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: <20090702144422.GA8972@hmsreliant.think-freely.org>
Date:	Thu, 2 Jul 2009 10:44:22 -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, Roland McGrath <roland@...hat.com>
Subject: Re: [PATCH 3/3] exec: Allow do_coredump to wait for user space
	pipe readers to complete (v6)

On Thu, Jul 02, 2009 at 01:36:10PM +0200, Oleg Nesterov wrote:
> 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.
> 
Thats true, but consider the converse situation, in which the userspace app does
read the pipe, so that we return from ->core_dump().  If the user app then
queries the /proc/<pid> directory of the crashing  process we are open to race.
Thats what this wait helps with.

> > (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.
> 
Ok, but you asked the question as to why I added that check, this is the answer.

> As for implementation, my only complaint is that wait_for_dump_helpers() lacks
> signal_pending() check, this wasn't answered.
> 
I'll have to defer to others on this.  It seems to me that, given that we are
waiting here in the context of process that has already received a fatal signal,
theres no opportunity to handle subsequent signals, so we don't really need to
check for them.  As for the user space helper, I'm not sure what detecting a
pending signal will do for us here.  I agree we busy wait if a signal is
pending, but if we drop out of the loop if a signal is pending then we cancel
the wait early, leading to the early removal of the /proc file for the crashing
process.  Could we add a schedule to the loop to allow the user space helper to
run if a signal is pending instead of just dropping the loop?

Neil

> Oleg.
> 
> 
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