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From: Peter Zijlstra <peterz@infradead.org>
To: Petr Mladek <pmladek@suse.com>
Cc: gor@linux.ibm.com, jpoimboe@redhat.com, jikos@kernel.org,
	mbenes@suse.cz, mingo@kernel.org, linux-kernel@vger.kernel.org,
	joe.lawrence@redhat.com, fweisbec@gmail.com, tglx@linutronix.de,
	hca@linux.ibm.com, svens@linux.ibm.com, sumanthk@linux.ibm.com,
	live-patching@vger.kernel.org, paulmck@kernel.org,
	rostedt@goodmis.org, x86@kernel.org
Subject: Re: [RFC][PATCH v2 09/11] context_tracking,livepatch: Dont disturb NOHZ_FULL
Date: Wed, 6 Oct 2021 13:41:39 +0200	[thread overview]
Message-ID: <20211006114139.GG174703@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <YV16jKrB5Azu/nD+@alley>

On Wed, Oct 06, 2021 at 12:29:32PM +0200, Petr Mladek wrote:
> On Wed 2021-10-06 11:04:26, Peter Zijlstra wrote:

> > So it needs to be something like:
> > 
> > 
> > 	CPU0				CPU1
> > 
> > 					<user>
> > 
> > 	if (context_tracking_set_cpu_work(task_cpu(), CT_WORK_KLP))
> > 
> > 					<kernel-entry>
> > 	  klp_update_patch_state	  klp_update_patch_state()
> > 
> > 
> > So that CPU0 and CPU1 race to complete klp_update_patch_state() *before*
> > any regular (!noinstr) code gets run.
> 
> Grr, you are right. I thought that we migrated the task when entering
> kernel even before. But it seems that we do it only when leaving
> the kernel in exit_to_user_mode_loop().

Yep... :-)

> > Which then means it needs to look something like:
> > 
> > noinstr void klp_update_patch_state(struct task_struct *task)
> > {
> > 	struct thread_info *ti = task_thread_info(task);
> > 
> > 	preempt_disable_notrace();
> > 	if (arch_test_bit(TIF_PATCH_PENDING, (unsigned long *)&ti->flags)) {
> > 		/*
> > 		 * Order loads of TIF_PATCH_PENDING vs klp_target_state.
> > 		 * See klp_init_transition().
> > 		 */
> > 		smp_rmb();
> > 		task->patch_state = __READ_ONCE(klp_target_state);
> > 		/*
> > 		 * Concurrent against self; must observe updated
> > 		 * task->patch_state if !TIF_PATCH_PENDING.
> > 		 */
> > 		smp_mb__before_atomic();
> 
> IMHO, smp_wmb() should be enough. We are here only when this
> CPU set task->patch_state right above. So that CPU running
> this code should see the correct task->patch_state.

Yes, I think smp_wmb() and smp_mb__before_atomic() are NOPS for all the
same architectures, so that might indeed be a better choice.

> The read barrier is needed only when @task is entering kernel and
> does not see TIF_PATCH_PENDING. It is handled by smp_rmb() in
> the "else" branch below.
> 
> It is possible that both CPUs see TIF_PATCH_PENDING and both
> set task->patch_state. But it should not cause any harm
> because they set the same value. Unless something really
> crazy happens with the internal CPU busses and caches.

Right, not our problem :-) Lots would be broken beyond repair in that
case.

> > 		arch_clear_bit(TIF_PATCH_PENDING, (unsigned long *)&ti->flags);
> > 	} else {
> > 		/*
> > 		 * Concurrent against self, see smp_mb__before_atomic()
> > 		 * above.
> > 		 */
> > 		smp_rmb();
> 
> Yeah, this is the counter part against the above smp_wmb().
> 
> > 	}
> > 	preempt_enable_notrace();
> > }
> 
> Now, I am scared to increase my paranoia level and search for even more
> possible races. I feel overwhelmed at the moment ;-)

:-)

Anyway, I still need to figure out how to extract this context tracking
stuff from RCU and not make a giant mess of things, so until that
time....

  reply	other threads:[~2021-10-06 11:43 UTC|newest]

