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author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2015-03-08 14:52:27 -0700 |
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committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2015-03-20 08:27:33 -0700 |
commit | a77da14ce9afb338040b405f6ab8afddc310411d (patch) | |
tree | 49c0ee20b6d871ef64f565e67c8ee2503f84d38a /kernel | |
parent | 5c60d25fa1b22fdcf141f8006d31c32b08db7311 (diff) | |
download | op-kernel-dev-a77da14ce9afb338040b405f6ab8afddc310411d.zip op-kernel-dev-a77da14ce9afb338040b405f6ab8afddc310411d.tar.gz |
rcu: Yet another fix for preemption and CPU hotplug
As noted earlier, the following sequence of events can occur when
running PREEMPT_RCU and HOTPLUG_CPU on a system with a multi-level
rcu_node combining tree:
1. A group of tasks block on CPUs corresponding to a given leaf
rcu_node structure while within RCU read-side critical sections.
2. All CPUs corrsponding to that rcu_node structure go offline.
3. The next grace period starts, but because there are still tasks
blocked, the upper-level bits corresponding to this leaf rcu_node
structure remain set.
4. All the tasks exit their RCU read-side critical sections and
remove themselves from the leaf rcu_node structure's list,
leaving it empty.
5. But because there now is code to check for this condition at
force-quiescent-state time, the upper bits are cleared and the
grace period completes.
However, there is another complication that can occur following step 4 above:
4a. The grace period starts, and the leaf rcu_node structure's
gp_tasks pointer is set to NULL because there are no tasks
blocked on this structure.
4b. One of the CPUs corresponding to the leaf rcu_node structure
comes back online.
4b. An endless stream of tasks are preempted within RCU read-side
critical sections on this CPU, such that the ->blkd_tasks
list is always non-empty.
The grace period will never end.
This commit therefore makes the force-quiescent-state processing check only
for absence of tasks blocking the current grace period rather than absence
of tasks altogether. This will cause a quiescent state to be reported if
the current leaf rcu_node structure is not blocking the current grace period
and its parent thinks that it is, regardless of how RCU managed to get
itself into this state.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: <stable@vger.kernel.org> # 4.0.x
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Diffstat (limited to 'kernel')
-rw-r--r-- | kernel/rcu/tree.c | 43 |
1 files changed, 27 insertions, 16 deletions
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 17b5abf..b3684b2 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2199,8 +2199,8 @@ static void rcu_report_unblock_qs_rnp(struct rcu_state *rsp, unsigned long mask; struct rcu_node *rnp_p; - WARN_ON_ONCE(rsp == &rcu_bh_state || rsp == &rcu_sched_state); - if (rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) { + if (rcu_state_p == &rcu_sched_state || rsp != rcu_state_p || + rnp->qsmask != 0 || rcu_preempt_blocked_readers_cgp(rnp)) { raw_spin_unlock_irqrestore(&rnp->lock, flags); return; /* Still need more quiescent states! */ } @@ -2208,9 +2208,8 @@ static void rcu_report_unblock_qs_rnp(struct rcu_state *rsp, rnp_p = rnp->parent; if (rnp_p == NULL) { /* - * Either there is only one rcu_node in the tree, - * or tasks were kicked up to root rcu_node due to - * CPUs going offline. + * Only one rcu_node structure in the tree, so don't + * try to report up to its nonexistent parent! */ rcu_report_qs_rsp(rsp, flags); return; @@ -2713,8 +2712,29 @@ static void force_qs_rnp(struct rcu_state *rsp, return; } if (rnp->qsmask == 0) { - rcu_initiate_boost(rnp, flags); /* releases rnp->lock */ - continue; + if (rcu_state_p == &rcu_sched_state || + rsp != rcu_state_p || + rcu_preempt_blocked_readers_cgp(rnp)) { + /* + * No point in scanning bits because they + * are all zero. But we might need to + * priority-boost blocked readers. + */ + rcu_initiate_boost(rnp, flags); + /* rcu_initiate_boost() releases rnp->lock */ + continue; + } + if (rnp->parent && + (rnp->parent->qsmask & rnp->grpmask)) { + /* + * Race between grace-period + * initialization and task exiting RCU + * read-side critical section: Report. + */ + rcu_report_unblock_qs_rnp(rsp, rnp, flags); + /* rcu_report_unblock_qs_rnp() rlses ->lock */ + continue; + } } cpu = rnp->grplo; bit = 1; @@ -2729,15 +2749,6 @@ static void force_qs_rnp(struct rcu_state *rsp, if (mask != 0) { /* Idle/offline CPUs, report. */ rcu_report_qs_rnp(mask, rsp, rnp, flags); - } else if (rnp->parent && - list_empty(&rnp->blkd_tasks) && - !rnp->qsmask && - (rnp->parent->qsmask & rnp->grpmask)) { - /* - * Race between grace-period initialization and task - * existing RCU read-side critical section, report. - */ - rcu_report_unblock_qs_rnp(rsp, rnp, flags); } else { /* Nothing to do here, so just drop the lock. */ raw_spin_unlock_irqrestore(&rnp->lock, flags); |