linux-kernel - Re: frequent softlockups with 3.10rc6.

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Date:	Mon, 24 Jun 2013 13:04:36 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dave Jones <davej@...hat.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrey Vagin <avagin@...nvz.org>
Subject: Re: frequent softlockups with 3.10rc6.

On Mon, 2013-06-24 at 12:51 -0400, Dave Jones wrote:
> On Mon, Jun 24, 2013 at 12:24:39PM -0400, Steven Rostedt wrote:
> 
>  > > Ah, this is the first victim of my new 'check sanity of nodes during list walks' patch.
>  > > It's doing the same prev->next next->prev checking as list_add and friends.
>  > > I'm looking at getting it into shape for a 3.12 merge after some other preparatory patches
>  > > go into 3.11
>  > 
>  > OK, and you may need to make an exception for the ring buffer. To do a
>  > lockless swap out of the reader page for one of the pages in the buffer,
>  > it uses the 2 LSB as flags. Notice the "next=ffff880243288001", that "1"
>  > is a flag that states the next page is the "header" page (next to be
>  > read). We use cmpxchg to update the pages to handle races between the
>  > reader and writer.
> 
> I just had a plumber come visit to replace my toilet.
> I think even he would say "dude, gross" about that hack.
> 

Wow, that hack made you so sick you needed to replace your toilet?

Note, the idea of using the 2 LSB bits of pointers came from -rt. Where
we do the same with the rt_mutex owner.

-- Steve


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