Message-ID: <1372093476.18733.170.camel@gandalf.local.home>
Subject: Re: frequent softlockups with 3.10rc6.
From: Steven Rostedt <rostedt@goodmis.org>
To: Dave Jones <davej@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Andrey Vagin <avagin@openvz.org>
Date: Mon, 24 Jun 2013 13:04:36 -0400
In-Reply-To: <20130624165140.GB8572@redhat.com>
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	 <20130623150603.GA32313@redhat.com> <20130623160452.GA11740@redhat.com>
	 <20130624020014.GB12811@redhat.com> <20130624143928.GA20659@redhat.com>
	 <1372085549.18733.162.camel@gandalf.local.home>
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	 <20130624165140.GB8572@redhat.com>
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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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