Add tracepoint_enabled() macro

[dmitri-shubin]

Allows application to check if given probe is enabled.
It's useful when data it passes to tracepoint is expensive to compute.

Signed-off-by: Dmitri Shubin dmitri.shubin@orc-group.com

[compudj]

I'd prefer if we can introduce the following APIs:

• tracepoint_enabled()
• do_tracepoint()

The required usage would be:

if (tracepoint_enabled(provider, name)) {
prepare_tracepoint(...);
do_tracepoint(provider, name, args);
}

Along with clear documentation about the fact that the do_tracepoint() call should be within the same block as the code preparing the tracepoint arguments. do_tracepoint() would not contain any check for tracepoint_enabled().

I'm really concerned that users could introduces races if not documented properly. For instance, the following code snippet is racy:

if (tracepoint_enabled(provider, name)) {
prepare(...);
}
tracepoint(provider, name, args);

because it contains two separate "enabled" checks. We should also document this possible race condition to make sure no user fall into that trap if they use tracepoint_enabled().

Also, about the STAP_PROBEV(), it needs the parameters to be prepared, but it's meant to be activated without having to activate the tracepoint. Therefore, I would recommend to document that do_tracepoint() does not contain call to STAP_PROBEV(), and that users who care about emitting one should do it themselves.

Thanks,

Mathieu

[dmitri-shubin]

Hi Mathieu,

Thanks for suggestions!
I will rework my change.

Am I right that this code is racy

if (tracepoint_enabled(provider, name)) {
    prepare(...);
}
tracepoint(provider, name, args);

due to args maybe non-initialized if the tracepoint is enabled between check in tracepoint_enabled() and in tracepoint() ?

[compudj]

Yes, this is the race condition I'm worried about.

Thanks,

Mathieu

[dmitri-shubin]

Hi @compudj

I reworked my commit, please have a look.

Thanks!

[compudj]

Merged with a small fix: removed the extra parenthesis
within the tracepoint_enabled macro.

Thanks!

Mathieu

----- Original Message -----

Hi @compudj

I reworked my commit, please have a look.

Thanks!

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Reply to this email directly or view it on GitHub .

Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

[dmitri-shubin]

Thank you!