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Date: Tue, 22 Oct 2002 00:48:06 +0530
From: Dipankar Sarma <dipankar@in.ibm.com>
To: mingming cao <cmm@us.ibm.com>
Cc: Hugh Dickins <hugh@veritas.com>, Andrew Morton <akpm@digeo.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH]IPC locks breaking down with RCU
Message-ID: <20021022004806.A10573@in.ibm.com>
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In-Reply-To: <3DB44343.701B7EFD@us.ibm.com>; from cmm@us.ibm.com on Mon, Oct 21, 2002 at 11:11:15AM -0700
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On Mon, Oct 21, 2002 at 11:11:15AM -0700, mingming cao wrote:
> A simple solution I could think of for this problem, moving the per_id
> lock out of the kern_ipc_perm structure, and put it in the ipc_id
> structure. Actually I did this way at the first time,  then I agreed
> with you that moving the per_id lock into there kern_ipc_perm structure
> will help reduce cacheline bouncing.  
> 
> I think that having the per_id lock stay out of the structure it
> protects will easy the job of ipc_rmid(), also will avoid the wrong
> preempt count problem caused by the additional check "if (out)" in
> ipc_unlock() as you mentioned above.

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 ?

Thanks
-- 
Dipankar Sarma  <dipankar@in.ibm.com> http://lse.sourceforge.net
Linux Technology Center, IBM Software Lab, Bangalore, India.
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