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

[Oleg Nesterov <oleg@redhat.com>]

On 07/02, Neil Horman wrote:
>
> On Thu, Jul 02, 2009 at 10:29:14AM +0200, Oleg Nesterov wrote:
> > (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?
> >
> I messed this up in v4 and am fixing it here.  If you read the documentation I
> added in patch 2, you can see that my intent with the core_pipe_limit sysctl was
> to designate 0 as a special value allowing unlimited parallel core_dumps in
> which we do not wait for any user space process completion

We do wait in any case. If core_dump app doesn't read the data from the pipe
->core_dump() can't complete. OK, unless all data fits into pipe buffers.

> (so that current
> system behavior can be maintained, which I think is desireable for those user
> space helpers who don't need access to a crashing processes meta data via proc.
> If you look above in the second patch where we do an atomic_inc_return, you'll
> see that we only honor the core_pipe_limit value if its non-zero.  This addional
> check restores the behavior I documented in that patch.

If you you look at my message you will see I am not arguing, but I am asking
others to ack this behaviour.

As for implementation, my only complaint is that wait_for_dump_helpers() lacks
signal_pending() check, this wasn't answered.

Oleg.