linux-ext4 - spinlocks in ext4

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Message-ID: <CAK5=ZRLQErM1dOT_Tuf-SC4-3VK_o6TRg-0k8A=iQGgFY6S+aw@mail.gmail.com>
Date:	Mon, 8 Oct 2012 16:33:45 -0700
From:	Peter Fordham <peter.fordham@...il.com>
To:	linux-ext4@...r.kernel.org
Subject: spinlocks in ext4

Hi,

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.

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.

thanks,

-Pete Fordham
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