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

Neil Horman
2009-07-02T10:29:49Z

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 (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.

Neil

> Oleg.
>
>