linux-kernel - Re: [PATCH 2/2] ARC: show_regs: fix lockdep splat for good

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Message-ID: <20181220130450.GB17350@dhcp22.suse.cz>
Date:   Thu, 20 Dec 2018 14:04:50 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vineet Gupta <vineet.gupta1@...opsys.com>
Cc:     linux-snps-arc@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] ARC: show_regs: fix lockdep splat for good

On Tue 18-12-18 10:53:59, Vineet Gupta wrote:
> signal handling core calls ARCH show_regs() with preemption disabled
> which causes __might_sleep functions such as mmput leading to lockdep
> splat.  Workaround by re-enabling preemption temporarily.
> 
> This may not be as bad as it sounds since the preemption disabling
> itself was introduced for a supressing smp_processor_id() warning in x86
> code by commit 3a9f84d354ce ("signals, debug: fix BUG: using
> smp_processor_id() in preemptible code in print_fatal_signal()")

The commit you are referring to here sounds dubious in itself. We do not
want to stick a preempt_disable just to silence a warning. show_regs is
called from preemptible context at several places (e.g. __warn). Maybe
this was not the case in 2009 when the change was introduced but this
seems like a relict from the past. So can we fix the actual problem
rather than build on top of it instead?

Or maybe I am just missing something here.
-- 
Michal Hocko
SUSE Labs