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    Subject[tip:core/urgent] rcu: Need to update rnp->gpnum if preemptable RCU is to be reliable
    Commit-ID:  08ce258a8c263a8b87c652a5661e37b0e7238454
    Gitweb: http://git.kernel.org/tip/08ce258a8c263a8b87c652a5661e37b0e7238454
    Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    AuthorDate: Tue, 8 Sep 2009 15:54:36 -0700
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Sat, 12 Sep 2009 09:47:06 +0200

    rcu: Need to update rnp->gpnum if preemptable RCU is to be reliable

    Without this patch, tasks preempted in RCU read-side critical
    sections can fail to block the grace period, given that
    rnp->gpnum is used to determine which rnp->blocked_tasks[]
    element the preempted task is enqueued on.

    Before the patch, rnp->gpnum is always zero, so preempted tasks
    are always enqueued on rnp->blocked_tasks[0], which is correct
    only when the current CPU has not checked into the current
    grace period and the grace-period number is even, or,
    similarly, if the current CPU -has- checked into the current
    grace period and the grace-period number is odd.

    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Acked-by: Steven Rostedt <rostedt@goodmis.org>
    Cc: laijs@cn.fujitsu.com
    Cc: dipankar@in.ibm.com
    Cc: akpm@linux-foundation.org
    Cc: mathieu.desnoyers@polymtl.ca
    Cc: josht@linux.vnet.ibm.com
    Cc: dvhltc@us.ibm.com
    Cc: niv@us.ibm.com
    Cc: peterz@infradead.org
    LKML-Reference: <12524504771622-git-send-email->
    Signed-off-by: Ingo Molnar <mingo@elte.hu>


    ---
    kernel/rcutree.c | 6 +++++-
    1 files changed, 5 insertions(+), 1 deletions(-)

    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index 6b11b07..c634a92 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -632,6 +632,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
    /* Special-case the common single-level case. */
    if (NUM_RCU_NODES == 1) {
    rnp->qsmask = rnp->qsmaskinit;
    + rnp->gpnum = rsp->gpnum;
    rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
    spin_unlock_irqrestore(&rnp->lock, flags);
    return;
    @@ -657,8 +658,10 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
    */

    rnp_end = rsp->level[NUM_RCU_LVLS - 1];
    - for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++)
    + for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++) {
    rnp_cur->qsmask = rnp_cur->qsmaskinit;
    + rnp->gpnum = rsp->gpnum;
    + }

    /*
    * Now set up the leaf nodes. Here we must be careful. First,
    @@ -679,6 +682,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
    for (; rnp_cur < rnp_end; rnp_cur++) {
    spin_lock(&rnp_cur->lock); /* irqs already disabled. */
    rnp_cur->qsmask = rnp_cur->qsmaskinit;
    + rnp->gpnum = rsp->gpnum;
    spin_unlock(&rnp_cur->lock); /* irqs already disabled. */
    }


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