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Date: Mon, 21 Oct 2002 20:36:24 +0100 (BST)
From: Hugh Dickins <hugh@veritas.com>
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To: Dipankar Sarma <dipankar@in.ibm.com>
cc: mingming cao <cmm@us.ibm.com>, Andrew Morton <akpm@digeo.com>,
       <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH]IPC locks breaking down with RCU
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On Tue, 22 Oct 2002, Dipankar Sarma wrote:
> 
> I took a quick look at the original ipc code and I don't understand
> something - it seems to me the ipc_ids structs are protected by the semaphore
> inside for all operations, so why do we need the spinlock in the
> first place ? Am I missing something here ?

I made that mistake too at first, Mingming set me straight.
Many of the entry points down() the ipc_ids.sem semaphore, but the
most significant ones do not.  ipc/sem.c is probably the best example
(if confusing, since it involves quite different meanings of semaphore):
sys_semop() is the frequent, fast entry point, uses sem_lock without down.

Hugh

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