linux-kernel - Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space pipe readers to complete (v6)

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Message-ID: <20090702082854.GA15003@redhat.com>
Date:	Thu, 2 Jul 2009 10:29:14 +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, Roland McGrath <roland@...hat.com>
Subject: Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space
	pipe readers to complete (v6)

(add Roland)

Neil, I guess we both are tired of this thread, but I still have questions ;)

On 07/01, Neil Horman wrote:
>
> +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);
> +	}
> +
> +	pipe->readers--;
> +	pipe->writers++;
> +	pipe_unlock(pipe);
> +
> +}

OK, I think this is simple enough and should work.

This is not exactly correct wrt signals, if we get TIF_SIGPENDING this
becomes a busy-wait loop.

I'd suggest to do while (->readers && !signal_pending()), this is not
exactly right too because we have other problems with signals, but
this is another story.

>  void do_coredump(long signr, int exit_code, struct pt_regs *regs)
>  {
>  	struct core_state core_state;
> @@ -1862,6 +1886,8 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
>  		current->signal->group_exit_code |= 0x80;
>
>  close_fail:
> +	if (ispipe && core_pipe_limit)
> +		wait_for_dump_helpers(file);

Oh. I thought I misread the first version, but now I see I got it right.
And now I confused again.

So, we only wait if core_pipe_limit != 0. Why?

The previous version, v4, called wait_for_dump_helpers() unconditionally.
And this looks more right to me. Once again, even without wait_for_dump()
the coredumping process can't be reaped until core_pattern app reads all
data from the pipe.

I won't insist. However, anybody else please take a look?

core_pipe_limit != 0 limits the number of coredump-via-pipe in flight, OK.

But, should wait_for_dump_helpers() depend on core_limit_pipe != 0?

Oleg.

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