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McKenney wrote: > On Sat, Jul 05, 2025 at 04:39:15PM -0400, Joel Fernandes wrote: >> Signed-off-by: Joel Fernandes > > Definitely headed in the right direction, though it does need just a > little bit more detail in the commit log. ;-) > > Also a few comments and questions interspersed below. > > Thanx, Paul > >> --- >> kernel/rcu/tree.h | 11 ++++++++++- >> kernel/rcu/tree_plugin.h | 29 ++++++++++++++++++++++------- >> 2 files changed, 32 insertions(+), 8 deletions(-) >> >> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h >> index 3830c19cf2f6..f8f612269e6e 100644 >> --- a/kernel/rcu/tree.h >> +++ b/kernel/rcu/tree.h >> @@ -174,6 +174,15 @@ struct rcu_snap_record { >> unsigned long jiffies; /* Track jiffies value */ >> }; >> >> +/* >> + * The IRQ work (deferred_qs_iw) is used by RCU to get scheduler's attention. >> + * It can be in one of the following states: >> + * - DEFER_QS_IDLE: An IRQ work was never scheduled. >> + * - DEFER_QS_PENDING: An IRQ work was scheduler but never run. >> + */ >> +#define DEFER_QS_IDLE 0 >> +#define DEFER_QS_PENDING 1 > > Having names for the states is good! > >> + >> /* Per-CPU data for read-copy update. */ >> struct rcu_data { >> /* 1) quiescent-state and grace-period handling : */ >> @@ -192,7 +201,7 @@ struct rcu_data { >> /* during and after the last grace */ >> /* period it is aware of. */ >> struct irq_work defer_qs_iw; /* Obtain later scheduler attention. */ >> - bool defer_qs_iw_pending; /* Scheduler attention pending? */ >> + int defer_qs_iw_pending; /* Scheduler attention pending? */ >> struct work_struct strict_work; /* Schedule readers for strict GPs. */ >> >> /* 2) batch handling */ >> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h >> index dd1c156c1759..baf57745b42f 100644 >> --- a/kernel/rcu/tree_plugin.h >> +++ b/kernel/rcu/tree_plugin.h >> @@ -486,13 +486,16 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) >> struct rcu_node *rnp; >> union rcu_special special; >> >> + rdp = this_cpu_ptr(&rcu_data); >> + if (rdp->defer_qs_iw_pending == DEFER_QS_PENDING) >> + rdp->defer_qs_iw_pending = DEFER_QS_IDLE; > > Good, this is where the request actually gets serviced. > >> + >> /* >> * If RCU core is waiting for this CPU to exit its critical section, >> * report the fact that it has exited. Because irqs are disabled, >> * t->rcu_read_unlock_special cannot change. >> */ >> special = t->rcu_read_unlock_special; >> - rdp = this_cpu_ptr(&rcu_data); >> if (!special.s && !rdp->cpu_no_qs.b.exp) { >> local_irq_restore(flags); >> return; >> @@ -623,12 +626,24 @@ notrace void rcu_preempt_deferred_qs(struct task_struct *t) >> */ >> static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp) >> { >> - unsigned long flags; >> - struct rcu_data *rdp; >> + volatile unsigned long flags; > > I don't understand why this wants to be volatile. > > Unless maybe you want to make sure that gdb can see it, in > which case, is there an existing Kconfig option for that? Maybe > CONFIG_DEBUG_INFO_NONE=n? This does not need to be volatile, sorry it was an older remnant (back when the handler was a NOOP in the v1, and I was afraid of compiler optimizations ;-)). But its no longer needed so I shall drop it (the volatile) :) > >> + struct rcu_data *rdp = this_cpu_ptr(&rcu_data); >> >> - rdp = container_of(iwp, struct rcu_data, defer_qs_iw); >> local_irq_save(flags); >> - rdp->defer_qs_iw_pending = false; >> + >> + /* >> + * Requeue the IRQ work on next unlock in following situation: >> + * 1. rcu_read_unlock() queues IRQ work (state -> DEFER_QS_PENDING) >> + * 2. CPU enters new rcu_read_lock() >> + * 3. IRQ work runs but cannot report QS due to rcu_preempt_depth() > 0 >> + * 4. rcu_read_unlock() does not re-queue work (state still PENDING) >> + * 5. Deferred QS reporting does not happen. >> + */ >> + if (rcu_preempt_depth() > 0) { >> + WRITE_ONCE(rdp->defer_qs_iw_pending, DEFER_QS_IDLE); > > Shouldn't we have just this WRITE_ONCE() in this then-clause? No, because if we let the IRQ work handler do that before we can execute rcu_preempt_deferred_qs_handler(), then it will cause infinite recursion, because an RCU read-side critical section can again try to queue the IRQ work (before entering the scheduler). Also testing shows doing that will reproduce the hang we're fixing. I think we should rename defer_qs_iw_pending to defer_qs_pending to better clarify that we are tracking the "Deferred QS" reporting than if the IRQ work actually ran? Thanks, - Joel