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Date:   Mon, 1 Feb 2021 12:46:49 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Miroslav Benes <mbenes@suse.cz>
Cc:     Christoph Hellwig <hch@lst.de>,
        Frederic Barrat <fbarrat@linux.ibm.com>,
        Andrew Donnellan <ajd@linux.ibm.com>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>, Jessica Yu <jeyu@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>, Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        dri-devel@lists.freedesktop.org, live-patching@vger.kernel.org,
        linux-kbuild@vger.kernel.org
Subject: Re: [PATCH 04/13] module: use RCU to synchronize find_module
Message-ID: <20210201114649.GA19696@lst.de>
References: <20210128181421.2279-1-hch@lst.de> <20210128181421.2279-5-hch@lst.de> <alpine.LSU.2.21.2101291626080.22237@pobox.suse.cz>
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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.