linux-kernel - Re: [PATCH v4 15/15] livepatch: allow removal of a disabled patch

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Date:   Fri, 3 Feb 2017 17:48:52 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Jessica Yu <jeyu@...hat.com>, Jiri Kosina <jikos@...nel.org>,
        Petr Mladek <pmladek@...e.com>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>, x86@...nel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        Vojtech Pavlik <vojtech@...e.com>, Jiri Slaby <jslaby@...e.cz>,
        Chris J Arges <chris.j.arges@...onical.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>
Subject: Re: [PATCH v4 15/15] livepatch: allow removal of a disabled patch

On Thu, 19 Jan 2017, Josh Poimboeuf wrote:

> From: Miroslav Benes <mbenes@...e.cz>
> 
> Currently we do not allow patch module to unload since there is no
> method to determine if a task is still running in the patched code.
> 
> The consistency model gives us the way because when the unpatching
> finishes we know that all tasks were marked as safe to call an original
> function. Thus every new call to the function calls the original code
> and at the same time no task can be somewhere in the patched code,
> because it had to leave that code to be marked as safe.
> 
> We can safely let the patch module go after that.
> 
> Completion is used for synchronization between module removal and sysfs
> infrastructure in a similar way to commit 942e443127e9 ("module: Fix
> mod->mkobj.kobj potentially freed too early").
> 
> Note that we still do not allow the removal for immediate model, that is
> no consistency model. The module refcount may increase in this case if
> somebody disables and enables the patch several times. This should not
> cause any harm.
> 
> With this change a call to try_module_get() is moved to
> __klp_enable_patch from klp_register_patch to make module reference
> counting symmetric (module_put() is in a patch disable path) and to
> allow to take a new reference to a disabled module when being enabled.
> 
> Finally, we need to be very careful about possible races between
> klp_unregister_patch(), kobject_put() functions and operations
> on the related sysfs files.
> 
> kobject_put(&patch->kobj) must be called without klp_mutex. Otherwise,
> it might be blocked by enabled_store() that needs the mutex as well.
> In addition, enabled_store() must check if the patch was not
> unregisted in the meantime.
> 
> There is no need to do the same for other kobject_put() callsites
> at the moment. Their sysfs operations neiter take the lock nor

s/neiter/neither/

Thanks,
Miroslav