linux-kernel - Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space pipe readers to complete (v5)

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Message-ID: <20090701160610.GA32012@redhat.com>
Date:	Wed, 1 Jul 2009 18:06:10 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk,
	andi@...stfloor.org, akpm@...ux-foundation.org,
	earl_chew@...lent.com
Subject: Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space
	pipe readers to complete (v5)

Neil, the changelog is not right,

On 07/01, Neil Horman wrote:
>
> We basically add ourselves as an additional
> reader (to prevent cleanup of the pipe), write the dump in ->core_dump(),

This is not what we do. We do not and must not inc readers before
->core_dump(). We only do this in wait_for_dump_helpers().

> note that we add ourselves as a reader before writing the
> file.  this closes the race in the window between the time we write the dump and
> the time we start checking for the user space process to be done with the pipe.

again, this doesn't match the patch.

> +static void wait_for_dump_helpers(struct file *file)
> +{
> +	struct pipe_inode_info *pipe;
> + 
> +	pipe = file->f_path.dentry->d_inode->i_pipe;
> + 
> +	pipe_lock(pipe);
> +	pipe->readers++;
> +	pipe->writers--;
> +	while (pipe->readers > 1) {
> +		wake_up_interruptible_sync(&pipe->wait);
> +		kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
> +		pipe_wait(pipe);
> +	}
> + 
> +	/*
> +	 * This reclaims the additional readers count we took in
> +	 * do_coredump
> +	 */

now the comment is wrong.

Can't understand why do you change readers/writers unconditianally,
we shouldn't have the false wakeups since ->writers == 0, but OK.

Oleg.

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