linux-kernel - Q: semaphore and kmalloc(GFP

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Message-Id: <200905110230.n4B2UUXk082125@www262.sakura.ne.jp>
Date:	Mon, 11 May 2009 11:30:30 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:	linux-kernel@...r.kernel.org
Subject: 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?

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

Regards.
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