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From: Miroslav Benes <mbenes@suse.cz>
To: Petr Mladek <pmladek@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	gor@linux.ibm.com, jpoimboe@redhat.com, jikos@kernel.org,
	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 13:48:01 +0200 (CEST)	[thread overview]
Message-ID: <alpine.LSU.2.21.2110061323300.2311@pobox.suse.cz> (raw)
In-Reply-To: <YV16jKrB5Azu/nD+@alley>

> > That's not sufficient, you need to tag the remote task with a ct_work
> > item to also runs klp_update_patch_state(), otherwise the remote CPU can
> > enter kernel space between checking is_ct_user() and doing
> > klp_update_patch_state():
> > 
> > 	CPU0				CPU1
> > 
> > 					<user>
> > 
> > 	if (is_ct_user()) // true
> > 					<kernel-entry>
> > 					  // run some kernel code
> > 	  klp_update_patch_state()
> > 	  *WHOOPSIE*
> > 
> > 
> > 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().

That is correct. You are probably confused by the old kGraft 
implementation which added the TIF also to _TIF_WORK_SYSCALL_ENTRY so 
that syscall_trace_enter() processed it too. But upstream solution has 
always switched tasks only on their exit to user mode, because it was 
deemed sufficient.

Btw, just for fun, the old kGraft in one of its first incarnations also 
had the following horrible hack to exactly "solve" the problem.

+/*
+ * Tasks which are running in userspace after the patching has been started
+ * can immediately be marked as migrated to the new universe.
+ *
+ * If this function returns non-zero (i.e. also when error happens), the task
+ * needs to be migrated using kgraft lazy mechanism.
+ */
+static inline bool kgr_needs_lazy_migration(struct task_struct *p)
+{
+       unsigned long s[3];
+       struct stack_trace t = {
+               .nr_entries = 0,
+               .skip = 0,
+               .max_entries = 3,
+               .entries = s,
+       };
+
+       save_stack_trace_tsk(p, &t);
+
+       return t.nr_entries > 2;
+}

Miroslav

  parent reply	other threads:[~2021-10-06 11:48 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
2021-10-06 11:48         ` Miroslav Benes [this message]
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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