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    Subject[PATCH v2 tip/core/rcu 2/6] rcu: Abstract the dynticks snapshot operation
    Date
    This commit is the second step towards full abstraction of all accesses to
    the ->dynticks counter, implementing the previously open-coded atomic
    add of zero in a new rcu_dynticks_snap() function. This abstraction will
    ease changes o the ->dynticks counter operation.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Reviewed-by: Josh Triplett <josh@joshtriplett.org>
    ---
    kernel/rcu/tree.c | 19 ++++++++++++++++---
    kernel/rcu/tree_exp.h | 6 ++----
    2 files changed, 18 insertions(+), 7 deletions(-)

    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index 14e283c351f6..805d55ee0b2a 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -282,6 +282,17 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
    };

    /*
    + * Snapshot the ->dynticks counter with full ordering so as to allow
    + * stable comparison of this counter with past and future snapshots.
    + */
    +static int rcu_dynticks_snap(struct rcu_dynticks *rdtp)
    +{
    + int snap = atomic_add_return(0, &rdtp->dynticks);
    +
    + return snap;
    +}
    +
    +/*
    * Do a double-increment of the ->dynticks counter to emulate a
    * momentary idle-CPU quiescent state.
    */
    @@ -1049,7 +1060,9 @@ void rcu_nmi_exit(void)
    */
    bool notrace __rcu_is_watching(void)
    {
    - return atomic_read(this_cpu_ptr(&rcu_dynticks.dynticks)) & 0x1;
    + struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
    +
    + return atomic_read(&rdtp->dynticks) & 0x1;
    }

    /**
    @@ -1132,7 +1145,7 @@ static int rcu_is_cpu_rrupt_from_idle(void)
    static int dyntick_save_progress_counter(struct rcu_data *rdp,
    bool *isidle, unsigned long *maxj)
    {
    - rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
    + rdp->dynticks_snap = rcu_dynticks_snap(rdp->dynticks);
    rcu_sysidle_check_cpu(rdp, isidle, maxj);
    if ((rdp->dynticks_snap & 0x1) == 0) {
    trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
    @@ -1157,7 +1170,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
    int *rcrmp;
    unsigned int snap;

    - curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
    + curr = (unsigned int)rcu_dynticks_snap(rdp->dynticks);
    snap = (unsigned int)rdp->dynticks_snap;

    /*
    diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
    index e59e1849b89a..011f626b2fd8 100644
    --- a/kernel/rcu/tree_exp.h
    +++ b/kernel/rcu/tree_exp.h
    @@ -356,10 +356,9 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
    mask_ofl_test = 0;
    for_each_leaf_node_possible_cpu(rnp, cpu) {
    struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
    - struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);

    rdp->exp_dynticks_snap =
    - atomic_add_return(0, &rdtp->dynticks);
    + rcu_dynticks_snap(rdp->dynticks);
    if (raw_smp_processor_id() == cpu ||
    !(rdp->exp_dynticks_snap & 0x1) ||
    !(rnp->qsmaskinitnext & rdp->grpmask))
    @@ -380,12 +379,11 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
    for_each_leaf_node_possible_cpu(rnp, cpu) {
    unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
    struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
    - struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);

    if (!(mask_ofl_ipi & mask))
    continue;
    retry_ipi:
    - if (atomic_add_return(0, &rdtp->dynticks) !=
    + if (rcu_dynticks_snap(rdp->dynticks) !=
    rdp->exp_dynticks_snap) {
    mask_ofl_test |= mask;
    continue;
    --
    2.5.2
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