diff options
| author | Joel Fernandes <joelagnelf@nvidia.com> | 2025-07-15 16:01:55 -0400 |
|---|---|---|
| committer | Neeraj Upadhyay <neeraj.iitr10@gmail.com> | 2025-07-22 17:10:50 +0530 |
| commit | 5d71c2b53f1790c2ca09d03848839c610653d278 (patch) | |
| tree | 1396a629a5ae3943ab8998a7c23f5fbd1d193b30 /kernel/rcu/tree.c | |
| parent | 186779c036468038b0d077ec5333a51512f867e5 (diff) | |
rcu: Document concurrent quiescent state reporting for offline CPUs
The synchronization of CPU offlining with GP initialization is confusing
to put it mildly (rightfully so as the issue it deals with is complex).
Recent discussions brought up a question -- what prevents the
rcu_implicit_dyntick_qs() from warning about QS reports for offline
CPUs (missing QS reports for offline CPUs causing indefinite hangs).
QS reporting for now-offline CPUs should only happen from:
- gp_init()
- rcutree_cpu_report_dead()
Add some documentation on this and refer to it from comments in the code
explaining how QS reporting is not missed when these functions are
concurrently running.
I referred heavily to this post [1] about the need for the ofl_lock.
[1] https://lore.kernel.org/all/20180924164443.GF4222@linux.ibm.com/
[ Applied paulmck feedback on moving documentation to Requirements.rst ]
Link: https://lore.kernel.org/all/01b4d228-9416-43f8-a62e-124b92e8741a@paulmck-laptop/
Co-developed-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>
Diffstat (limited to 'kernel/rcu/tree.c')
| -rw-r--r-- | kernel/rcu/tree.c | 19 |
1 files changed, 18 insertions, 1 deletions
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index b206db119546..dd282a984fec 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1886,6 +1886,10 @@ static noinline_for_stack bool rcu_gp_init(void) /* Exclude CPU hotplug operations. */ rcu_for_each_leaf_node(rnp) { local_irq_disable(); + /* + * Serialize with CPU offline. See Requirements.rst > Hotplug CPU > + * Concurrent Quiescent State Reporting for Offline CPUs. + */ arch_spin_lock(&rcu_state.ofl_lock); raw_spin_lock_rcu_node(rnp); if (rnp->qsmaskinit == rnp->qsmaskinitnext && @@ -1960,7 +1964,12 @@ static noinline_for_stack bool rcu_gp_init(void) trace_rcu_grace_period_init(rcu_state.name, rnp->gp_seq, rnp->level, rnp->grplo, rnp->grphi, rnp->qsmask); - /* Quiescent states for tasks on any now-offline CPUs. */ + /* + * Quiescent states for tasks on any now-offline CPUs. Since we + * released the ofl and rnp lock before this loop, CPUs might + * have gone offline and we have to report QS on their behalf. + * See Requirements.rst > Hotplug CPU > Concurrent QS Reporting. + */ mask = rnp->qsmask & ~rnp->qsmaskinitnext; rnp->rcu_gp_init_mask = mask; if ((mask || rnp->wait_blkd_tasks) && rcu_is_leaf_node(rnp)) @@ -4386,6 +4395,13 @@ void rcutree_report_cpu_dead(void) /* Remove outgoing CPU from mask in the leaf rcu_node structure. */ mask = rdp->grpmask; + + /* + * Hold the ofl_lock and rnp lock to avoid races between CPU going + * offline and doing a QS report (as below), versus rcu_gp_init(). + * See Requirements.rst > Hotplug CPU > Concurrent QS Reporting section + * for more details. + */ arch_spin_lock(&rcu_state.ofl_lock); raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order guarantee. */ rdp->rcu_ofl_gp_seq = READ_ONCE(rcu_state.gp_seq); @@ -4396,6 +4412,7 @@ void rcutree_report_cpu_dead(void) rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags); raw_spin_lock_irqsave_rcu_node(rnp, flags); } + /* Clear from ->qsmaskinitnext to mark offline. */ WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask); raw_spin_unlock_irqrestore_rcu_node(rnp, flags); arch_spin_unlock(&rcu_state.ofl_lock); |