Date: Sun, 16 Jan 2005 12:32:12 -0800
From: Andrew Morton <akpm@osdl.org>
To: Robert Wisniewski <bob@watson.ibm.com>
Cc: karim@opersys.com, hch@infradead.org, tglx@linutronix.de,
       linux-kernel@vger.kernel.org, bob@watson.ibm.com
Subject: Re: 2.6.11-rc1-mm1
Message-Id: <20050116123212.1b22495b.akpm@osdl.org>
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	<16874.50688.68959.36156@kix.watson.ibm.com>
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Robert Wisniewski <bob@watson.ibm.com> wrote:
>
> modify_val_spin()
>  {
>  	acquire_spin_lock()
>  	// calculate some_value based on global_val
>  	// for example c=global_val; if (c%0) some_value=10; else some_value=20;
>  	global_val = global_val + some_value
>  	release_spin_lock()
>  }
> 
>  modify_val_atomic()
>  {
>  	do
>  	// calculate some_value based on global_val
>  	// for example c=global_val; if (c%0) some_value=10; else some_value=20;
>  	global_val = global_val + some_value
>  	while (compare_and_store(global_val, , ))
>  }
> 
>  What's the difference.  The deal is if two processes execute this code
>  simultaneously and one gets interrupted in the middle of modify_val_spin,
>  then the other wastes its entire quantum spinning for the lock.  In the
>  modify_val_atomic if one process gets interrupted, no problem, the other
>  process can proceed through, then when the first one runs again the CAS
>  will fail, and it will go around the loop again.

One could use spin_lock_irq().  The performance would be similar.

> Now imagine it was the kernel involved...

Or are you saying that userspace does the above as well?
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