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From: Petr Mladek <pmladek@suse.com>
To: Peter Zijlstra <peterz@infradead.org>
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
Subject: Re: [RFC][PATCH 7/7] livepatch,context_tracking: Avoid disturbing NOHZ_FULL tasks
Date: Fri, 24 Sep 2021 09:33:59 +0200	[thread overview]
Message-ID: <YU1/Z/+IviZXog5w@alley> (raw)
In-Reply-To: <YUyBGJGCgrR56C7r@hirez.programming.kicks-ass.net>

On Thu 2021-09-23 15:28:56, Peter Zijlstra wrote:
> On Thu, Sep 23, 2021 at 03:14:48PM +0200, Petr Mladek wrote:
> 
> > IMHO, this is not safe:
> > 
> > CPU0				CPU1
> > 
> > klp_check_task(A)
> >   if (context_tracking_state_cpu(task_cpu(task)) == CONTEXT_USER)
> >      goto complete;
> > 
> >   clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
> > 
> > 				# task switching to kernel space
> > 				klp_update_patch_state(A)
> > 				       if (test_and_clear_tsk_thread_flag(task,	TIF_PATCH_PENDING))
> > 				       //false
> > 
> > 				# calling kernel code with old task->patch_state
> > 
> > 	task->patch_state = klp_target_state;
> > 
> > BANG: CPU0 sets task->patch_state when task A is already running
> > 	kernel code on CPU1.
> 
> Why is that a problem? That is, who actually cares about
> task->patch_state ? I was under the impression that state was purely for
> klp itself, to track which task has observed the new state.

It is the other way. The patch_state is used in klp_ftrace_handler()
to decide which code must be used (old or new).

The state must change only when the given task is _not_ using
any patched function. Hence we do it when:

     + no patched function is on the stack  (needed primary for kthreads)
     + entering/leaving kernel              (reliable way for user space)

See "Consistency model" in Documentation/livepatch/livepatch.rst
for more details.

Best Regards,
Petr

  reply	other threads:[~2021-09-24  7:34 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-22 11:05 [RFC][PATCH 0/7] sched,rcu,context_tracking,livepatch: Improve livepatch task transitions for idle and NOHZ_FULL cpus Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 1/7] sched,rcu: Rework try_invoke_on_locked_down_task() Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 2/7] sched: Fix task_try_func() Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 3/7] sched,livepatch: Use task_try_func() Peter Zijlstra
2021-09-23 12:05   ` Petr Mladek
2021-09-23 13:17     ` Peter Zijlstra
2021-09-23 13:47       ` Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 4/7] sched: Simplify wake_up_*idle*() Peter Zijlstra
2021-09-22 11:05 ` [RFC][PATCH 5/7] sched,livepatch: Use wake_up_if_idle() Peter Zijlstra
2021-09-22 13:05   ` Miroslav Benes
2021-09-23 12:19     ` Petr Mladek
2021-09-22 11:05 ` [RFC][PATCH 6/7] context_tracking: Provide SMP ordering using RCU Peter Zijlstra
2021-09-22 15:17   ` Paul E. McKenney
2021-09-22 19:33     ` Peter Zijlstra
2021-09-22 19:47       ` Peter Zijlstra
2021-09-22 19:59         ` Paul E. McKenney
2021-09-22 19:53       ` Paul E. McKenney
2021-09-22 20:02         ` Peter Zijlstra
2021-09-23 12:10   ` Petr Mladek
2021-09-24 18:57   ` Joel Savitz
2021-09-27 14:33     ` Petr Mladek
2021-09-22 11:05 ` [RFC][PATCH 7/7] livepatch,context_tracking: Avoid disturbing NOHZ_FULL tasks Peter Zijlstra
2021-09-23 13:14   ` Petr Mladek
2021-09-23 13:28     ` Peter Zijlstra
2021-09-24  7:33       ` Petr Mladek [this message]
2021-09-23 13:22 ` [RFC][PATCH 0/7] sched,rcu,context_tracking,livepatch: Improve livepatch task transitions for idle and NOHZ_FULL cpus Petr Mladek

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