Question about sparse arrays/atomic operations in systemtap

[Alan Conway]

I have a question about systemtap. I have a script (attached) for
measuring mutex contention (not the futex trick, I am trying to measure
directly from pthread_mutex_* calls). Aside from the merits of the
script, it has a problem which seems like it would be quite common, I'm
wondering if there is a well known solution.

To summarize: I am keeping count of the number of threads that are
attempting to lock a mutex at any given time but there is a race:

probe process("/lib64/libpthread.so.0").function("pthread_mutex_lock") {
    if (++mutex_locks[$mutex] > 1) // Contended
       ...
probe process("/lib64/libpthread.so.0").function("pthread_mutex_unlock")
{
    // TODO: There is a race here:
    // - this thread does --mutex_locks[$mutex] and sees 0
    // - another thread hits the lock probe and does
++mutex_locks[$mutex]
    // - this thread does delete mutex_locks[$mutex] and nukes a non-0
value
    // Consequence: we could miss contentions. However we can't drop
    // the delete or we overrun the mutex_locks array.
    if (--mutex_locks[$mutex] <= 0)
      delete mutex_locks[$mutex];

I think what I want is an atomic (decrement and delete if 0) operation
on an array.

Another way to solve it (which might have more general use) would be to
have a new array type: a 'sparse array' which automatically deletes
values when they are set to the array default (0, "", empty statistic
etc) Such an array type would be very useful for arrays that are
potentially huge (e.g. an entry for every mutex in a process) but are
mostly 0.

Is there already a solution to this problem in systemtap, or would the
sparse array be a useful addition?

Cheers,
Alan.


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