From mboxrd@z Thu Jan 1 00:00:00 1970 From: Juri Lelli Subject: Re: [PATCH v6 4/5] sched/core: Prevent race condition between cpuset and __sched_setscheduler() Date: Tue, 5 Feb 2019 12:49:11 +0100 Message-ID: <20190205114911.GH30905@localhost.localdomain> References: <20190117084739.17078-1-juri.lelli@redhat.com> <20190117084739.17078-5-juri.lelli@redhat.com> <20190204121029.GD17550@hirez.programming.kicks-ass.net> <20190205095143.GG30905@localhost.localdomain> <20190205112049.GO17528@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20190205112049.GO17528@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Zijlstra 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 On 05/02/19 12:20, Peter Zijlstra wrote: > On Tue, Feb 05, 2019 at 10:51:43AM +0100, Juri Lelli wrote: > > On 04/02/19 13:10, Peter Zijlstra wrote: > > > On Thu, Jan 17, 2019 at 09:47:38AM +0100, Juri Lelli wrote: > > > > No synchronisation mechanism exists between the cpuset subsystem and calls > > > > to function __sched_setscheduler(). As such, it is possible that new root > > > > domains are created on the cpuset side while a deadline acceptance test > > > > is carried out in __sched_setscheduler(), leading to a potential oversell > > > > of CPU bandwidth. > > > > > > > > Grab callback_lock from core scheduler, so to prevent situations such as > > > > the one described above from happening. > > > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > > index f5263383170e..d928a42b8852 100644 > > > > --- a/kernel/sched/core.c > > > > +++ b/kernel/sched/core.c > > > > @@ -4224,6 +4224,13 @@ static int __sched_setscheduler(struct task_struct *p, > > > > rq = task_rq_lock(p, &rf); > > > > update_rq_clock(rq); > > > > > > > > + /* > > > > + * Make sure we don't race with the cpuset subsystem where root > > > > + * domains can be rebuilt or modified while operations like DL > > > > + * admission checks are carried out. > > > > + */ > > > > + cpuset_read_only_lock(); > > > > + > > > > /* > > > > * Changing the policy of the stop threads its a very bad idea: > > > > */ > > > > @@ -4285,6 +4292,7 @@ static int __sched_setscheduler(struct task_struct *p, > > > > /* Re-check policy now with rq lock held: */ > > > > if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) { > > > > policy = oldpolicy = -1; > > > > + cpuset_read_only_unlock(); > > > > task_rq_unlock(rq, p, &rf); > > > > goto recheck; > > > > } > > > > @@ -4342,6 +4350,7 @@ static int __sched_setscheduler(struct task_struct *p, > > > > > > > > /* Avoid rq from going away on us: */ > > > > preempt_disable(); > > > > + cpuset_read_only_unlock(); > > > > task_rq_unlock(rq, p, &rf); > > > > > > > > if (pi) > > > > @@ -4354,6 +4363,7 @@ static int __sched_setscheduler(struct task_struct *p, > > > > return 0; > > > > > > > > unlock: > > > > + cpuset_read_only_unlock(); > > > > task_rq_unlock(rq, p, &rf); > > > > return retval; > > > > } > > > > > > Why take callback_lock inside rq->lock and not the other way around? > > > AFAICT there is no pre-existing order so we can pick one here. > > > > I dediced to go for this order because if we do the other way around > > grabbing callback_lock should have to also disable irqs, no? And I > > didn't want to modify task_rq_lock; or at least this approach seemed > > less intrusive code-wide. > > Ah, but this way around we add the wait-time of callback_lock to > rq_lock, which seems undesirable because rq_lock is a far hotter lock in > general, right? Eh, indeed. OK, I'll work on it.