linux-kernel - Re: Q: semaphore and kmalloc(GFP

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Message-Id: <20090511115410.B0EA.A69D9226@jp.fujitsu.com>
Date:	Mon, 11 May 2009 11:56:02 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org
Subject: Re: Q: semaphore and kmalloc(GFP_KERNEL)?

> Hello.
> I got a question.
> 
> When we use kmalloc(GFP_KERNEL), we cannot use spinlock.
> We use semaphore instead.
> 
> What happens if a writer process got into sleep state at kmalloc()
> without releasing a rw_semaphore for writing,
> 
>     down_write(&sem);
>     list_for_each()
>         ...;
>     ptr = kmalloc(size, GFP_KERNEL);
>     list_add();
>     up_write(&sem);
> 
> and then reader processes tried to hold that rw_semaphore?
> 
>     down_read(&sem);
>     list_for_each()
>         ...;
>     up_read(&sem);
> 
> Can the kernel detect that we need to let kmalloc() fail and
> wake up the writer process so that the writer process releases
> the rw_semaphore and reader processes can continue?

it can't. Plus it shouldn't.
some caller don't want kmalloc() failure.


> (If the kernel can't detect, I worry that we experience
> all processes sleeping for undefined duration.)




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