I'll assume the lock_retrieve() and lock_nstore() both obtain a lock and then operate on the cachefile.

First off, you have some problematic races. There is one between -e $lockfile and open (LOCK, ">$lockfile"); where multiple script instances can fall through to the non-cached path (and then sequentially update the cache file).

Secondly, I think there's a race between close LOCK and unlink $lockfile. After the close, another process can obtain the lockfile, only for this to be promptly deleted, allowing a third process to grab the lock as well...

Thirdly, the locking will needlessly hinder cached reads when it happens.

Now about the fix. 1) Open the cache-file. 2) Stat the open file. 3) If age is less than 2*thresh, use the cached content. 4) If age is more than 1*thresh, trigger $update_needed. 5) When update is needed, launch the updater, or, if access delays are acceptable, perform the locked-update right there. 6) The updater can LOCK_EX|LOCK_NB the cache file, write a new tmpfile and rename that. Coded this way, the cached fast path needs no locking at all. HtH.

Edit. added LOCK_NB above, probably the simplest way to ensure a single updater is run. flock+fork should be acceptable for running the update.

In reply to Re: Faster locking by oiskuu
in thread Faster locking by bbs2web

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