[lttng-dev] Test if tracepoint is enabled

[David Bryant]

On 05/12/12 00:32, Mathieu Desnoyers wrote:
> Hrm. So what you would like is something like:
>
> int tracepoint_get_state(provider, name);
>
> So you could use it for your setup and cleanup. However, let's see the
> possible use of this construct:
>
> 1 - this construct would be OK:
>
> if (tracepoint_get_state(myprovider, myname)) {
>          setup();
>          tracepoint(myprovider, myname, args...);
>          cleanup();
> }
>
> but we see that the state test would happen twice, which is somewhat
> inefficient.
>
> But the actual problem comes from this construct, which is _not_ OK, and
> racy:
>
> 2 -
>
> if (tracepoint_get_state(myprovider, myname))
>          setup();
> tracepoint(myprovider, myname, args...);
> if (tracepoint_get_state(myprovider, myname))
>          cleanup();
>
> The problem is that users would expect that some synchronization
> magically happen when enabling/disabling tracepoints, but it's not the
> case. So we could very well end up executing just the cleanup, or just
> the tracepoint, or 2 of the 3, if we hit the tracepoint while it is
> being enabled/disabled.
>
> Providing this level of deep access into the tracepoint structures seems
> error-prone (as shown by construct #2), and inefficient (as shown by the
> 2 tests performed by construct #1).
>
> This is why I am tempted to introduce a tracepoint_cond() or something
> similar, to ensure that no racy construct can be created, and ensure
> that we only test the tracepoint state once per site.
>
> Thoughts ?
This construct avoids the inefficiency of checking tracepoint state twice:

if (tracepoint_get_state(myprovider, myname) {
     setup();
     tracepoint_force(myprovider, myname, args...);
     cleanup();
}

ie, tracepoint_force() doesn't re-check the state. However, it relies on 
the user adhering to the pattern. tracepoint_force() could also be 
misused, violating the state of enabled/disabled tracepoints.

I take your point about the user being easily misled. It's not obvious 
enough that tracepoints are enabled/disabled asynchronously and we 
should only provide safe constructs that are compatible with that model.

So, building on your example:

#define tracepoint_cond(provider, name, pre, post, ...) \
     do { \
         if (caa_unlikely(__tracepoint_##provider##___##name.state)) { \
             pre \
             __tracepoint_cb_##provider##___##name(__VA_ARGS__); \
             post \
         } \
     } \
     while (0)

And a contrived example:

#include <stdio.h>
#include <stdlib.h>

int main() {
     char * a;
     tracepoint_cond(provider, name,
                     { printf("pre\n");  a = malloc(16); },
                     { printf("post\n"); free(a);        }
                    );
     return 0;
}

This is ok. But more work with the macro handling would be required to 
get the pre/post code fragments through as parameters. The example only 
compiles because its pre/post arguments don't contain any commas.

What do you think?

Thanks,
Dave

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