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

From: Manfred Spraul <manfred@colorfullife.com>
To: Vasily Averin <vvs@virtuozzo.com>,
	cgel.zte@gmail.com, akpm@linux-foundation.org
Cc: shakeelb@google.com, rdunlap@infradead.org, dbueso@suse.de,
	unixbhaskar@gmail.com, chi.minghao@zte.com.cn, arnd@arndb.de,
	linux-kernel@vger.kernel.org, Zeal Robot <zealci@zte.com.cn>
Subject: Re: [PATCH] ipc/sem: do not sleep with a spin lock held
Date: Wed, 22 Dec 2021 18:06:25 +0100	[thread overview]
Message-ID: <4d786a02-1560-25ed-4603-8419672ec044@colorfullife.com> (raw)
In-Reply-To: <02171384-9d1e-19fa-5fa1-5f84b720a03e@virtuozzo.com>

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@gmail.com wrote:
>>>> From: Minghao Chi <chi.minghao@zte.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