linux-ext4 - Re: spinlocks in ext4

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Message-ID: <20121009005425.GB20682@thunk.org>
Date:	Mon, 8 Oct 2012 20:54:25 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Peter Fordham <peter.fordham@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: spinlocks in ext4

On Mon, Oct 08, 2012 at 04:33:45PM -0700, Peter Fordham wrote:
> 
> Can someone give me a quick outline of why spinlocks are required in
> the EXT4 code? Don't all file-system requests originate from user
> context, hence meaning all locking be done with mutexes or semaphores.

Mutexes are incredibly expensive in the contended case, since you
basically have to take a trip through the scheduler.  If the other CPU
is only going to be holding the lock for a few dozen cycles, a
spinlock is far preferable to a mutex.

> I'm doing some profiling on an ARM device it's showing up spin unlock
> taking a lot of time and I'd like to migrate to using mutex's instead
> since they don't incur penalties from synchronization instructions
> like DMB.  I'm guessing there's some underlying reason why this isn't
> safe and I'd like to understand it.

Why in the world does ARM have expensive spinlocks?  ARM64 is *doomed*
if this is a fundamental property of the ARM processor design...

   	     		 	     	 - Ted
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