From: "Paul E. McKenney" <paulmck@kernel.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Valentin Schneider <valentin.schneider@arm.com>,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, rcu@vger.kernel.org,
linux-rt-users@vger.kernel.org,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Frederic Weisbecker <frederic@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
Anshuman Khandual <anshuman.khandual@arm.com>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Steven Price <steven.price@arm.com>,
Ard Biesheuvel <ardb@kernel.org>,
Boqun Feng <boqun.feng@gmail.com>, Mike Galbraith <efault@gmx.de>,
Scott Wood <swood@redhat.com>
Subject: Re: [PATCH] rcutorture: Avoid problematic critical section nesting on RT
Date: Thu, 19 Aug 2021 11:20:35 -0700 [thread overview]
Message-ID: <20210819182035.GF4126399@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210819154708.3efz6jtgwtuhpeds@linutronix.de>
On Thu, Aug 19, 2021 at 05:47:08PM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-08-19 17:39:29 [+0200], To Paul E. McKenney wrote:
> > up with following which I can explain:
> >
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 40ef5417d9545..5c8b31b7eff03 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -1432,28 +1432,34 @@ static void rcutorture_one_extend(int *readstate, int newstate,
> > /* First, put new protection in place to avoid critical-section gap. */
> > if (statesnew & RCUTORTURE_RDR_BH)
> > local_bh_disable();
> > + if (statesnew & RCUTORTURE_RDR_RBH)
> > + rcu_read_lock_bh();
> > if (statesnew & RCUTORTURE_RDR_IRQ)
> > local_irq_disable();
> > if (statesnew & RCUTORTURE_RDR_PREEMPT)
> > preempt_disable();
> > - if (statesnew & RCUTORTURE_RDR_RBH)
> > - rcu_read_lock_bh();
> > if (statesnew & RCUTORTURE_RDR_SCHED)
> > rcu_read_lock_sched();
> > if (statesnew & RCUTORTURE_RDR_RCU)
> > idxnew = cur_ops->readlock() << RCUTORTURE_RDR_SHIFT;
>
> So the ordering in the enable and disable part regarding BH is
> important. First BH, then preemption or IRQ.
>
> > - /* Next, remove old protection, irq first due to bh conflict. */
> > + /*
> > + * Next, remove old protection, in decreasing order of strength
> > + * to avoid unlock paths that aren't safe in the stronger
> > + * context. Namely: BH can not be enabled with disabled interrupts.
> > + * Additionally PREEMPT_RT requires that BH is enabled in preemptible
> > + * context.
> > + */
> > if (statesold & RCUTORTURE_RDR_IRQ)
> > local_irq_enable();
> > - if (statesold & RCUTORTURE_RDR_BH)
> > - local_bh_enable();
> > if (statesold & RCUTORTURE_RDR_PREEMPT)
> > preempt_enable();
> > - if (statesold & RCUTORTURE_RDR_RBH)
> > - rcu_read_unlock_bh();
> > if (statesold & RCUTORTURE_RDR_SCHED)
> > rcu_read_unlock_sched();
> > + if (statesold & RCUTORTURE_RDR_BH)
> > + local_bh_enable();
> > + if (statesold & RCUTORTURE_RDR_RBH)
> > + rcu_read_unlock_bh();
> > if (statesold & RCUTORTURE_RDR_RCU) {
> > bool lockit = !statesnew && !(torture_random(trsp) & 0xffff);
>
> The same in the unlock part so that BH is unlocked in preemptible
> context.
> Now if you need bh lock/unlock in atomic context (either with disabled
> IRQs or preemption) then I would dig out the atomic-bh part again and
> make !RT only without the preempt_disable() section around about which
> one you did complain.
>
> > @@ -1496,6 +1502,9 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
> > int mask = rcutorture_extend_mask_max();
> > unsigned long randmask1 = torture_random(trsp) >> 8;
> > unsigned long randmask2 = randmask1 >> 3;
> > + unsigned long preempts = RCUTORTURE_RDR_PREEMPT | RCUTORTURE_RDR_SCHED;
> > + unsigned long preempts_irq = preempts | RCUTORTURE_RDR_IRQ;
> > + unsigned long bhs = RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
> >
> > WARN_ON_ONCE(mask >> RCUTORTURE_RDR_SHIFT);
> > /* Mostly only one bit (need preemption!), sometimes lots of bits. */
> > @@ -1503,11 +1512,37 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
> > mask = mask & randmask2;
> > else
> > mask = mask & (1 << (randmask2 % RCUTORTURE_RDR_NBITS));
> > - /* Can't enable bh w/irq disabled. */
> > - if ((mask & RCUTORTURE_RDR_IRQ) &&
> > - ((!(mask & RCUTORTURE_RDR_BH) && (oldmask & RCUTORTURE_RDR_BH)) ||
> > - (!(mask & RCUTORTURE_RDR_RBH) && (oldmask & RCUTORTURE_RDR_RBH))))
> > - mask |= RCUTORTURE_RDR_BH | RCUTORTURE_RDR_RBH;
> > +
> > + /*
> > + * Can't enable bh w/irq disabled.
