Usage of DTRACE_PROBE macro from sdt.h in kernel modules

[Shrikanth Kamath]

On Wed, Sep 3, 2014 at 4:49 PM, Mark Johnston <markj at freebsd.org> wrote:
> On Wed, Sep 03, 2014 at 02:09:48PM -0700, Shrikanth Kamath wrote:
>> Thanks  Mark/Andriy for helping with this query. I checked the define
>> KDTRACE_HOOKS define that is set. Rather I tried this, I defined a
>> fake provider just to check if that prompts sdt_kld_load to create the
>> SDT probes from my kernel module.
>>
>> + SDT_PROVIDER_DEFINE(fake);
>> And it does help load the SDT probes I created, even though I am not
>> using the fake provider I defined. I feel that sdt_kld_load is flawed
>> when it is looking for sdt_providers_set in the module. It expects at
>> least a provider define and cannot use one of the defined ones in
>> kernel by just declaring it.
>>
>>          if (linker_file_lookup_set(lf, "sdt_providers_set", &begin,
>> &end, NULL))
>>              return;
>
> You're completely right; this issue was fixed in
> http://svnweb.freebsd.org/base?view=revision&revision=267706
>
>>
>> I intended to use DTRACE_PROBE() instead of the conventional
>> SDT_PROBE_DEFINE/SDT_PROBE usage because I wanted to create probes
>> which have probe names based on __LINE__ macro...disclaimer...this was
>> just for experiment sakes...
>>
>> E.g
>> func_foo()
>> {
>>     ....
>>     if ()
>>         return EINVAL;
>>     ...
>>     if ()
>>         return EINVAL;
>>     ...
>>     if ()
>>         return EINVAL;
>> }
>>
>> which I replaced with
>> func_foo()
>> {
>>     ...
>>     if ()
>>         RETSDT(func_foo, EINVAL);
>>     ...
>>     if ()
>>         RETSDT(func_foo, EINVAL);
>>     ...
>>     if ()
>>         RETSDT(func_foo, EINVAL);
>> }
>> where RETSDT macro and other macros are defined as
>>
>> #define PROBENAME1(func, __LINE__) func ## __LINE__
>> #define PROBENAME(func, line) PROBENAME1(func, line)
>>
>> #define RETSDT(func, error_code)       \
>>     do {                                                    \
>>         DTRACE_PROBE(PROBENAME(func, __LINE__));\
>>         return (error_code);               \
>>     } while (0)
>>
>> With the fake provider I defined I get to see and execute my SDT probes
>> % dtrace -l | grep func_foo
>> 56455        sdt          netstack                 func_foo1592
>>
>> Here netstack is my module, and I have a probe name based on __LINE__
>> which is 1592.
>
> Why not just use a single probe and make the line number and function
> name arguments to the probe? That is, write something like
>
> #define RETSDT(error_code) do { \
>         DTRACE_PROBE2(error__return, __func__, __LINE__)
>         return (error_code);
> } while (0)
>
Mark, for some reason using a single probe does not seem to fire the probe.

--
Shrikanth R K