Subject: Re: [PATCH net-next 02/12] net: sched: flower: refactor fl_change

From: Vlad Buslov <[email-masked]>
Date: Fri, 15 Feb 2019 16:25:52 +0000

On Fri 15 Feb 2019 at 10:47, Stefano Brivio <[email-masked]> wrote:
> On Fri, 15 Feb 2019 10:38:04 +0000
> Vlad Buslov <[email-masked]> wrote:
>
>> On Thu 14 Feb 2019 at 20:34, Stefano Brivio <[email-masked]> wrote:
>> > On Thu, 14 Feb 2019 09:47:02 +0200
>> > Vlad Buslov <[email-masked]> wrote:
>> >
>> >> As a preparation for using classifier spinlock instead of relying on
>> >> external rtnl lock, rearrange code in fl_change. The goal is to group the
>> >> code which changes classifier state in single block in order to allow
>> >> following commits in this set to protect it from parallel modification with
>> >> tp->lock. Data structures that require tp->lock protection are mask
>> >> hashtable and filters list, and classifier handle_idr.
>> >>
>> >> fl_hw_replace_filter() is a sleeping function and cannot be called while
>> >> holding a spinlock. In order to execute all sequence of changes to shared
>> >> classifier data structures atomically, call fl_hw_replace_filter() before
>> >> modifying them.
>> >>
>> >> Signed-off-by: Vlad Buslov <[email-masked]>
>> >> Acked-by: Jiri Pirko <[email-masked]>
>> >> ---
>> >>  net/sched/cls_flower.c | 85 ++++++++++++++++++++++++++------------------------
>> >>  1 file changed, 44 insertions(+), 41 deletions(-)
>> >>
>> >> diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
>> >> index 88d7af78ba7e..91596a6271f8 100644
>> >> --- a/net/sched/cls_flower.c
>> >> +++ b/net/sched/cls_flower.c
>> >> @@ -1354,90 +1354,93 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
>> >>  	if (err < 0)
>> >>  		goto errout;
>> >>
>> >> -	if (!handle) {
>> >> -		handle = 1;
>> >> -		err = idr_alloc_u32(&head->handle_idr, fnew, &handle,
>> >> -				    INT_MAX, GFP_KERNEL);
>> >> -	} else if (!fold) {
>> >> -		/* user specifies a handle and it doesn't exist */
>> >> -		err = idr_alloc_u32(&head->handle_idr, fnew, &handle,
>> >> -				    handle, GFP_KERNEL);
>> >> -	}
>> >> -	if (err)
>> >> -		goto errout;
>> >> -	fnew->handle = handle;
>> >> -
>> >>
>> >> [...]
>> >>
>> >>  	if (fold) {
>> >> +		fnew->handle = handle;
>> >
>> > I'm probably missing something, but what if fold is passed and the
>> > handle isn't specified? That can still happen, right? In that case we
>> > wouldn't be allocating the handle.
>>
>> Hi Stefano,
>>
>> Thank you for reviewing my code.
>>
>> Cls API lookups fold by handle, so this pointer can only be not NULL
>> when user specified a handle and filter with such handle exists on tp.
>
> Ah, of course. Thanks for clarifying. By the way, what tricked me here
> was this check in fl_change():
>
> 	if (fold && handle && fold->handle != handle)
> 		...
>
> which could be turned into:
>
> 	if (fold && fold->handle != handle)
> 		...
>
> at this point.

At this point I don't think this check is needed at all because fold
can't suddenly change its handle in between this check and initial
lookup in cls API. Looking at commit history, this check is present
since original commit by Jiri that implements flower classifier. Maybe
semantics of cls API was different back then?