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Date:   Fri, 1 Apr 2022 13:45:03 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org,
        Oleg Nesterov <oleg@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ben Segall <bsegall@google.com>,
        Borislav Petkov <bp@alien8.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Mel Gorman <mgorman@suse.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [PATCH] signal/x86: Delay calling signals in atomic
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On 2022-03-30 13:10:05 [-0500], Eric W. Biederman wrote:
> But it looks like if we are coming from userspace then we use the same
> stack as any other time we would come from userspace.  AKA a stack
> that allows the kernel to sleep.
> 
> So I don't see what the problem is that is trying to be fixed.

It is not only the stack. In atomic context / disabled interrupts it is
not possible to acquire a spinlock_t (sighand_struct::siglock) which is
done later.

> I know that code has been changed over the years, perhaps this is
> something that was fixed upstream and the real time tree didn't realize
> there was no longer a need to fix anything?
> 
> Or am I missing something subtle when reading the idtentry assembly?

It certainly is true that the code changed over the years. The per-CPU
stack is one problem, the siglock in atomic context is the other one.
Thank you for the input. Let me digest the informations I have here and
get back.

> Eric

Sebastian