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Groups > linux.kernel > #1397151
| From | Miroslav Benes <mbenes@suse.cz> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model |
| Date | 2016-05-09 17:50 +0200 |
| Message-ID | <rwVBw-8t-3@gated-at.bofh.it> (permalink) |
| References | <rt12N-1W5-3@gated-at.bofh.it> <rt12O-1W5-9@gated-at.bofh.it> <rv4fV-3Mb-29@gated-at.bofh.it> <rv5OF-5wR-1@gated-at.bofh.it> <rv8Mz-8iq-31@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
On Wed, 4 May 2016, Josh Poimboeuf wrote:
> On Wed, May 04, 2016 at 04:12:05PM +0200, Petr Mladek wrote:
> > On Wed 2016-05-04 14:39:40, Petr Mladek wrote:
> > > *
> > > * Note that the task must never be migrated to the target
> > > * state when being inside this ftrace handler.
> > > */
> > >
> > > We might want to move the second paragraph on top of the function.
> > > It is a basic and important fact. It actually explains why the first
> > > read barrier is not needed when the patch is being disabled.
> >
> > I wrote the statement partly intuitively. I think that it is really
> > somehow important. And I am slightly in doubts if we are on the safe side.
> >
> > First, why is it important that the task->patch_state is not switched
> > when being inside the ftrace handler?
> >
> > If we are inside the handler, we are kind-of inside the called
> > function. And the basic idea of this consistency model is that
> > we must not switch a task when it is inside a patched function.
> > This is normally decided by the stack.
> >
> > The handler is a bit special because it is called right before the
> > function. If it was the only patched function on the stack, it would
> > not matter if we choose the new or old code. Both decisions would
> > be safe for the moment.
> >
> > The fun starts when the function calls another patched function.
> > The other patched function must be called consistently with
> > the first one. If the first function was from the patch,
> > the other must be from the patch as well and vice versa.
> >
> > This is why we must not switch task->patch_state dangerously
> > when being inside the ftrace handler.
> >
> > Now I am not sure if this condition is fulfilled. The ftrace handler
> > is called as the very first instruction of the function. Does not
> > it break the stack validity? Could we sleep inside the ftrace
> > handler? Will the patched function be detected on the stack?
> >
> > Or is my brain already too far in the fantasy world?
>
> I think this isn't a possibility.
>
> In today's code base, this can't happen because task patch states are
> only switched when sleeping or when exiting the kernel. The ftrace
> handler doesn't sleep directly.
>
> If it were preempted, it couldn't be switched there either because we
> consider preempted stacks to be unreliable.
And IIRC ftrace handlers cannot sleep and are called with preemption
disabled as of now. The code is a bit obscure, but see
__ftrace_ops_list_func for example. This is "main" ftrace handler that
calls all the registered ones in case FTRACE_OPS_FL_DYNAMIC is set (which
is always true for handlers coming from modules) and CONFIG_PREEMPT is
on. If it is off and there is only one handler registered for a function
dynamic trampoline is used. See commit 12cce594fa8f ("ftrace/x86: Allow
!CONFIG_PREEMPT dynamic ops to use allocated trampolines"). I think
Steven had a plan to implement dynamic trampolines even for
CONFIG_PREEMPT case but he still hasn't done it. It should use RCU_TASKS
infrastructure.
The reason for all the mess is that ftrace needs to be sure that no task
is in the handler when the handler/trampoline is freed.
So we should be safe for now even from this side.
