Re: associative array synchronization question

[Josh Stone <jistone at redhat dot com>]

On 07/20/2012 12:47 PM, Nicholas Murphy wrote:
> Sorry for the very basic question:

No need to apologize.

> it says in the documentation that any probe using an associative
> array (which is necessarily global) will automatically hold a lock on
> that array for the duration of the probe...is that true regardless of
> whether the probe actually touches the array?  Or does it only lock
> on first access?

When we take locks, we use the trylock functions (with a little spinning
and waiting), so we won't ever block forever.  That means sometimes we
can fail to take a lock.

We also chose to make probe handlers atomic, such that if a probe
handler runs at all, it must run in full.  We don't want to get into a
situation where a lock in the middle of a probe can't be obtained, thus
ruining that probe's execution flow.

Therefore, all locks needed for a handler are attempted before anything
is started in that handler, and held until the handler is done.  If any
of the locks can't be obtained, the probe is skipped entirely.

So for your case, where you may be conditionally writing an array, we
will be grabbing the write lock regardless of the condition (which we
don't know at that time).

If your data values can fit stats types (various numeric accumulation),
and you'll be writing values more often than you read, then a stats
array may be a better locking choice, because it only needs a fully
exclusive lock when reading instead.

Hope that helps, and feel free to ask followup questions.

Josh