linux-kernel - Re: [patch 2/4] tree rcu: Add debug RCU head option

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Message-ID: <20091006160948.GB15904@Krystal>
Date:	Tue, 6 Oct 2009 12:09:48 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	akpm@...ux-foundation.org, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [patch 2/4] tree rcu: Add debug RCU head option

* Eric Dumazet (eric.dumazet@...il.com) wrote:
> Mathieu Desnoyers a écrit :
> > Poisoning the rcu_head callback list. Only for rcu tree for now.
> > 
> > Helps finding racy users of call_rcu(), which results in hangs because list
> > entries are overwritten and/or skipped. Using the lower bit to poison because
> > include/net/dst.h __pad_to_align_refcnt complains when struct rcu_head grows.
> > 
> 
> I see :)
> 

I don't know if there is an easy fix for __pad_to_align_refcnt ?

> > --- linux-2.6-lttng.orig/include/linux/rcupdate.h	2009-10-06 10:35:15.000000000 -0400
> > +++ linux-2.6-lttng/include/linux/rcupdate.h	2009-10-06 10:35:18.000000000 -0400
> > @@ -45,6 +45,8 @@
> >   * struct rcu_head - callback structure for use with RCU
> >   * @next: next update requests in a list
> >   * @func: actual update function to call after the grace period.
> > + *
> > + * Debug mode assumes func pointer value is word-aligned.
> 
> Ouch, I guess you never tried CONFIG_CC_OPTIMIZE_FOR_SIZE=y ?
> 

oops. Well, simple fix would be : depends on !CC_OPTIMIZE_FOR_SIZE

It's a debug option after all... but I'd prefer growing the structure
with a supplementary field if possible.

Mathieu

> random extract from "nm -v vmlinux"
> 
> c0415e58 T in4_pton
> c0415f66 T in6_pton
> c04161f0 T in_aton
> c0416240 T net_ratelimit
> c0416250 t linkwatch_add_event
> c0416281 t linkwatch_schedule_work
> c0416301 T linkwatch_fire_event
> c0416368 t __linkwatch_run_queue
> c04164d2 t linkwatch_event
> c04164f4 T linkwatch_run_queue
> c0416500 T sk_chk_filter
> c0416703 t sk_filter_rcu_release
> c041671d T sk_detach_filter
> c041676a T sk_attach_filter
> c0416862 T sk_run_filter
> c0416f5d T sk_filter
> c0416fc4 t __flow_cache_shrink
> 
> gcc -v
> Using built-in specs.
> Target: i686-pc-linux-gnu
> Configured with: ../gcc-4.4.1/configure --enable-languages=c,c++ --prefix=/usr
> Thread model: posix
> gcc version 4.4.1 (GCC)
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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