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 <[email protected]>
---
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
On Thu, Jun 09, 2022 at 12:43:40PM +0530, Neeraj Upadhyay wrote:
> 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 <[email protected]>
Thank you, Neeraj! As usual, I could not resist the urge to wordsmith
as shown below. Could you please check to see if I messed something up?
Thanx, Paul
------------------------------------------------------------------------
commit bdf3a744d3ad21336a390bfcc2e41de63f193eaf
Author: Neeraj Upadhyay <[email protected]>
Date: Thu Jun 9 12:43:40 2022 +0530
rcu/tree: Add comment to describe GP-done condition in fqs loop
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 <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a93c5f4d7e092..9a941e7ee6109 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2083,7 +2083,15 @@ 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. */
+ /*
+ * Exit the loop if the root rcu_node structure indicates that the grace period
+ * has ended, leave the loop. The rcu_preempt_blocked_readers_cgp(rnp) check
+ * is required only for single-node rcu_node trees because readers blocking
+ * the current grace period are queued only on leaf rcu_node structures.
+ * For multi-node trees, checking the root node's ->qsmask suffices, because a
+ * given root node's ->qsmask bit is cleared only when all CPUs and tasks from
+ * the corresponding leaf node have passed through their quiescent state.
+ */
if (!READ_ONCE(rnp->qsmask) &&
!rcu_preempt_blocked_readers_cgp(rnp))
break;
On 6/10/2022 1:53 AM, Paul E. McKenney wrote:
> On Thu, Jun 09, 2022 at 12:43:40PM +0530, Neeraj Upadhyay wrote:
>> 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 <[email protected]>
>
> Thank you, Neeraj! As usual, I could not resist the urge to wordsmith
> as shown below. Could you please check to see if I messed something up?
Thanks!
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit bdf3a744d3ad21336a390bfcc2e41de63f193eaf
> Author: Neeraj Upadhyay <[email protected]>
> Date: Thu Jun 9 12:43:40 2022 +0530
>
> rcu/tree: Add comment to describe GP-done condition in fqs loop
>
> 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 <[email protected]>
> Signed-off-by: Paul E. McKenney <[email protected]>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a93c5f4d7e092..9a941e7ee6109 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2083,7 +2083,15 @@ 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. */
> + /*
> + * Exit the loop if the root rcu_node structure indicates that the grace period
> + * has ended, leave the loop. The rcu_preempt_blocked_readers_cgp(rnp) check
We can remove ", leave the loop" ?
> + * is required only for single-node rcu_node trees because readers blocking
> + * the current grace period are queued only on leaf rcu_node structures.
> + * For multi-node trees, checking the root node's ->qsmask suffices, because a
> + * given root node's ->qsmask bit is cleared only when all CPUs and tasks from
Do we need to say "a given root node's" , we have only single RCU node
in the system, so we can just say, "because root node's ->qsmask bit is
cleared..." ?
> + * the corresponding leaf node have passed through their quiescent state.
Change "the corresponding leaf node" to "their corresponding leaf nodes"
or "the corresponding leaf nodes"?
Thanks
Neeraj
> + */
> if (!READ_ONCE(rnp->qsmask) &&
> !rcu_preempt_blocked_readers_cgp(rnp))
> break;
On Fri, Jun 10, 2022 at 08:45:42AM +0530, Neeraj Upadhyay wrote:
>
>
> On 6/10/2022 1:53 AM, Paul E. McKenney wrote:
> > On Thu, Jun 09, 2022 at 12:43:40PM +0530, Neeraj Upadhyay wrote:
> > > 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 <[email protected]>
> >
> > Thank you, Neeraj! As usual, I could not resist the urge to wordsmith
> > as shown below. Could you please check to see if I messed something up?
>
> Thanks!
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit bdf3a744d3ad21336a390bfcc2e41de63f193eaf
> > Author: Neeraj Upadhyay <[email protected]>
> > Date: Thu Jun 9 12:43:40 2022 +0530
> >
> > rcu/tree: Add comment to describe GP-done condition in fqs loop
> > 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 <[email protected]>
> > Signed-off-by: Paul E. McKenney <[email protected]>
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index a93c5f4d7e092..9a941e7ee6109 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2083,7 +2083,15 @@ 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. */
> > + /*
> > + * Exit the loop if the root rcu_node structure indicates that the grace period
> > + * has ended, leave the loop. The rcu_preempt_blocked_readers_cgp(rnp) check
>
> We can remove ", leave the loop" ?
>
> > + * is required only for single-node rcu_node trees because readers blocking
> > + * the current grace period are queued only on leaf rcu_node structures.
> > + * For multi-node trees, checking the root node's ->qsmask suffices, because a
> > + * given root node's ->qsmask bit is cleared only when all CPUs and tasks from
>
> Do we need to say "a given root node's" , we have only single RCU node in
> the system, so we can just say, "because root node's ->qsmask bit is
> cleared..." ?
>
> > + * the corresponding leaf node have passed through their quiescent state.
>
> Change "the corresponding leaf node" to "their corresponding leaf nodes" or
> "the corresponding leaf nodes"?
And the winner is "the corresponding leaf nodes"! Good catch, and
thank you!
Thanx, Paul
> Thanks
> Neeraj
>
> > + */
> > if (!READ_ONCE(rnp->qsmask) &&
> > !rcu_preempt_blocked_readers_cgp(rnp))
> > break;
On Thu, Jun 09, 2022 at 12:43:40PM +0530, Neeraj Upadhyay wrote:
> 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 <[email protected]>
> ---
> 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;
Paul's wording changes are OK with me.
For the patch:
Reviewed-by: Joel Fernandes (Google) <[email protected]>
thanks,
- Joel
On Fri, Jun 10, 2022 at 04:37:39PM +0000, Joel Fernandes wrote:
> On Thu, Jun 09, 2022 at 12:43:40PM +0530, Neeraj Upadhyay wrote:
> > 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 <[email protected]>
> > ---
> > 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;
>
> Paul's wording changes are OK with me.
>
> For the patch:
> Reviewed-by: Joel Fernandes (Google) <[email protected]>
Applied, thank you!
Thanx, Paul