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

[Eric Dumazet <eric.dumazet@gmail.com>]

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 :)

> --- 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 ?

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)