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    SubjectRe: [PATCH 1/4] rcu: Don't check irq nesting from rcu idle entry/exit
    On Mon, Nov 28, 2011 at 10:24:44PM +0100, Frederic Weisbecker wrote:
    > rcu_idle_enter() and rcu_idle_exit() don't need to check
    > the irq nesting as they are not called from irq.
    >
    > Only check dyntick_nesting from rcu_irq_enter() and
    > rcu_irq_exit().
    >
    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > Cc: Josh Triplett <josh@joshtriplett.org>
    > ---
    > kernel/rcutree.c | 18 ++++++++----------
    > 1 files changed, 8 insertions(+), 10 deletions(-)

    I have queued this. I had to update it to deal with a recent conflicting
    update, please see below for the new version.

    Thanx, Paul

    ------------------------------------------------------------------------

    rcu: Don't check irq nesting from rcu idle entry/exit

    Because tasks do not nest, rcu_idle_enter() and rcu_idle_exit() do
    not need to check for nesting. This commit therefore moves nesting
    checks from rcu_idle_enter_common() to rcu_irq_exit() and from
    rcu_idle_exit_common() to rcu_irq_enter().

    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    Cc: Josh Triplett <josh@joshtriplett.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index bf085d7..54d9b4f 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -350,10 +350,6 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
    */
    static void rcu_idle_enter_common(struct rcu_dynticks *rdtp, long long oldval)
    {
    - if (rdtp->dynticks_nesting) {
    - trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
    - return;
    - }
    trace_rcu_dyntick("Start", oldval, rdtp->dynticks_nesting);
    if (!is_idle_task(current)) {
    struct task_struct *idle = idle_task(smp_processor_id());
    @@ -426,7 +422,10 @@ void rcu_irq_exit(void)
    oldval = rdtp->dynticks_nesting;
    rdtp->dynticks_nesting--;
    WARN_ON_ONCE(rdtp->dynticks_nesting < 0);
    - rcu_idle_enter_common(rdtp, oldval);
    + if (rdtp->dynticks_nesting)
    + trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
    + else
    + rcu_idle_enter_common(rdtp, oldval);
    local_irq_restore(flags);
    }

    @@ -439,10 +438,6 @@ void rcu_irq_exit(void)
    */
    static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval)
    {
    - if (oldval) {
    - trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
    - return;
    - }
    smp_mb__before_atomic_inc(); /* Force ordering w/previous sojourn. */
    atomic_inc(&rdtp->dynticks);
    /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
    @@ -519,6 +514,10 @@ void rcu_irq_enter(void)
    rdtp->dynticks_nesting++;
    WARN_ON_ONCE(rdtp->dynticks_nesting == 0);
    rcu_idle_exit_common(rdtp, oldval);
    + if (oldval)
    + trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
    + else
    + rcu_idle_exit_common(rdtp, oldval);
    local_irq_restore(flags);
    }



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