linux-kernel - Re: [PATCH] ipc/sem: do not sleep with a spin lock held

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Date:   Wed, 22 Dec 2021 18:06:25 +0100
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Vasily Averin <vvs@...tuozzo.com>, cgel.zte@...il.com,
        akpm@...ux-foundation.org
Cc:     shakeelb@...gle.com, rdunlap@...radead.org, dbueso@...e.de,
        unixbhaskar@...il.com, chi.minghao@....com.cn, arnd@...db.de,
        linux-kernel@...r.kernel.org, Zeal Robot <zealci@....com.cn>
Subject: Re: [PATCH] ipc/sem: do not sleep with a spin lock held

Hi Vasily,

On 12/22/21 16:50, Vasily Averin wrote:
> On 22.12.2021 18:31, Vasily Averin wrote:
>> On 22.12.2021 14:45, Manfred Spraul wrote:
>>> Hi Minghao,
>>>
>>> On 12/22/21 09:10, cgel.zte@...il.com wrote:
>>>> From: Minghao Chi <chi.minghao@....com.cn>
>>>>
>>>> We can't call kvfree() with a spin lock held, so defer it.
>> I'm sorry, but I do not understand why exactly we cannot use kvfree?
>> Could you explain it in more details?
> Got it,
> there is cond_resched() called in __vfree() -> __vunmap()
>
> However I'm still not sure that in_interrupt() is used correctly here.

I see three different topics:

- is the current code violating the API? I think yes, thus there is a 
bug that needs to be fixed.

- Where is __vunmap() sleeping? Would it be possible to make __vunmap() 
safe to be called when owning a spinlock?

- should kvfree() use vfree() [i.e. unsafe when owning a spinlock] or 
vfree_atomic [i.e. a bit slower, but safe]


As we did quite many s/kfree/kvfree/ changes, perhaps just switching to 
vfree_atomic() is the best solution.

@Andrew: What would you prefer?

In addition, if we do not use vfree_atomic(): Then I would propose to 
copy the might_sleep_if() from vfree() into kvfree()

--

     Manfred