Messages in this thread Patch in this message | | | From | Neeraj Upadhyay <> | Subject | [PATCH] rcu/tree: Add comment to describe GP done condition in fqs loop | Date | Thu, 9 Jun 2022 12:43:40 +0530 |
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Add a comment to explain why !rcu_preempt_blocked_readers_cgp() condition is required on root rnp node, for GP completion check in rcu_gp_fqs_loop().
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com> --- kernel/rcu/tree.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index a93c5f4d7e09..9cd1ba512fdc 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2083,7 +2083,17 @@ static noinline_for_stack void rcu_gp_fqs_loop(void) rcu_gp_torture_wait(); WRITE_ONCE(rcu_state.gp_state, RCU_GP_DOING_FQS); /* Locking provides needed memory barriers. */ - /* If grace period done, leave loop. */ + /* + * If grace period done, leave loop. rcu_preempt_blocked_readers_cgp(rnp) + * check is required for the case where we only have single node in the + * rcu_node tree; so, root rcu node is also the leaf node, where readers + * blocking current gp are queued. For multi-node tree, checking ->qsmask + * on the root node is sufficient, as root rcu node's ->qsmask is only + * cleared, when all leaf rcu nodes have propagated their quiescent + * state to their parent node, which happens only after both ->qsmask + * and rcu_preempt_blocked_readers_cgp(rnp_leaf) are cleared for those + * leaf nodes. + */ if (!READ_ONCE(rnp->qsmask) && !rcu_preempt_blocked_readers_cgp(rnp)) break; -- 2.17.1
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