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    Subject[tip:core/rcu] rcu: Remove leg of force_quiescent_state() switch statement
    Commit-ID:  ee47eb9f4da6f44af965d6d049e77ee8c8a4b822
    Gitweb: http://git.kernel.org/tip/ee47eb9f4da6f44af965d6d049e77ee8c8a4b822
    Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    AuthorDate: Mon, 4 Jan 2010 15:09:07 -0800
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Wed, 13 Jan 2010 09:06:04 +0100

    rcu: Remove leg of force_quiescent_state() switch statement

    The comparisons of rsp->gpnum nad rsp->completed in
    rcu_process_dyntick() and force_quiescent_state() can be
    replaced by the much more clear rcu_gp_in_progress() predicate
    function. After doing this, it becomes clear that the
    RCU_SAVE_COMPLETED leg of the force_quiescent_state() function's
    switch statement is almost completely a no-op. A small change
    to the RCU_SAVE_DYNTICK leg renders it a complete no-op, after
    which it can be removed. Doing so also eliminates the forcenow
    local variable from force_quiescent_state().

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: laijs@cn.fujitsu.com
    Cc: dipankar@in.ibm.com
    Cc: mathieu.desnoyers@polymtl.ca
    Cc: josh@joshtriplett.org
    Cc: dvhltc@us.ibm.com
    Cc: niv@us.ibm.com
    Cc: peterz@infradead.org
    Cc: rostedt@goodmis.org
    Cc: Valdis.Kletnieks@vt.edu
    Cc: dhowells@redhat.com
    LKML-Reference: <12626465501781-git-send-email->
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    ---
    kernel/rcutree.c | 22 +++++-----------------
    kernel/rcutree.h | 5 ++---
    2 files changed, 7 insertions(+), 20 deletions(-)

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index e497119..6268f37 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -1144,6 +1144,7 @@ void rcu_check_callbacks(int cpu, int user)
    /*
    * Scan the leaf rcu_node structures, processing dyntick state for any that
    * have not yet encountered a quiescent state, using the function specified.
    + * The caller must have suppressed start of new grace periods.
    */
    static void rcu_process_dyntick(struct rcu_state *rsp,
    int (*f)(struct rcu_data *))
    @@ -1157,7 +1158,7 @@ static void rcu_process_dyntick(struct rcu_state *rsp,
    rcu_for_each_leaf_node(rsp, rnp) {
    mask = 0;
    spin_lock_irqsave(&rnp->lock, flags);
    - if (rnp->completed != rsp->gpnum - 1) {
    + if (!rcu_gp_in_progress(rsp)) {
    spin_unlock_irqrestore(&rnp->lock, flags);
    return;
    }
    @@ -1171,7 +1172,7 @@ static void rcu_process_dyntick(struct rcu_state *rsp,
    if ((rnp->qsmask & bit) != 0 && f(rsp->rda[cpu]))
    mask |= bit;
    }
    - if (mask != 0 && rnp->completed == rsp->gpnum - 1) {
    + if (mask != 0 && rcu_gp_in_progress(rsp)) {

    /* rcu_report_qs_rnp() releases rnp->lock. */
    rcu_report_qs_rnp(mask, rsp, rnp, flags);
    @@ -1189,7 +1190,6 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
    {
    unsigned long flags;
    struct rcu_node *rnp = rcu_get_root(rsp);
    - u8 forcenow;

    if (!rcu_gp_in_progress(rsp))
    return; /* No grace period in progress, nothing to force. */
    @@ -1224,21 +1224,9 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
    /* Record dyntick-idle state. */
    rcu_process_dyntick(rsp, dyntick_save_progress_counter);
    spin_lock(&rnp->lock); /* irqs already disabled */
    - if (!rcu_gp_in_progress(rsp))
    - break;
    - /* fall into next case. */
    -
    - case RCU_SAVE_COMPLETED:
    -
    - /* Update state, record completion counter. */
    - forcenow = 0;
    - if (rsp->gpnum - 1 == rsp->completed) {
    - forcenow = rsp->signaled == RCU_SAVE_COMPLETED;
    + if (rcu_gp_in_progress(rsp))
    rsp->signaled = RCU_FORCE_QS;
    - }
    - if (!forcenow)
    - break;
    - /* fall into next case. */
    + break;

    case RCU_FORCE_QS:

    diff --git a/kernel/rcutree.h b/kernel/rcutree.h
    index 5348561..edb6fae 100644
    --- a/kernel/rcutree.h
    +++ b/kernel/rcutree.h
    @@ -237,12 +237,11 @@ struct rcu_data {
    #define RCU_GP_IDLE 0 /* No grace period in progress. */
    #define RCU_GP_INIT 1 /* Grace period being initialized. */
    #define RCU_SAVE_DYNTICK 2 /* Need to scan dyntick state. */
    -#define RCU_SAVE_COMPLETED 3 /* Need to save rsp->completed. */
    -#define RCU_FORCE_QS 4 /* Need to force quiescent state. */
    +#define RCU_FORCE_QS 3 /* Need to force quiescent state. */
    #ifdef CONFIG_NO_HZ
    #define RCU_SIGNAL_INIT RCU_SAVE_DYNTICK
    #else /* #ifdef CONFIG_NO_HZ */
    -#define RCU_SIGNAL_INIT RCU_SAVE_COMPLETED
    +#define RCU_SIGNAL_INIT RCU_FORCE_QS
    #endif /* #else #ifdef CONFIG_NO_HZ */

    #define RCU_JIFFIES_TILL_FORCE_QS 3 /* for rsp->jiffies_force_qs */

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