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rcu: demote SRCU_SYNCHRONIZE_DELAY from kernel-parameter status

Because the adaptive synchronize_srcu_expedited() approach has
worked very well in testing, remove the kernel parameter and
replace it by a C-preprocessor macro.  If someone finds problems
with this approach, a more complex and aggressively adaptive
approach might be required.

Longer term, SRCU will be merged with the other RCU implementations,
at which point synchronize_srcu_expedited() will be event driven,
just as synchronize_sched_expedited() currently is.  At that point,
there will be no need for this adaptive approach.

Reported-by: Linus Torvalds <>
Signed-off-by: Paul E. McKenney <>
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1 parent 37bc46e commit 38253e011545b6f67d42dc136360d269c8742de2 @paulmck paulmck committed with
Showing with 13 additions and 17 deletions.
  1. +0 −15 init/Kconfig
  2. +13 −2 kernel/srcu.c
15 init/Kconfig
@@ -481,21 +481,6 @@ config RCU_BOOST_DELAY
Accept the default if unsure.
- int "Microseconds to delay before waiting for readers"
- range 0 20
- default 10
- help
- This option controls how long SRCU delays before entering its
- loop waiting on SRCU readers. The purpose of this loop is
- to avoid the unconditional context-switch penalty that would
- otherwise be incurred if there was an active SRCU reader,
- in a manner similar to adaptive locking schemes. This should
- be set to be a bit longer than the common-case SRCU read-side
- critical-section overhead.
- Accept the default if unsure.
endmenu # "RCU Subsystem"
15 kernel/srcu.c
@@ -158,6 +158,16 @@ void __srcu_read_unlock(struct srcu_struct *sp, int idx)
+ * We use an adaptive strategy for synchronize_srcu() and especially for
+ * synchronize_srcu_expedited(). We spin for a fixed time period
+ * (defined below) to allow SRCU readers to exit their read-side critical
+ * sections. If there are still some readers after 10 microseconds,
+ * we repeatedly block for 1-millisecond time periods. This approach
+ * has done well in testing, so there is no need for a config parameter.
+ */
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
@@ -209,11 +219,12 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
* will have finished executing. We initially give readers
* an arbitrarily chosen 10 microseconds to get out of their
* SRCU read-side critical sections, then loop waiting 1/HZ
- * seconds per iteration.
+ * seconds per iteration. The 10-microsecond value has done
+ * very well in testing.
if (srcu_readers_active_idx(sp, idx))
while (srcu_readers_active_idx(sp, idx))

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