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Date: Fri, 24 Sep 2021 09:33:59 +0200 From: Petr Mladek <pmladek@...e.com> To: Peter Zijlstra <peterz@...radead.org> Cc: gor@...ux.ibm.com, jpoimboe@...hat.com, jikos@...nel.org, mbenes@...e.cz, mingo@...nel.org, linux-kernel@...r.kernel.org, joe.lawrence@...hat.com, fweisbec@...il.com, tglx@...utronix.de, hca@...ux.ibm.com, svens@...ux.ibm.com, sumanthk@...ux.ibm.com, live-patching@...r.kernel.org, paulmck@...nel.org Subject: Re: [RFC][PATCH 7/7] livepatch,context_tracking: Avoid disturbing NOHZ_FULL tasks 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
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