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-29 15:17 [PATCH v2 00/11] sched,rcu,context_tracking,livepatch: Improve livepatch task transitions for idle and NOHZ_FULL Peter Zijlstra
2021-09-29 15:17 ` [PATCH v2 01/11] sched: Improve try_invoke_on_locked_down_task() Peter Zijlstra
2021-09-29 15:17 ` [PATCH v2 02/11] sched,rcu: Rework try_invoke_on_locked_down_task() Peter Zijlstra
2021-09-29 15:17 ` [PATCH v2 03/11] sched,livepatch: Use task_call_func() Peter Zijlstra
2021-10-05 11:40   ` Petr Mladek
2021-10-05 14:03     ` Peter Zijlstra
2021-10-06  8:59   ` Miroslav Benes
2021-09-29 15:17 ` [PATCH v2 04/11] sched: Simplify wake_up_*idle*() Peter Zijlstra
2021-10-13 14:32   ` Qian Cai
2021-10-19  3:47     ` Qian Cai
2021-10-19  8:56       ` Peter Zijlstra
2021-10-19  9:10         ` Peter Zijlstra
2021-10-19 15:32           ` Qian Cai
2021-10-19 15:50             ` Peter Zijlstra
2021-10-19 19:22               ` Qian Cai
2021-10-19 20:27                 ` Peter Zijlstra
     [not found]   ` <CGME20211022134630eucas1p2e79e2816587d182c580459d567c1f2a9@eucas1p2.samsung.com>
2021-10-22 13:46     ` Marek Szyprowski
2021-09-29 15:17 ` [PATCH v2 05/11] sched,livepatch: Use wake_up_if_idle() Peter Zijlstra
2021-10-05 12:00   ` Petr Mladek
2021-10-06  9:16   ` Miroslav Benes
2021-10-07  9:18     ` Vasily Gorbik
2021-10-07 10:02       ` Peter Zijlstra
2021-10-13 19:37   ` Arnd Bergmann
2021-10-14 10:42     ` Peter Zijlstra
2021-09-29 15:17 ` [RFC][PATCH v2 06/11] context_tracking: Prefix user_{enter,exit}*() Peter Zijlstra
2021-09-29 15:17 ` [RFC][PATCH v2 07/11] context_tracking: Add an atomic sequence/state count Peter Zijlstra
2021-09-29 15:17 ` [RFC][PATCH v2 08/11] context_tracking,rcu: Replace RCU dynticks counter with context_tracking Peter Zijlstra
2021-09-29 18:37   ` Paul E. McKenney
2021-09-29 19:09     ` Peter Zijlstra
2021-09-29 19:11     ` Peter Zijlstra
2021-09-29 19:13     ` Peter Zijlstra
2021-09-29 19:24       ` Peter Zijlstra
2021-09-29 19:45         ` Paul E. McKenney
2021-09-29 18:54   ` Peter Zijlstra
2021-09-29 15:17 ` [RFC][PATCH v2 09/11] context_tracking,livepatch: Dont disturb NOHZ_FULL Peter Zijlstra
2021-10-06  8:12   ` Petr Mladek
2021-10-06  9:04     ` Peter Zijlstra
2021-10-06 10:29       ` Petr Mladek
2021-10-06 11:41         ` Peter Zijlstra [this message]
2021-10-06 11:48         ` Miroslav Benes
2021-09-29 15:17 ` [RFC][PATCH v2 10/11] livepatch: Remove klp_synchronize_transition() Peter Zijlstra
2021-10-06 12:30   ` Petr Mladek
2021-09-29 15:17 ` [RFC][PATCH v2 11/11] context_tracking,x86: Fix text_poke_sync() vs NOHZ_FULL Peter Zijlstra
2021-10-21 18:39   ` Marcelo Tosatti
2021-10-21 18:40     ` Marcelo Tosatti
2021-10-21 19:25     ` Peter Zijlstra
2021-10-21 19:57       ` Marcelo Tosatti
2021-10-21 20:18         ` Peter Zijlstra
2021-10-26 18:19           ` Marcelo Tosatti
2021-10-26 19:38             ` Peter Zijlstra
2021-09-29 18:03 ` [PATCH v2 00/11] sched,rcu,context_tracking,livepatch: Improve livepatch task transitions for idle and NOHZ_FULL Paul E. McKenney

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