Re: [PATCH v3] Allow user probes on versioned symbols

[Masami Hiramatsu <mhiramat@kernel.org>]

On Mon, 3 Apr 2017 11:46:58 -0300
Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> > > But apart from those problems, I think that one should be able to ask
> > > for a versioned symbol, to probe just apps using that specific version,
>  
> > I agree, but wasn't trying to tackle that at the moment.  I can look into it, though.
>  
> > > for instance, we should consider the whole name as two functions, which
> > > in fact, they are, no?
>  
> > I'm not sure I understand what you mean here.  Do you mean we should set a probe at every version of a given symbol name?  For example, if there are symbols:
> > a@@V2
> > a@V1.1
> > a@V1
>  
> > ...for a request to set a probe at "a", we'd actually set a probe at all 3?
> 
> I think that we should just probe the default for that symbol and have a
> way to probe all of them, perhaps using the wildcard, i.e.:
> 
> [root@jouet linux]# nm /lib64/libpthread-2.24.so  | grep ' pthread_cond_timedwait'
> 000000000000dd90 T pthread_cond_timedwait@GLIBC_2.2.5
> 000000000000d6e0 T pthread_cond_timedwait@@GLIBC_2.3.2
> [root@jouet linux]#
> 
>   # perf probe -x /lib64/libpthread-2.24.so pthread_cond_timedwait
> 
> should be equivalent to:
> 
>   # perf probe -x /lib64/libpthread-2.24.so pthread_cond_timedwait@@GLIBC_2.3.2
> 
> Which matches how these versioned symbols are resolved by the linker,
> no?
> 
> I.e. when 'pthread_cond_timedwait' is specified and the symbol table
> lookup fails, I think we should re-lookup for
> 'pthread_cond_timedwait@@*', i.e. we should have a
> symbol__find_default_by_name(), which will take the
> "pthread_cond_timedwait" and use a symbol comparison using
> strncmp(strlen(key)), matching, should then look at right after the
> common part looking for the double @@.

Hm, this 'fallback'process sounds good idea to me.
BTW, how would we support other SYMBOL@VERSION, since we already
use '@' for specifying source code?
One possible way is to support it directly in perf-probe. If it
failed to find probe point from dwarf, try to find from symbol
map by using '@VERSION' suffix.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>