> On Nov 7, 2019, at 7:11 PM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Thu, Nov 7, 2019 at 5:10 PM Song Liu <liu.song.a23@xxxxxxxxx> wrote:
>>>>>>>>> + goto out;
>>>>>>>>> + tr->selector++;
>>>>>>>>
>>>>>>>> Shall we do selector-- for unlink?
>>>>>>>
>>>>>>> It's a bit flip. I think it would be more confusing with --
>>>>>>
>>>>>> Right.. Maybe should use int instead of u64 for selector?
>>>>>
>>>>> No, since int can overflow.
>>>>
>>>> I guess it is OK to overflow, no?
>>>
>>> overflow is not ok, since transition 0->1 should use nop->call patching
>>> whereas 1->2, 2->3 should use call->call.
>>>
>>> In my initial implementation (one I didn't share with anyone) I had
>>> trampoline_mutex taken inside bpf_trampoline_update(). And multiple link()
>>> operation were allowed. The idea was to attach multiple progs and update
>>> trampoline once. But then I realized that I cannot do that since 'unlink +
>>> update' where only 'update' is taking lock will not guarantee success. Since
>>> other 'link' operations can race and 'update' can potentially fail in
>>> arch_prepare_bpf_trampoline() due to new things that 'link' brought in. In that
>>> version (since there several fentry/fexit progs can come in at once) I used
>>> separate 'selector' ticker to pick the side of the page. Once I realized the
>>> issue (to guarantee that unlink+update == always success) I moved mutex all the
>>> way to unlink and link and left 'selector' as-is. Just now I realized that
>>> 'selector' can be removed. fentry_cnt + fexit_cnt can be used instead. This
>>> sum of counters will change 1 bit at a time. Am I right?
>>
>> Yeah, I think fentry_cnt + fexit_cnt is cleaner.
>
> ... and that didn't work.
> It's transition that matters. Either need to remember previous sum value
> or have separate selector. imo selector is cleaner, so I'm back to that.
Hmm.. is this because of the error handling path?
+ tr->progs_cnt[kind]++;
+ err = bpf_trampoline_update(prog);
+ if (err) {
+ hlist_del(&prog->aux->tramp_hlist);
+ tr->progs_cnt[kind]--;
+ }
Thanks,
Song
