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    SubjectRe: [PATCH 4/7] ufs: remove the BKL
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    On Wednesday 02 March 2011, Nick Bowler wrote:
    > On 2011-03-02 00:13 +0100, Arnd Bergmann wrote:
    > > This introduces a new per-superblock mutex in UFS to replace
    > > the big kernel lock. I have been careful to avoid nested
    > > calls to lock_ufs and to get the lock order right with
    > > respect to other mutexes, in particular lock_super.
    > >
    > > I did not make any attempt to prove that the big kernel
    > > lock is not needed in a particular place in the code,
    > > which is very possible.
    > >
    > > The code is still only compile-tested,
    >
    > This isn't true anymore; I've been running with this patch (well, the
    > previous versions thereof) for some time now. On the other hand, I
    > don't use all of this driver's features.

    I'll updated the comment. Can I add your Tested-by tag?

    > > but it should at least be harmless on non-SMP systems, since the new
    > > mutex is not taken on those.
    >
    > I think this part of the patch is strange. It seems like a gratuitous
    > difference between SMP/preempt and other systems to #if out the code
    > that takes the mutex. This might make problems with the conversion fly
    > under the radar longer because people with older systems won't encounter
    > them.

    I agree it is strange, but the mutex has some serious performance impact
    that I wanted to minimize on the systems where we know it is not needed.
    The BKL was only active on those systems, so we know that non-SMP
    non-preempt kernels don't need the mutex.

    Arnd


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