[v2,1/2] net: extend drop reasons for multiple subsystems

From: Johannes Berg <johannes.berg@intel.com>

Extend drop reasons to make them usable by subsystems
other than core by reserving the high 16 bits for a
new subsystem ID, of which 0 of course is used for the
existing reasons immediately.

To still be able to have string reasons, restructure
that code a bit to make the loopup under RCU, the only
user of this (right now) is drop_monitor.

Link: https://lore.kernel.org/netdev/00659771ed54353f92027702c5bbb84702da62ce.camel@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
 include/net/dropreason.h | 37 ++++++++++++++++++++++---
 net/core/drop_monitor.c  | 32 +++++++++++++++-------
 net/core/skbuff.c        | 58 +++++++++++++++++++++++++++++++++++++---
 3 files changed, 111 insertions(+), 16 deletions(-)

On Thu, 30 Mar 2023 23:22:26 +0200 Johannes Berg wrote:
> diff --git a/include/net/dropreason.h b/include/net/dropreason.h
> index c0a3ea806cd5..d7a134c108ad 100644
> --- a/include/net/dropreason.h
> +++ b/include/net/dropreason.h
> @@ -339,10 +339,28 @@ enum skb_drop_reason {
>  	 */
>  	SKB_DROP_REASON_IPV6_NDISC_NS_OTHERHOST,
>  	/**
> -	 * @SKB_DROP_REASON_MAX: the maximum of drop reason, which shouldn't be
> -	 * used as a real 'reason'
> -	 */
> +	 * @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
> +	 * shouldn't be used as a real 'reason' - only for tracing code gen
> +         */
>  	SKB_DROP_REASON_MAX,
> +
> +	/** @SKB_DROP_REASON_SUBSYS_MASK: subsystem mask in drop reasons,
> +	 * see &enum skb_drop_reason_subsys
> +	 */
> +	SKB_DROP_REASON_SUBSYS_MASK = 0xffff0000,
> +};
> +
> +#define SKB_DROP_REASON_SUBSYS_SHIFT	16
> +
> +/**
> + * enum skb_drop_reason_subsys - subsystem tag for (extended) drop reasons
> + */
> +enum skb_drop_reason_subsys {
> +	/** @SKB_DROP_REASON_SUBSYS_CORE: core drop reasons defined above */
> +	SKB_DROP_REASON_SUBSYS_CORE,
> +
> +	/** @SKB_DROP_REASON_SUBSYS_NUM: number of subsystems defined */
> +	SKB_DROP_REASON_SUBSYS_NUM
>  };
>  
>  #define SKB_DR_INIT(name, reason)				\
> @@ -358,6 +376,17 @@ enum skb_drop_reason {
>  			SKB_DR_SET(name, reason);		\
>  	} while (0)
>  
> -extern const char * const drop_reasons[];
> +struct drop_reason_list {
> +	const char * const *reasons;
> +	size_t n_reasons;
> +};
> +
> +/* Note: due to dynamic registrations, access must be under RCU */
> +extern const struct drop_reason_list __rcu *
> +drop_reasons_by_subsys[SKB_DROP_REASON_SUBSYS_NUM];
> +
> +void drop_reasons_register_subsys(enum skb_drop_reason_subsys subsys,
> +				  const struct drop_reason_list *list);
> +void drop_reasons_unregister_subsys(enum skb_drop_reason_subsys subsys);

dropreason.h is included by skbuff.h because history, but I don't think
any of the new stuff must be visible in skbuff.h.

Could you make a new header, and put as much of this stuff there as
possible? Our future selves will thank us for shorter rebuild times..

