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    SubjectRe: linux-next ppc64: RCU mods cause __might_sleep BUGs
    On Wed, May 02, 2012 at 03:54:24PM -0700, Hugh Dickins wrote:
    > On Wed, May 2, 2012 at 2:54 PM, Paul E. McKenney
    > <> wrote:
    > > On Thu, May 03, 2012 at 07:20:15AM +1000, Benjamin Herrenschmidt wrote:
    > >> On Wed, 2012-05-02 at 13:25 -0700, Hugh Dickins wrote:
    > >> > Got it at last.  Embarrassingly obvious.  __rcu_read_lock() and
    > >> > __rcu_read_unlock() are not safe to be using __this_cpu operations,
    > >> > the cpu may change in between the rmw's read and write: they should
    > >> > be using this_cpu operations (or, I put preempt_disable/enable in the
    > >> > __rcu_read_unlock below).  __this_cpus there work out fine on x86,
    > >> > which was given good instructions to use; but not so well on PowerPC.
    > >> >
    > >> > I've been running successfully for an hour now with the patch below;
    > >> > but I expect you'll want to consider the tradeoffs, and may choose a
    > >> > different solution.
    > >>
    > >> Didn't Linus recently rant about these __this_cpu vs this_cpu nonsense ?
    > >>
    > >> I thought that was going out..
    > >
    > > Linus did rant about __raw_get_cpu_var() because it cannot use the x86
    > > %fs segement overrides a bit more than a month ago.  The __this_cpu
    > > stuff is useful if you have preemption disabled -- avoids the extra
    > > layer of preempt_disable().
    > >
    > > Or was this a different rant?
    > I think it ended up with Christoph removing the more egregious
    > variants, but this_cpu_that and __this_cpu_the_other remaining.

    Ah, thank you for the pointer.

    It would be nice to have the CPU transparency of x86 on other
    architectures, but from what I can see, that would require dedicating
    a register to this purpose -- and even then requires that the arch
    have indexed addressing modes. There are some other approaches, for
    example, having __this_cpu_that() be located at a special address that
    the scheduler treated as implicitly preempt_disable(). Or I suppose
    that the arch-specific trap-handling code could fake it. A little
    bit messy, but the ability to access a given CPU's per-CPU variable
    while running on that CPU does appear to have at least a couple of
    uses -- inlining RCU and also making preempt_disable() use per-CPU

    In any case, I must confess that I feel quite silly about my series
    of patches. I have reverted them aside from a couple that did useful
    optimizations, and they should show up in -next shortly.

    Thanx, Paul

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