Miroslav
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[RFC PATCH v2 00/18] livepatch: hybrid consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
[RFC PATCH v2 14/18] livepatch: remove unnecessary object loaded check Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
[RFC PATCH v2 10/18] livepatch/powerpc: add TIF_PATCH_PENDING thread flag Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
Re: [RFC PATCH v2 10/18] livepatch/powerpc: add TIF_PATCH_PENDING thread flag Petr Mladek <pmladek@suse.com> - 2016-05-03 11:10 +0200
Re: [RFC PATCH v2 10/18] livepatch/powerpc: add TIF_PATCH_PENDING thread flag Miroslav Benes <mbenes@suse.cz> - 2016-05-03 14:10 +0200
[RFC PATCH v2 16/18] livepatch: store function sizes Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
[RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-29 20:10 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-29 22:20 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 22:30 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-29 22:40 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 23:30 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-29 23:40 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Jiri Kosina <jikos@kernel.org> - 2016-04-30 00:20 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-30 01:00 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-30 02:20 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-30 00:50 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-04-30 02:10 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-02 16:00 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-05-02 18:00 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-02 19:40 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-05-02 20:20 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Ingo Molnar <mingo@kernel.org> - 2016-05-02 20:40 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-02 21:50 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Jiri Kosina <jikos@kernel.org> - 2016-05-02 22:00 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Jiri Kosina <jikos@kernel.org> - 2016-05-02 22:10 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Andy Lutomirski <luto@amacapital.net> - 2016-05-03 02:50 +0200
RE: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking David Laight <David.Laight@ACULAB.COM> - 2016-05-04 17:20 +0200
Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 22:20 +0200
[RFC PATCH v2 09/18] livepatch/x86: add TIF_PATCH_PENDING thread flag Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
Re: [RFC PATCH v2 09/18] livepatch/x86: add TIF_PATCH_PENDING thread flag Andy Lutomirski <luto@amacapital.net> - 2016-04-29 20:10 +0200
Re: [RFC PATCH v2 09/18] livepatch/x86: add TIF_PATCH_PENDING thread flag Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 22:20 +0200
[RFC PATCH v2 02/18] x86/asm/head: use a common function for starting CPUs Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
[RFC PATCH v2 13/18] livepatch: separate enabled and patched states Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
Re: [RFC PATCH v2 13/18] livepatch: separate enabled and patched states Petr Mladek <pmladek@suse.com> - 2016-05-03 11:40 +0200
Re: [RFC PATCH v2 13/18] livepatch: separate enabled and patched states Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-03 15:50 +0200
[RFC PATCH v2 06/18] x86: dump_trace() error handling Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 22:50 +0200
Re: [RFC PATCH v2 06/18] x86: dump_trace() error handling Minfei Huang <mnghuan@gmail.com> - 2016-04-29 15:50 +0200
Re: [RFC PATCH v2 06/18] x86: dump_trace() error handling Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 16:10 +0200
[RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 23:00 +0200
Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Brian Gerst <brgerst@gmail.com> - 2016-04-29 20:50 +0200
Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 22:30 +0200
Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Andy Lutomirski <luto@kernel.org> - 2016-04-29 21:40 +0200
Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-29 23:00 +0200
Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Andy Lutomirski <luto@amacapital.net> - 2016-04-29 23:40 +0200
Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-30 01:30 +0200
Re: [RFC PATCH v2 03/18] x86/asm/head: standardize the bottom of the stack for idle tasks Andy Lutomirski <luto@amacapital.net> - 2016-04-30 02:20 +0200
[RFC PATCH v2 04/18] x86: move _stext marker before head code Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 23:00 +0200
[RFC PATCH v2 01/18] x86/asm/head: clean up initial stack variable Josh Poimboeuf <jpoimboe@redhat.com> - 2016-04-28 23:00 +0200
Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-04 10:50 +0200
Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 18:00 +0200
Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Miroslav Benes <mbenes@suse.cz> - 2016-05-05 11:50 +0200
Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-05 15:10 +0200
barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-04 14:40 +0200
Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Peter Zijlstra <peterz@infradead.org> - 2016-05-04 16:00 +0200
Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 19:00 +0200
Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-04 16:20 +0200
Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 19:30 +0200
Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-05 13:30 +0200
Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Miroslav Benes <mbenes@suse.cz> - 2016-05-09 17:50 +0200
Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 19:10 +0200
Re: barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-05 12:30 +0200
klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-04 16:50 +0200
Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Jiri Kosina <jikos@kernel.org> - 2016-05-04 17:00 +0200
Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-04 20:00 +0200
Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-05 14:00 +0200
Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-06 14:40 +0200
Re: klp_task_patch: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-09 14:30 +0200
Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Petr Mladek <pmladek@suse.com> - 2016-05-06 13:40 +0200
Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Josh Poimboeuf <jpoimboe@redhat.com> - 2016-05-06 14:50 +0200
Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Miroslav Benes <mbenes@suse.cz> - 2016-05-09 11:50 +0200
Re: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model Miroslav Benes <mbenes@suse.cz> - 2016-05-10 13:50 +0200
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