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    SubjectRe: [PATCH RFC tip/core/rcu 0/28] Preview of RCU changes for 3.3
    On Wed, Nov 02, 2011 at 01:30:17PM -0700, Paul E. McKenney wrote:
    > This patchset permits idle tasks to use RCU read-side critical sections,
    > although they are still prohibited between tick_nohz_idle_exit_norcu()
    > and tick_nohz_idle_exit_norcu(); makes synchronize_sched_expedited()
    > better able to share work among concurrent callers, allows ftrace_dump()
    > to be invoked from modules, dumps tracing upon detection of an rcutorture
    > failure, detects illegal use of RCU read-side critical sections from
    > extended quiescent states, legitimizes the pre-existin use of RCU in the
    > idle notifiers, fixes a memory-barrier botch, introduces an SRCU-like bulk
    > reference count, improve dynticks entry/exit tracing, further improves
    > RCU's ability to allow a given CPU to enter dyntick-idle mode quickly,
    > fixes idle-task checks, updates documentation, and additional fixes
    > from a still-ongoing top-to-bottom inspection of RCU. The patches are
    > as follows:

    I've reviewed all of these patches. For all of them except those
    indicated below:
    Reviewed-by: Josh Triplett <josh@joshtriplett.org>

    > 1. Strengthen memory barriers used in PowerPC value-returning
    > atomics and locking primitives. It is likely that this
    > commit will be superseded by something from the powerpc
    > maintainers. The need for this strengthening was validated
    > by tooling from Peter Sewell's group at the University of
    > Cambridge.

    As before, I don't have the background on powerpc to provide review on
    this one. However, I trust Peter Sewell's group to have gotten the
    details right. :)

    > 5. Document the troubleshooting of lockdep lock-class leaks.

    I replied with a few comments and a typo fix.

    > 11. Remove a useless self-awaken when setting up expedited grace
    > periods, courtesy of Thomas Gleixner and the -rt effort.

    Replied with a fix: commit needs splitting.

    > 12-17. Make lockdep-RCU warn when RCU read-side primitives are
    > invoked from an idle RCU extended quiescent state, mostly
    > courtesy of Frederic Weisbecker.

    Replied to 17 with a minor nit, but Reviewed-by still applies to all
    five with or without that nit fixed.

    > 18-23. Separate out the scheduler-clock tick's idea of dyntick
    > idle from RCU's notion of an idle extended quiescent state, mostly
    > courtesy of Frederic Weisbecker. These commits are needed for
    > Frederic's work to suppress the scheduler-clock tick when there
    > is but one runnable task on a given CPU.

    Very much looking forward to that work. Any pointer to more information
    on tickless-when-one-task?

    Replied to patch 19 with some naming comments; the rest seem fine.

    > 24. Introduce a bulk reference count, which is related to SRCU,
    > but which allows a reference to be acquired in an irq handler
    > and released by the task that was interrupted.

    Replied with comments.

    > 27. Allow CPUs with pending RCU callbacks to enter dyntick-idle
    > mode. Beware this commit, as it compiled and passed rcutorture
    > on the first try, which historically has indicated the presence
    > of subtle and highly destructive bugs.

    Heh. I reviewed this one particularly carefully, then, but I didn't
    find any logic errors. I did reply with a couple of comments, though.

    > 28. Fix RCU's determination of whether or not it is running in the
    > context of an idle task.

    Replied with concerns.

    - Josh Triplett


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