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: <20090701181951.GB31414@hmsreliant.think-freely.org>
Date:	Wed, 1 Jul 2009 14:19:51 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Oleg Nesterov <oleg@...hat.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)

On Wed, Jul 01, 2009 at 06:06:10PM +0200, Oleg Nesterov wrote:
> 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.
> 
Gahhh!  Stupid commit message reuse.

> > +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.
> 
Removing.

> Can't understand why do you change readers/writers unconditianally,
> we shouldn't have the false wakeups since ->writers == 0, but OK.
> 
I do it for exactly the reason I said I was doing it, because We remove
ourselves as a writer (to get the EOF condition that closing would otherwise
give us), and then we add ourselves as a reader (to prevent reaping when the
real reader in userspace actually exits).  Your patch does exactly the same
thing, I just moved the increment/decrement outside of the while loop so we dont
needlessly increment and decrement those values multiple times.  Its just an
efficiency change.

Neil

> Oleg.
> 
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