linux-kernel - Re: [PATCH] preempt: Move PREEMPT

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Message-ID: <21f75fd1-58f3-4c80-8527-55282518c802@suse.com>
Date: Tue, 10 Dec 2024 14:52:23 +0100
From: Petr Pavlu <petr.pavlu@...e.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev, Ben Segall <bsegall@...gle.com>,
 Clark Williams <clrkwllms@...nel.org>, Daniel Gomez <da.gomez@...sung.com>,
 Dietmar Eggemann <dietmar.eggemann@....com>, Ingo Molnar <mingo@...hat.com>,
 Juri Lelli <juri.lelli@...hat.com>, Luis Chamberlain <mcgrof@...nel.org>,
 Mel Gorman <mgorman@...e.de>, Peter Zijlstra <peterz@...radead.org>,
 Petr Pavlu <petr.pavlu@...e.com>, Sami Tolvanen <samitolvanen@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Valentin Schneider <vschneid@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] preempt: Move PREEMPT_RT before PREEMPT in vermagic.

On 12/5/24 17:06, Sebastian Andrzej Siewior wrote:
> Since the dynamic preemption has been enabled for PREEMPT_RT we have now
> CONFIG_PREEMPT and CONFIG_PREEMPT_RT set simultaneously. This affects
> the vermagic strings which comes now PREEMPT with PREEMPT_RT enabled.
> 
> The PREEMPT_RT module usually can not be loaded on a PREEMPT kernel
> because some symbols are missing.
> However if the symbols are fine then it continues and it crashes later.
> The problem is that the struct module has a different layout and the
> num_exentries or init members are at a different position leading to a
> crash later on. This is not necessary caught by the size check in
> elf_validity_cache_index_mod() because the mem member has an alignment
> requirement of __module_memory_align which is big enough keep the total
> size unchanged. Therefore we should keep the string accurate instead of
> removing it.
> 
> Move the PREEMPT_RT check before the PREEMPT so that it takes precedence
> if both symbols are enabled.
> 
> Fixes: 35772d627b55c ("sched: Enable PREEMPT_DYNAMIC for PREEMPT_RT")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Reviewed-by: Petr Pavlu <petr.pavlu@...e.com>

I'm going to wait for a few days if others want to comment and then plan
to take this through the modules tree for 6.13-rcX fixes.

[Luis asked me to look after the modules tree for a while, with the plan
to rotate it.]

-- 
Thanks,
Petr