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srcu: Crude control of expedited grace periods
SRCU's implementation of expedited grace periods has always assumed that the SRCU instance is idle when the expedited request arrives. This commit improves this a bit by maintaining a count of the number of outstanding expedited requests, thus allowing prior non-expedited grace periods accommodate these requests by shifting to expedited mode. However, any non-expedited wait already in progress will still wait for the full duration. Improved control of expedited grace periods is planned, but one step at a time. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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@ -42,6 +42,7 @@ struct srcu_array {
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struct srcu_struct {
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unsigned long completed;
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unsigned long srcu_gp_seq;
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atomic_t srcu_exp_cnt;
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struct srcu_array __percpu *per_cpu_ref;
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spinlock_t queue_lock; /* protect ->srcu_cblist */
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struct rcu_segcblist srcu_cblist;
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@ -43,6 +43,7 @@ static int init_srcu_struct_fields(struct srcu_struct *sp)
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{
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sp->completed = 0;
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sp->srcu_gp_seq = 0;
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atomic_set(&sp->srcu_exp_cnt, 0);
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spin_lock_init(&sp->queue_lock);
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rcu_segcblist_init(&sp->srcu_cblist);
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INIT_DELAYED_WORK(&sp->work, process_srcu);
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@ -179,7 +180,6 @@ static bool srcu_readers_active(struct srcu_struct *sp)
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return sum;
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}
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#define SRCU_CALLBACK_BATCH 10
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#define SRCU_INTERVAL 1
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/**
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@ -197,6 +197,7 @@ static bool srcu_readers_active(struct srcu_struct *sp)
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*/
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void cleanup_srcu_struct(struct srcu_struct *sp)
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{
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WARN_ON_ONCE(atomic_read(&sp->srcu_exp_cnt));
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if (WARN_ON(srcu_readers_active(sp)))
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return; /* Leakage unless caller handles error. */
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if (WARN_ON(!rcu_segcblist_empty(&sp->srcu_cblist)))
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@ -244,13 +245,10 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
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* We use an adaptive strategy for synchronize_srcu() and especially for
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* synchronize_srcu_expedited(). We spin for a fixed time period
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* (defined below) to allow SRCU readers to exit their read-side critical
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* sections. If there are still some readers after 10 microseconds,
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* we repeatedly block for 1-millisecond time periods. This approach
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* has done well in testing, so there is no need for a config parameter.
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* sections. If there are still some readers after a few microseconds,
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* we repeatedly block for 1-millisecond time periods.
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*/
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#define SRCU_RETRY_CHECK_DELAY 5
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#define SYNCHRONIZE_SRCU_TRYCOUNT 2
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#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12
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/*
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* Start an SRCU grace period.
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@ -267,16 +265,16 @@ static void srcu_gp_start(struct srcu_struct *sp)
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}
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/*
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* Wait until all readers counted by array index idx complete, but loop
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* a maximum of trycount times. The caller must ensure that ->completed
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* is not changed while checking.
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* Wait until all readers counted by array index idx complete, but
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* loop an additional time if there is an expedited grace period pending.
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* The caller must ensure that ->completed is not changed while checking.
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*/
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static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
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{
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for (;;) {
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if (srcu_readers_active_idx_check(sp, idx))
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return true;
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if (--trycount <= 0)
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if (--trycount + !!atomic_read(&sp->srcu_exp_cnt) <= 0)
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return false;
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udelay(SRCU_RETRY_CHECK_DELAY);
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}
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@ -364,7 +362,7 @@ static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
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/*
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* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
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*/
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static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
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static void __synchronize_srcu(struct srcu_struct *sp)
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{
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struct rcu_synchronize rcu;
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struct rcu_head *head = &rcu.head;
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@ -400,6 +398,32 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
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smp_mb(); /* Caller's later accesses after GP. */
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}
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/**
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* synchronize_srcu_expedited - Brute-force SRCU grace period
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* @sp: srcu_struct with which to synchronize.
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*
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* Wait for an SRCU grace period to elapse, but be more aggressive about
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* spinning rather than blocking when waiting.
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*
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* Note that synchronize_srcu_expedited() has the same deadlock and
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* memory-ordering properties as does synchronize_srcu().
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*/
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void synchronize_srcu_expedited(struct srcu_struct *sp)
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{
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bool do_norm = rcu_gp_is_normal();
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if (!do_norm) {
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atomic_inc(&sp->srcu_exp_cnt);
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smp_mb__after_atomic(); /* increment before GP. */
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}
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__synchronize_srcu(sp);
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if (!do_norm) {
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smp_mb__before_atomic(); /* GP before decrement. */
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atomic_dec(&sp->srcu_exp_cnt);
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}
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
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/**
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* synchronize_srcu - wait for prior SRCU read-side critical-section completion
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* @sp: srcu_struct with which to synchronize.
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@ -441,28 +465,13 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
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*/
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void synchronize_srcu(struct srcu_struct *sp)
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{
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__synchronize_srcu(sp, (rcu_gp_is_expedited() && !rcu_gp_is_normal())
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? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
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: SYNCHRONIZE_SRCU_TRYCOUNT);
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if (rcu_gp_is_expedited())
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synchronize_srcu_expedited(sp);
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else
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__synchronize_srcu(sp);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu);
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/**
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* synchronize_srcu_expedited - Brute-force SRCU grace period
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* @sp: srcu_struct with which to synchronize.
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*
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* Wait for an SRCU grace period to elapse, but be more aggressive about
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* spinning rather than blocking when waiting.
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*
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* Note that synchronize_srcu_expedited() has the same deadlock and
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* memory-ordering properties as does synchronize_srcu().
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*/
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void synchronize_srcu_expedited(struct srcu_struct *sp)
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{
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__synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
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/**
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* srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
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* @sp: srcu_struct on which to wait for in-flight callbacks.
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@ -490,7 +499,7 @@ EXPORT_SYMBOL_GPL(srcu_batches_completed);
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* Core SRCU state machine. Advance callbacks from ->batch_check0 to
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* ->batch_check1 and then to ->batch_done as readers drain.
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*/
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static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
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static void srcu_advance_batches(struct srcu_struct *sp)
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{
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int idx;
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@ -521,8 +530,8 @@ static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
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if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
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idx = 1 ^ (sp->completed & 1);
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if (!try_check_zero(sp, idx, trycount))
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return; /* readers present, retry after SRCU_INTERVAL */
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if (!try_check_zero(sp, idx, 1))
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return; /* readers present, retry later. */
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srcu_flip(sp);
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rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2);
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}
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@ -534,9 +543,8 @@ static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
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* so check at least twice in quick succession after a flip.
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*/
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idx = 1 ^ (sp->completed & 1);
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trycount = trycount < 2 ? 2 : trycount;
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if (!try_check_zero(sp, idx, trycount))
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return; /* readers present, retry after SRCU_INTERVAL */
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if (!try_check_zero(sp, idx, 2))
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return; /* readers present, retry after later. */
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srcu_gp_end(sp);
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}
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}
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@ -602,8 +610,8 @@ void process_srcu(struct work_struct *work)
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sp = container_of(work, struct srcu_struct, work.work);
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srcu_advance_batches(sp, 1);
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srcu_advance_batches(sp);
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srcu_invoke_callbacks(sp);
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srcu_reschedule(sp, SRCU_INTERVAL);
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srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
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}
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EXPORT_SYMBOL_GPL(process_srcu);
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