>  #undef FN
>  #define FN(reason) [SKB_DROP_REASON_##reason] = #reason,
> -const char * const drop_reasons[] = {
> +static const char * const drop_reasons[] = {
>  	[SKB_CONSUMED] = "CONSUMED",
>  	DEFINE_DROP_REASON(FN, FN)
>  };
> -EXPORT_SYMBOL(drop_reasons);
> +
> +static const struct drop_reason_list drop_reasons_core = {
> +	.reasons = drop_reasons,
> +	.n_reasons = ARRAY_SIZE(drop_reasons),
> +};
> +
> +const struct drop_reason_list __rcu *
> +drop_reasons_by_subsys[SKB_DROP_REASON_SUBSYS_NUM] = {
> +	[SKB_DROP_REASON_SUBSYS_CORE] = &drop_reasons_core,
> +};
> +EXPORT_SYMBOL(drop_reasons_by_subsys);
> +
> +/**
> + * drop_reasons_register_subsys - register another drop reason subsystem
> + * @subsys: the subsystem to register, must not be the core
> + * @list: the list of drop reasons within the subsystem, must point to
> + *	a statically initialized list
> + */
> +void drop_reasons_register_subsys(enum skb_drop_reason_subsys subsys,
> +				  const struct drop_reason_list *list)
> +{
> +	if (WARN(subsys <= SKB_DROP_REASON_SUBSYS_CORE ||
> +		 subsys >= ARRAY_SIZE(drop_reasons_by_subsys),
> +		 "invalid subsystem %d\n", subsys))
> +		return;
> +
> +	/* must point to statically allocated memory, so INIT is OK */
> +	RCU_INIT_POINTER(drop_reasons_by_subsys[subsys], list);
> +}
> +EXPORT_SYMBOL_GPL(drop_reasons_register_subsys);
> +
> +/**
> + * drop_reasons_unregister_subsys - unregister a drop reason subsystem
> + * @subsys: the subsystem to remove, must not be the core
> + *
> + * Note: This will synchronize_rcu() to ensure no users when it returns.
> + */
> +void drop_reasons_unregister_subsys(enum skb_drop_reason_subsys subsys)
> +{
> +	if (WARN(subsys <= SKB_DROP_REASON_SUBSYS_CORE ||
> +		 subsys >= ARRAY_SIZE(drop_reasons_by_subsys),
> +		 "invalid subsystem %d\n", subsys))
> +		return;
> +
> +	RCU_INIT_POINTER(drop_reasons_by_subsys[subsys], NULL);
> +
> +	synchronize_rcu();
> +}
> +EXPORT_SYMBOL_GPL(drop_reasons_unregister_subsys);

Weak preference to also take the code out of skbuff.c but that's not as
important.


You To'd both wireless and netdev, who are you expecting to apply this?
:S

Hi Johannes,

kernel test robot noticed the following build warnings:

[auto build test WARNING on wireless-next/main]
[also build test WARNING on wireless/main horms-ipvs/master net/main net-next/main linus/master v6.3-rc6 next-20230413]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Johannes-Berg/mac80211-use-the-new-drop-reasons-infrastructure/20230331-052445
base:   https://git.kernel.org/pub/scm/linux/kernel/git/wireless/wireless-next.git main
patch link:    https://lore.kernel.org/r/20230330212227.928595-1-johannes%40sipsolutions.net
patch subject: [PATCH v2 1/2] net: extend drop reasons for multiple subsystems
config: m68k-randconfig-s042-20230413 (https://download.01.org/0day-ci/archive/20230414/202304141104.ixaAlfxh-lkp@intel.com/config)
compiler: m68k-linux-gcc (GCC) 12.1.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # apt-get install sparse
        # sparse version: v0.6.4-39-gce1a6720-dirty
        # https://github.com/intel-lab-lkp/linux/commit/64925179be74db98280d706236e37e05bf7b5cca
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Johannes-Berg/mac80211-use-the-new-drop-reasons-infrastructure/20230331-052445
        git checkout 64925179be74db98280d706236e37e05bf7b5cca
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=m68k olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-12.1.0 make.cross C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=m68k SHELL=/bin/bash net/core/

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
| Link: https://lore.kernel.org/oe-kbuild-all/202304141104.ixaAlfxh-lkp@intel.com/

sparse warnings: (new ones prefixed by >>)
>> net/core/skbuff.c:138:42: sparse: sparse: incorrect type in initializer (different address spaces) @@     expected struct drop_reason_list const [noderef] __rcu * @@     got struct drop_reason_list const * @@
   net/core/skbuff.c:138:42: sparse:     expected struct drop_reason_list const [noderef] __rcu *
   net/core/skbuff.c:138:42: sparse:     got struct drop_reason_list const *

vim +138 net/core/skbuff.c

   135	
   136	const struct drop_reason_list __rcu *
   137	drop_reasons_by_subsys[SKB_DROP_REASON_SUBSYS_NUM] = {
 > 138		[SKB_DROP_REASON_SUBSYS_CORE] = &drop_reasons_core,
   139	};
   140	EXPORT_SYMBOL(drop_reasons_by_subsys);
   141

