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    SubjectRe: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM
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    On Fri, 2011-05-27 at 21:52 +0100, Russell King - ARM Linux wrote:
    > On Fri, May 27, 2011 at 02:06:29PM +0200, Ingo Molnar wrote:
    > > The expectations are to have irqs off (we are holding the runqueue
    > > lock if !__ARCH_WANT_INTERRUPTS_ON_CTXSW), so that's not workable i
    > > suspect.
    >
    > Just a thought, but we _might_ be able to avoid a lot of this hastle if
    > we had a new arch hook in finish_task_switch(), after finish_lock_switch()
    > returns but before the old MM is dropped.

    I'd be more than willing to provide this.

    > For the new ASID-based switch_mm(), we currently do this:
    >
    > 1. check ASID validity
    > 2. flush branch predictor
    > 3. set reserved ASID value
    > 4. set new page tables
    > 5. set new ASID value
    >
    > This will be shortly changed to:
    >
    > 1. check ASID validity
    > 2. flush branch predictor
    > 3. set swapper_pg_dir tables
    > 4. set new ASID value
    > 5. set new page tables
    >
    > We could change switch_mm() to only do:
    >
    > 1. flush branch predictor
    > 2. set swapper_pg_dir tables
    > 3. check ASID validity
    > 4. set new ASID value
    >
    > At this point, we have no user mappings, and so nothing will be using the
    > ASID at this point. Then in a new post-finish_lock_switch() arch hook:
    >
    > 5. check whether we need to do flushing as a result of ASID change
    > 6. set new page tables
    >
    > I think this may simplify the ASID code. It needs prototyping out,
    > reviewing and testing, but I think it may work.
    >
    > And I think it may also be workable with the CPUs which need to flush
    > the caches on context switches - we can postpone their page table
    > switch to this new arch hook too, which will mean we wouldn't require
    > __ARCH_WANT_INTERRUPTS_ON_CTXSW on ARM at all.
    >
    > Any thoughts (if you've followed what I'm going on about) ?

    Yeah, definitely worth a try, you mentioned on IRC the problem of
    detecting if switch_mm() happened in the new arch hook. Since
    switch_mm() gets a @next pointer we can set a TIF flag there and have
    the new arch hook test for that and conditionally perform the required
    work.

    Now, supposing we can get ARM to not rely on
    __ARCH_WANT_INTERRUPTS_ON_CTXSW anymore, there's only microblaze left,
    Michal, would a similar scheme work for you? If so we can fully
    deprecate and remove this exception from the scheduler (yay!).





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