From: "Eric W. Biederman" <ebiederm@xmission.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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
Date: Mon, 04 Apr 2022 09:29:10 -0500 [thread overview]
Message-ID: <87y20kkjm1.fsf@email.froward.int.ebiederm.org> (raw)
In-Reply-To: <Ykblv2aKh3ekqpi4@linutronix.de> (Sebastian Andrzej Siewior's message of "Fri, 1 Apr 2022 13:45:03 +0200")
Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:
> 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.
Looking at do_int3_user the interrupts must be enabled.
>
>> 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.
Certainly. I case it helps this is the relevant bit of code:
static void do_int3_user(struct pt_regs *regs)
{
if (do_int3(regs))
return;
cond_local_irq_enable(regs);
do_trap(X86_TRAP_BP, SIGTRAP, "int3", regs, 0, 0, NULL);
cond_local_irq_disable(regs);
}
The signal delivery where siglock is take happens inside of do_trap. If
we are coming from kernel mode only do_int3 is called.
Coming from user_mode we switch to the task stack and
enable interrupts.
Unless I am misreading the code the cond_local_irq_{enable/disable} can
correctly be replaced local_irq_{enable/disable} as coming from user
mode interrupts are always enabled.
Unless I am misreading cond_local_irq_enable. If for some reason
cond_local_irq_enable doesn't enable interrupts when come from user
mode fixing that appears to be the fix that is needed.
Eric
next prev parent reply other threads:[~2022-04-04 14:29 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-14 20:19 [PATCH] signal/x86: Delay calling signals in atomic Sebastian Andrzej Siewior
2022-03-28 14:25 ` Eric W. Biederman
2022-03-28 14:41 ` Eric W. Biederman
2022-03-28 16:32 ` Sebastian Andrzej Siewior
2022-03-28 14:48 ` Eric W. Biederman
2022-03-28 16:17 ` Sebastian Andrzej Siewior
2022-03-28 22:07 ` Eric W. Biederman
2022-03-29 9:31 ` Sebastian Andrzej Siewior
2022-03-30 18:10 ` Eric W. Biederman
2022-04-01 11:45 ` Sebastian Andrzej Siewior
2022-04-04 14:29 ` Eric W. Biederman [this message]
2022-03-28 16:28 ` Thomas Gleixner
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