linux-kernel - Clarification re: outstanding IPC flaws

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Message-ID: <671c20540711260901w6662fcabv779ce4fc51ec3d14@mail.gmail.com>
Date:	Mon, 26 Nov 2007 12:01:22 -0500
From:	"Tom Burns" <tom.i.burns@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Clarification re: outstanding IPC flaws

Hi List,

  Reading current kernel git ipc/sem.c I see the following comment:

--
 * - The previous code had two flaws:
...
 *   2) It did not wake up all zero waiting processes. We try to do
 *      better but only get the semops right which only wait for zero or
 *      increase. If there are decrement operations in the operations
 *      array we do the same as before.

--

I have some questions about this comment.

Most importantly - does this mean that if I have multiple processes
queued inside semop() calls with sem_val = -1 (also, if it matters,
SEM_UNDO flag is set), I can expect to eventually see none of the
queued processes woken up when whoever has the semaphore releases it?

If that's the case, is there any semop best-use example I should be
following for multiple multi-threaded processes sharing multiple
semaphores?

Currently I have some globally reachable semaphores doing semop calls
with SEM_UNDO.  They usually work fine with multiple processes waiting
on each other to lock the semaphore, but under certain circumstances a
semaphore will be released and none of the processes waiting on the
semaphore will be woken up.

Thanks in advance,
Tom Burns
Software Developer
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