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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