel.org>, KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [4.2] commit d59cfc09c32 (sched, cgroup: replace
 signal_struct->group_rwsem with a global percpu_rwsem) causes regression for
 libvirt/kvm

On 15/09/2015 15:36, Christian Borntraeger wrote:
> I am wondering why the old code behaved in such fatal ways. Is there
> some interaction between waiting for a reschedule in the
> synchronize_sched writer and some fork code actually waiting for the
> read side to get the lock together with some rescheduling going on
> waiting for a lock that fork holds? lockdep does not give me an hints
> so I have no clue :-(

It may just be consuming too much CPU usage.  kernel/rcu/tree.c warns
about it:

 * if you are using synchronize_sched_expedited() in a loop, please
 * restructure your code to batch your updates, and then use a single
 * synchronize_sched() instead.

and you may remember that in KVM we switched from RCU to SRCU exactly to
avoid userspace-controlled synchronize_rcu_expedited().

In fact, I would say that any userspace-controlled call to *_expedited()
is a bug waiting to happen and a bad idea---because userspace can, with
little effort, end up calling it in a loop.

Paolo
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