linux-kernel - Re: [BUG] ptraced process waiting on syscall may return kernel internal errnos

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Date:	Fri, 15 Jun 2007 10:06:56 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Roland McGrath <roland@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
Subject: Re: [BUG] ptraced process waiting on syscall may return kernel
	internal errnos

On Thu, 2007-06-14 at 16:58 +0400, Oleg Nesterov wrote:
> 
> > Well, the only code path in which we'd want to call cancel_freezing() for kernel
> > threads is when the freezing of kernel threads.  However, this only happens if
> > one of the kernel threads declares itself as freezable and the fails to call
> > try_to_freeze(), which is a bug.
> 
> But this happens? We know a lot of reasons why try_to_freeze() can fail just
> because some kthread waits for already frozen task.

The freezer is a deadlock-o-matic by design. It's one of the major if
not the main reason why we don't use the code in kernel/power.c on
PowerBooks for suspend-to-ram. I don't like it and paulus hates it.

> Well, we can have very subtle problems because a kernel thread may run with
> TIF_SIGPENDING forever. This means in particualar that any wait_event_interruptible()
> can't succeed. I think this is worse than explicit failure (like -ERESSTART... leak),
> because it is hard to reproduce/debug.

Then just don't send signals to them...

> > Moreover, I'm not sure that it's a good idea at all to send signals to kernel
> > threads from the freezer, since in fact we only need to wake them up to make
> > them call try_to_freeze() (after we've set TIF_FREEZE for them).
> 
> Yes! I completely agree.

Sounds like a plan...

Freezer problem 1/102893264 solved.

Ben.


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