netdev - Re: rcu build on bug

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Date:   Fri, 7 Aug 2020 11:44:49 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: rcu build on bug

More clarification - this:

---
diff --git a/net/sched/cls_fw.c b/net/sched/cls_fw.c
index ec945294626a..06510c8c8281 100644
--- a/net/sched/cls_fw.c
+++ b/net/sched/cls_fw.c
@@ -22,12 +22,12 @@
  #include <net/pkt_cls.h>
  #include <net/sch_generic.h>

-#define HTSIZE 256
+#define HTSIZE 4096

  struct fw_head {
+       struct rcu_head         rcu;
         u32                     mask;
         struct fw_filter __rcu  *ht[HTSIZE];
-       struct rcu_head         rcu;
  };
----

Does not fix it..


cheers,
jamal

On 2020-08-07 11:16 a.m., Jamal Hadi Salim wrote:
> 
> Made this small change:
> 
> ------
> diff --git a/net/sched/cls_fw.c b/net/sched/cls_fw.c
> index ec945294626a..75d43ed10cd8 100644
> --- a/net/sched/cls_fw.c
> +++ b/net/sched/cls_fw.c
> @@ -22,7 +22,7 @@
>   #include <net/pkt_cls.h>
>   #include <net/sch_generic.h>
> 
> -#define HTSIZE 256
> +#define HTSIZE 4096
> 
>   struct fw_head {
>          u32                     mask;
> ---------
> 
> Generated compile errors as follows:
> 
> ------------
>    DESCEND  objtool
>    CALL    scripts/atomic/check-atomics.sh
>    CALL    scripts/checksyscalls.sh
>    CHK     include/generated/compile.h
>    CC      net/sched/cls_fw.o
> In file included from ./include/linux/export.h:43:0,
>                   from ./include/linux/linkage.h:7,
>                   from ./include/linux/kernel.h:8,
>                   from ./include/linux/list.h:9,
>                   from ./include/linux/module.h:12,
>                   from net/sched/cls_fw.c:13:
> net/sched/cls_fw.c: In function ‘fw_destroy’:
> ./include/linux/compiler.h:392:38: error: call to 
> ‘__compiletime_assert_415’ declared with attribute error: BUILD_BUG_ON 
> failed: !__is_kfree_rcu_offset(__builtin_offsetof(typeof(*(head)), rcu))
>    _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>                                        ^
> ./include/linux/compiler.h:373:4: note: in definition of macro 
> ‘__compiletime_assert’
>      prefix ## suffix();    \
>      ^~~~~~
> ./include/linux/compiler.h:392:2: note: in expansion of macro 
> ‘_compiletime_assert’
>    _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
>    ^~~~~~~~~~~~~~~~~~~
> ./include/linux/build_bug.h:39:37: note: in expansion of macro 
> ‘compiletime_assert’
>   #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
>                                       ^~~~~~~~~~~~~~~~~~
> ./include/linux/build_bug.h:50:2: note: in expansion of macro 
> ‘BUILD_BUG_ON_MSG’
>    BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
>    ^~~~~~~~~~~~~~~~
> ./include/linux/rcupdate.h:840:3: note: in expansion of macro 
> ‘BUILD_BUG_ON’
>     BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); \
>     ^~~~~~~~~~~~
> ./include/linux/rcupdate.h:875:3: note: in expansion of macro ‘__kfree_rcu’
>     __kfree_rcu(&((___p)->rhf), offsetof(typeof(*(ptr)), rhf)); \
>     ^~~~~~~~~~~
> net/sched/cls_fw.c:151:2: note: in expansion of macro ‘kfree_rcu’
>    kfree_rcu(head, rcu);
>    ^~~~~~~~~
> scripts/Makefile.build:280: recipe for target 'net/sched/cls_fw.o' failed
> make[2]: *** [net/sched/cls_fw.o] Error 1
> scripts/Makefile.build:497: recipe for target 'net/sched' failed
> make[1]: *** [net/sched] Error 2
> Makefile:1771: recipe for target 'net' failed
> make: *** [net] Error 2
> make: *** Waiting for unfinished jobs....
> 
> ----------------
> 
> Gets fixed if i reduce the hash buckets of course.
> Looking at include/linux/rcupdate.h I see:
> 
> ------
> /*
>   * Does the specified offset indicate that the corresponding rcu_head
>   * structure can be handled by kfree_rcu()?
>   */
> #define __is_kfree_rcu_offset(offset) ((offset) < 4096)
> 
> ------
> 
> I am guessing the hash table got too large.
> Smells like hard coded expectation?
> 
> How to fix?
> 
> cheers,
> jamal