> > + */
> > + if (mask & RCUTORTURE_RDR_IRQ)
> > + mask |= oldmask & bhs;
> > +
> > + /*
> > + * Ideally these sequences would be detected in debug builds
> > + * (regardless of RT), but until then don't stop testing
> > + * them on non-RT.
> > + */
> > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > + /*
> > + * Can't release the outermost rcu lock in an irq disabled
> > + * section without preemption also being disabled, if irqs
> > + * had ever been enabled during this RCU critical section
> > + * (could leak a special flag and delay reporting the qs).
> > + */
> > + if ((oldmask & RCUTORTURE_RDR_RCU) &&
> > + (mask & RCUTORTURE_RDR_IRQ) &&
> > + !(mask & preempts))
> > + mask |= RCUTORTURE_RDR_RCU;
>
> This piece above, I don't understand. I had it running for a while and
> it didn't explode. Let me try TREE01 for 30min without that piece.
This might be historical. There was a time when interrupts being
disabled across rcu_read_unlock() meant that preemption had to have
been disabled across the entire RCU read-side critical section.
I am not seeing a purpose for it now, but I could easily be missing
something, especially given my tenuous grasp of RT.
Either way, looking forward to the next version!
Thanx, Paul
> > + /* Can't modify bh in atomic context */
> > + if (oldmask & preempts_irq)
> > + mask &= ~bhs;
> > + if ((oldmask | mask) & preempts_irq)
> > + mask |= oldmask & bhs;
>
> And this is needed because we can't lock/unlock bh while atomic.
>
> > + }
> > +
> > return mask ?: RCUTORTURE_RDR_RCU;
> > }
> >
>
> Sebastian
next prev parent reply other threads:[~2021-08-19 18:20 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-11 20:13 [PATCH v3 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
2021-08-12 16:47 ` Paul E. McKenney
2021-08-17 12:13 ` Sebastian Andrzej Siewior
2021-08-17 13:17 ` Sebastian Andrzej Siewior
2021-08-17 14:40 ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Sebastian Andrzej Siewior
2021-08-18 22:46 ` Paul E. McKenney
2021-08-19 15:35 ` Sebastian Andrzej Siewior
2021-08-19 15:39 ` Sebastian Andrzej Siewior
2021-08-19 15:47 ` Sebastian Andrzej Siewior
2021-08-19 18:20 ` Paul E. McKenney [this message]
2021-08-19 18:45 ` Sebastian Andrzej Siewior
2021-08-20 4:11 ` Scott Wood
2021-08-20 7:11 ` Sebastian Andrzej Siewior
2021-08-20 7:42 ` [PATCH v2] rcutorture: Avoid problematic critical section nesting on PREEMPT_RT Sebastian Andrzej Siewior
2021-08-20 22:10 ` Paul E. McKenney
2021-08-20 3:23 ` [PATCH] rcutorture: Avoid problematic critical section nesting on RT Scott Wood
2021-08-20 6:54 ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 2/4] sched: Introduce migratable() Valentin Schneider
2021-08-17 14:43 ` Sebastian Andrzej Siewior
2021-08-22 17:31 ` Valentin Schneider
2021-08-17 17:09 ` Sebastian Andrzej Siewior
2021-08-17 19:30 ` Phil Auld
2021-08-22 18:14 ` Valentin Schneider
2022-01-26 16:56 ` Sebastian Andrzej Siewior
2022-01-26 18:10 ` Valentin Schneider
2022-01-27 10:07 ` Sebastian Andrzej Siewior
2022-01-27 18:23 ` Valentin Schneider
2022-01-27 19:27 ` Valentin Schneider
2022-02-04 9:24 ` Sebastian Andrzej Siewior
2021-08-11 20:13 ` [PATCH v3 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
2021-08-13 0:20 ` Paul E. McKenney
2021-08-13 18:48 ` Valentin Schneider
2021-08-17 15:36 ` Sebastian Andrzej Siewior
2021-08-22 18:15 ` Valentin Schneider
2021-09-21 14:05 ` Thomas Gleixner
2021-09-21 21:12 ` rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT Thomas Gleixner
2021-09-21 23:36 ` Frederic Weisbecker
2021-09-22 2:18 ` Paul E. McKenney
2021-09-22 11:31 ` Frederic Weisbecker
2021-09-21 23:45 ` Frederic Weisbecker
2021-09-22 6:32 ` Sebastian Andrzej Siewior
2021-09-22 11:10 ` Frederic Weisbecker
2021-09-22 11:27 ` Sebastian Andrzej Siewior
2021-09-22 11:38 ` Frederic Weisbecker
2021-09-22 13:02 ` Sebastian Andrzej Siewior
2021-09-23 10:02 ` Frederic Weisbecker
2021-09-30 9:00 ` Valentin Schneider
2021-09-30 10:53 ` Frederic Weisbecker
2021-09-30 13:22 ` Valentin Schneider
2021-08-11 20:13 ` [PATCH v3 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider
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