linux-kernel - Re: [PATCH 04/13] module: use RCU to synchronize find

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Date:   Mon, 1 Feb 2021 14:16:12 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Jessica Yu <jeyu@...nel.org>
cc:     Christoph Hellwig <hch@....de>,
        Frederic Barrat <fbarrat@...ux.ibm.com>,
        Andrew Donnellan <ajd@...ux.ibm.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>, Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        dri-devel@...ts.freedesktop.org, live-patching@...r.kernel.org,
        linux-kbuild@...r.kernel.org
Subject: Re: [PATCH 04/13] module: use RCU to synchronize find_module

On Mon, 1 Feb 2021, Jessica Yu wrote:

> +++ Miroslav Benes [29/01/21 16:29 +0100]:
> >On Thu, 28 Jan 2021, Christoph Hellwig wrote:
> >
> >> Allow for a RCU-sched critical section around find_module, following
> >> the lower level find_module_all helper, and switch the two callers
> >> outside of module.c to use such a RCU-sched critical section instead
> >> of module_mutex.
> >
> >That's a nice idea.
> >
> >> @@ -57,7 +58,7 @@ static void klp_find_object_module(struct klp_object
> >> *obj)
> >>   if (!klp_is_module(obj))
> >>    return;
> >>
> >> -	mutex_lock(&module_mutex);
> >> +	rcu_read_lock_sched();
> >>   /*
> >>    * We do not want to block removal of patched modules and therefore
> >>    * we do not take a reference here. The patches are removed by
> >> @@ -74,7 +75,7 @@ static void klp_find_object_module(struct klp_object
> >> *obj)
> >>   if (mod && mod->klp_alive)
> >
> >RCU always baffles me a bit, so I'll ask. Don't we need
> >rcu_dereference_sched() here? "mod" comes from a RCU-protected list, so I
> >wonder.
> 
> Same here :-) I had to double check the RCU documentation. For our
> modules list case I believe the rcu list API should take care of that
> for us. Worth noting is this snippet from Documentation/RCU/whatisRCU.txt:
> 
>    rcu_dereference() is typically used indirectly, via the _rcu
>    list-manipulation primitives, such as list_for_each_entry_rcu()

Ok, thanks to both for checking and explanation.

Ack to the patch then.

Miroslav