On Fri, 2023-03-31 at 21:36 -0700, Jakub Kicinski wrote:
> 
> > +/* Note: due to dynamic registrations, access must be under RCU */
> > +extern const struct drop_reason_list __rcu *
> > +drop_reasons_by_subsys[SKB_DROP_REASON_SUBSYS_NUM];
> > +
> > +void drop_reasons_register_subsys(enum skb_drop_reason_subsys subsys,
> > +				  const struct drop_reason_list *list);
> > +void drop_reasons_unregister_subsys(enum skb_drop_reason_subsys subsys);
> 
> dropreason.h is included by skbuff.h because history, but I don't think
> any of the new stuff must be visible in skbuff.h.
> 
> Could you make a new header, and put as much of this stuff there as
> possible? Our future selves will thank us for shorter rebuild times..

Sure. Not sure it'll make a big difference in rebuild, but we'll see :)

I ended up moving dropreason.h to dropreason-core.h first, that way we
also have a naming scheme for non-core dropreason files should they
become visible outside of the subsystem (i.e. mac80211 just has them
internally).

Dunno, let me know if you prefer something else, I just couldn't come up
with a non-confusing longer name for the new thing.

> Weak preference to also take the code out of skbuff.c but that's not as
> important.

I guess I can create a new dropreason.c, but is that worth it? It's only
a few lines. Let me know, then I can resend.

> You To'd both wireless and netdev, who are you expecting to apply this?
> :S

Good question :)

The first patch (patches in v3) really should go through net-next I
suppose, and I wouldn't mind if the other one did as well, it doesn't
right now touch anything likely to change.

johannes

On Fri, 14 Apr 2023 11:25:08 +0200 Johannes Berg wrote:
> On Fri, 2023-03-31 at 21:36 -0700, Jakub Kicinski wrote:
> >   
> > > +/* Note: due to dynamic registrations, access must be under RCU */
> > > +extern const struct drop_reason_list __rcu *
> > > +drop_reasons_by_subsys[SKB_DROP_REASON_SUBSYS_NUM];
> > > +
> > > +void drop_reasons_register_subsys(enum skb_drop_reason_subsys subsys,
> > > +				  const struct drop_reason_list *list);
> > > +void drop_reasons_unregister_subsys(enum skb_drop_reason_subsys subsys);  
> > 
> > dropreason.h is included by skbuff.h because history, but I don't think
> > any of the new stuff must be visible in skbuff.h.
> > 
> > Could you make a new header, and put as much of this stuff there as
> > possible? Our future selves will thank us for shorter rebuild times..  
> 
> Sure. Not sure it'll make a big difference in rebuild, but we'll see :)
> 
> I ended up moving dropreason.h to dropreason-core.h first, that way we
> also have a naming scheme for non-core dropreason files should they
> become visible outside of the subsystem (i.e. mac80211 just has them
> internally).
> 
> Dunno, let me know if you prefer something else, I just couldn't come up
> with a non-confusing longer name for the new thing.

Sounds good.

> > Weak preference to also take the code out of skbuff.c but that's not as
> > important.  
> 
> I guess I can create a new dropreason.c, but is that worth it? It's only
> a few lines. Let me know, then I can resend.

It's hard to tell. Most additions to the core are small at the start so
we end up chucking all of them into a handful of existing source files.
And those files grow and grow. Splitting the later is extra work and
makes backports harder.

It's a game of predicting which code will likely grow into a reasonable
~500+ LoC at some point, and which code will not. I have the feeling
that dropreason code will grow. But yes, it's still fairly small, we 
can defer.

> > You To'd both wireless and netdev, who are you expecting to apply this?
> > :S  
> 
> Good question :)
> 
> The first patch (patches in v3) really should go through net-next I
> suppose, and I wouldn't mind if the other one did as well, it doesn't
> right now touch anything likely to change.

SG!