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

Neil Horman
2009-07-01T18:20:12Z

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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