linux-kernel - Re: [PATCH v2] rt: x86: enable preemption in IST exception for x86-32

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Date:	Tue, 22 Dec 2015 10:12:42 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	tglx@...utronix.de, rostedt@...dmis.org,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH v2] rt: x86: enable preemption in IST exception for x86-32

On 12/22/2015 4:06 AM, Sebastian Andrzej Siewior wrote:
> * Yang Shi | 2015-12-14 15:06:44 [-0800]:
>
>> Mainline kernel commit 959274753857efe9c5f1ba35fe727f51e9aa128d
>> ("x86, traps: Track entry into and exit from IST context"), introduced
>> ist_enter which disables preemption uncondiontionally for both x86-64 and
>> x86-32. However, x86-32 does not have an IST and the stack still belongs to
>> the current task and there is no problem in scheduling out the task.
>
> no no. So from a quick look I *assumed* you merged your v1 and revert of the
> Steven's patch into one piece. But now I see that you don't disable preemption
> 64bit which means you revert upstream change.

No, I neither merged my v1 patch nor reverted Steven's patch.

Directed by Thomas's suggestion in v1 patch review, I looked into the 
code further.

Currently, the code does:

ist_enter disables preemption unconditionally for both x86-64 and x86-32 
(by mainline commit). So, for the x86-64 part, it is fine since the 
preemption should be disabled because ist exception will use per CPU 
stack and signal delay send is necessary.

But, disabling preemption on x86-32 is unnecessary since it doesn't have 
ist stacks, so the v2 patch re-enables preemption for x86-32.

upstream commit 959274753857efe9c5f1ba35fe727f51e9aa128d ("x86, traps: 
Track entry into and exit from IST context") commit log has more details 
for the ist stacks.

So, #1) v1 patch is not needed anymore since x86-32 still doesn't need 
signal delay send in v2, #2) don't need revert Steven's patch, the logic 
is just changed slightly (#ifdef CONFIG_X86_64 --> #if 
!defined(CONFIG_X86_64)) and adopted the mainline APIs.

So, this is definitely a new approach for fixing the same problem and 
*not* an incremental patch.

>
> Here is what happens:
> - I drop your v2
> - I merge your v1 with updated patch description
> - I revert "x86: Do not disable preemption in int3 on 32bit". If someone
>    wants to skip the delayed signal on 32bit please address this upstream
>    first (that is skip the preempt_disable() on 32bit if it is not
>    required there).
> - Yang Shi, please send a changelong if you send incremental patches.

Sorry for any inconvenience. I should made the comment clearer at the 
first place even though it is not an incremental patch.

Thanks,
Yang

>
> Sebastian
>

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