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From "Ian Holsman" <li...@holsman.net>
Subject Re: [PATCH] apr_queue race condition
Date Fri, 14 Feb 2003 04:40:30 GMT

Thanks for the patch, 
and thanks for using APR!

Regards
Ian

On Sun, 09 Feb 2003 13:28:13 -0800, Jacob Lewallen wrote:

>    Hi there, I've stumbled on a bug in the apr_queue_t code... When
> calling apr_queue_pop to retrieve an item from the queue the call may
> block indefinately despite items having been pushed in. Same things goes
> for calls to apr_queue_push that are blocking until there is room in the
> queue (they may stay blocked even though items have been popped from the
> queue). The problem lies in the logic that manages the two conditions that
> help operate the queue - NOT_EMPTY, and NOT_FULL. In apr_queue_push, the
> NOT_EMPTY condition is only signaled if the queue was empty prior to
> adding the new item. This can cause problems if there are multiple threads
> blocked in calls to apr_queue_pop and then two or more successive calls to
> apr_queue_push are handled prior to one of the apr_queue_pop awakening...
> for example:
> 
> apr_queue_pop1 [BLOCKED] (nelts = 0)
> apr_queue_pop2 [BLOCKED] (nelts = 0)
> apr_queue_pop3 [BLOCKED] (nelts = 0)
> apr_queue_push1 (nelts = 1) (nelts was 0, so signals) apr_queue_push2
> (nelts = 2)
> apr_queue_pop1 [UNBLOCKS] (nelts = 1) (awake from push1's signal)
> apr_queue_push3 (nelts = 2)
> apr_queue_push4 (nelts = 3)
> 
> pop2, pop3 as well as any others remained blocked.
> 
> Similar logic is used to handle blocked calls to apr_queue_push. My
> solution was to add two variables to apr_queue_t to track the number of
> threads waiting for a non-full and non-empty state, and choosing to signal
> the conditions based on those variables. So far this has worked fine for
> me. testqueue doesn't seem to catch this because of the way it's written
> (timing delays betwee calls make this harder to catch)
> 
> I've attached a patch. I appreciate any comments, it being my first patch
> and all.
> 
> ---
> jacob lewallen
> jlewalle@cs.ucr.edu
> Index: apr_queue.c
>


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