srcu: Make Tiny call_srcu() safe to call from any context

Give Tiny call_srcu() the same treatment as Tree SRCU.  When interrupts
are disabled and the scheduler is up, stage the callback on the
srcu_struct's lockless list for an irq_work to re-issue later.  Tiny SRCU
is uniprocessor, so there is no CPU-offline drain.  A draining flag drops
a deferring call_srcu() that re-enters mid-drain (unless from an NMI), as
in Tree SRCU; such a drop can strand state the caller tied to the callback,
not just the callback itself.

Interrupts stay off for the whole batch.  TINY_SRCU implies !SMP, where
arch_irq_work_has_interrupt() is false, so the drain always waits for the
tick and a batch is whatever one tick's worth of interrupts-disabled
call_srcu()s staged.  Unlike the other three flavors srcu_do_enqueue() here
has no debug_rcu_head_queue(), so nothing reports a double call_srcu();
termination of the drain rests on srcu_do_enqueue() clearing ->next, and
the callback list self-links at the tail exactly as a double call_srcu()
made it before.

srcu_barrier() (now out of line) and cleanup_srcu_struct() drain the
deferred list first, so a deferred callback is re-issued onto the callback
list and invoked by the grace-period work that cleanup_srcu_struct()
flushes, rather than stranded on a soon-to-be-freed srcu_struct.
cleanup_srcu_struct() also syncs ->defer_iw, since that irq_work is
embedded in the srcu_struct the caller is about to free.

The draining flag is global rather than per-srcu_struct: a re-entrant
call_srcu(B) inside a drain of A raises B's own ->defer_iw, whose drain can
stage back onto A, so a per-srcu_struct flag would not break the chain.
The cost is that a drain of A also drops a non-NMI call_srcu() to any
other srcu_struct for its duration.

Gated by CONFIG_RCU_DEFER like Tree SRCU, though the srcu_struct members
are unconditional.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
index fbcf13bc12d1..85b5de438450 100644
--- a/include/linux/srcutiny.h
+++ b/include/linux/srcutiny.h
@@ -12,6 +12,7 @@
 #define _LINUX_SRCU_TINY_H
 
 #include <linux/irq_work_types.h>
+#include <linux/llist.h>
 #include <linux/swait.h>
 
 struct srcu_struct {
@@ -26,6 +27,8 @@ struct srcu_struct {
 	struct rcu_head **srcu_cb_tail;	/* Pending callbacks: Tail. */
 	struct work_struct srcu_work;	/* For driving grace periods. */
 	struct irq_work srcu_irq_work;	/* Defer schedule_work() to irq work. */
+	struct llist_head defer_cbs;	/* Callbacks deferred on re-entry. */
+	struct irq_work defer_iw;	/* Re-issues defer_cbs later. */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
@@ -33,6 +36,7 @@ struct srcu_struct {
 
 void srcu_drive_gp(struct work_struct *wp);
 void srcu_tiny_irq_work(struct irq_work *irq_work);
+void srcu_defer_drain(struct irq_work *irq_work);
 
 #define __SRCU_STRUCT_INIT(name, __ignored, ___ignored, ____ignored)	\
 {									\
@@ -40,6 +44,9 @@ void srcu_tiny_irq_work(struct irq_work *irq_work);
 	.srcu_cb_tail = &name.srcu_cb_head,				\
 	.srcu_work = __WORK_INITIALIZER(name.srcu_work, srcu_drive_gp),	\
 	.srcu_irq_work = { .func = srcu_tiny_irq_work },		\
+	.defer_cbs = LLIST_HEAD_INIT(name.defer_cbs),			\
+	.defer_iw = { .node = { .u_flags = IRQ_WORK_HARD_IRQ },		\
+		      .func = srcu_defer_drain },			\
 	__SRCU_DEP_MAP_INIT(name)					\
 }
 
@@ -131,10 +138,7 @@ static inline void synchronize_srcu_expedited(struct srcu_struct *ssp)
 	synchronize_srcu(ssp);
 }
 
-static inline void srcu_barrier(struct srcu_struct *ssp)
-{
-	synchronize_srcu(ssp);
-}
+void srcu_barrier(struct srcu_struct *ssp);
 
 static inline void srcu_expedite_current(struct srcu_struct *ssp) { }
 #define srcu_check_read_flavor(ssp, read_flavor) do { } while (0)