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

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Date:   Mon, 1 Feb 2021 12:46:49 +0100
From:   Christoph Hellwig <hch@....de>
To:     Miroslav Benes <mbenes@...e.cz>
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>, Jessica Yu <jeyu@...nel.org>,
        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 Fri, Jan 29, 2021 at 04:29:02PM +0100, Miroslav Benes wrote:
> >  
> > -	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.

rcu_dereference* is only used for dereferencing points where that
reference itself is RCU protected, that is the lookup of mod itself down
in find_module_all in this case.