Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait helpers

From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Turner <pjt@google.com>, NeilBrown <nfbrown@novell.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Mike Galbraith <efault@gmx.de>, Ingo Molnar <mingo@kernel.org>,
	Peter Anvin <hpa@zytor.com>,
	vladimir.murzin@arm.com, linux-tip-commits@vger.kernel.org,
	jstancek@redhat.com
Subject: Re: [tip:locking/core] sched/wait: Fix signal handling in bit wait helpers
Date: Tue, 15 Dec 2015 19:16:29 +0100	[thread overview]
Message-ID: <20151215181629.GA19007@redhat.com> (raw)
In-Reply-To: <20151211113959.GI6356@twins.programming.kicks-ass.net>

On 12/11, Peter Zijlstra wrote:
>
> On Fri, Dec 11, 2015 at 03:30:33AM -0800, Paul Turner wrote:
>
> > > Blergh, all I've managed to far is to confuse myself further. Even
> > > something like the original (+- the EINTR) should work when we consider
> > > the looping, even when mixed with an occasional spurious wakeup.
> > >
> > >
> > > int bit_wait()
> > > {
> > >         if (signal_pending_state(current->state, current))
> > >                 return -EINTR;
> > >         schedule();
> > > }
>
> So I asked Vladimir to test that (simply changing the return from 1 to
> -EINTR) and it made his fail much less likely but it still failed in the
> same way.
>
> So I'm fairly sure I'm still missing something :/

Same here...

Yes, "return 1" in bit_wait_io() doesn't look right. For example
do_generic_file_read() can wrongly return if lock_page_killable() returns
this error code. But I fail to understand how this can read to rcu-stall.

Oleg.