From: Kunwu Chan <[email protected]> synchronize_srcu_atomic() is restricted to srcu_read_lock_atomic() and srcu_read_unlock_atomic(), whose read-side critical sections disable preemption. In the common case where there are no readers at all, the grace period therefore need not do the index flip. Add a fastpath that sums both ranks of the per-CPU ->srcu_ctrs[] counters and, if the lock counts match the unlock counts on both ranks, ends the grace period immediately, skipping the srcu_advance_state() scans, mirroring the similar Tiny SRCU fastpath.
Correctness requires the counter-sum proof to follow the grace-period anchor written by srcu_gp_start(); placing it before the anchor could let this grace period miss a pre-existing reader and return without waiting for it. The smp_mb() between the unlock and lock sums pairs with the smp_mb() in __srcu_read_lock(). The grace period is ended manually under ->lock and ->srcu_atomic_gp_flag. Signed-off-by: Kunwu Chan <[email protected]> --- kernel/rcu/srcutree.c | 48 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 48 insertions(+) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 7dd705eec573..533607de5728 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -2113,6 +2113,8 @@ void synchronize_srcu_atomic(struct srcu_struct *ssp) { unsigned long srcu_state; struct srcu_usage *sup = ssp->srcu_sup; + unsigned long rdm0, rdm1; + unsigned long unlocks0, unlocks1; // Initialize. Either init_srcu_struct() was invoked or // DEFINE_SRCU() or similar was used. Therefore, no allocation @@ -2149,6 +2151,52 @@ void synchronize_srcu_atomic(struct srcu_struct *ssp) srcu_gp_start(ssp); raw_spin_unlock_irq_rcu_node(sup); + // + // Fastpath: If there are no readers at all, neither grace-period + // scan need wait, so both can be satisfied at once without doing + // the index flip. The counter-sum proof is the same as that of + // srcu_readers_active_idx_check(), but spanning both indices. + // Atomic SRCU guarantees that all readers are of + // SRCU_READ_FLAVOR_ATOMIC, so the SLOWGP check never triggers and + // the ->srcu_reader_flavor masks returned by + // srcu_readers_unlock_idx() are unused. + // + // This proof must follow the grace-period anchor written by the + // srcu_gp_start() above, never precede it. With the anchor first, + // a reader whose lock increment is missed by the sums below cannot + // have incremented its lock counter before the anchor, and therefore + // cannot be a pre-existing reader of this grace period. Placing the + // proof before the anchor would let this grace period miss a + // pre-existing reader and return without waiting for it. + // + // The smp_mb() pairs with the smp_mb() in __srcu_read_lock() + // (store-buffering pattern), which guarantees that a lock is always + // counted if the corresponding unlock is counted, the same + // memory-ordering guarantee as is provided by + // srcu_readers_active_idx_check(). + // + unlocks0 = srcu_readers_unlock_idx(ssp, 0, &rdm0); + unlocks1 = srcu_readers_unlock_idx(ssp, 1, &rdm1); + smp_mb(); /* A */ + if (srcu_readers_lock_idx(ssp, 0, false, unlocks0) && + srcu_readers_lock_idx(ssp, 1, false, unlocks1)) { + // No readers, so end this grace period manually, skipping + // the index flip. Advancing the sequence number via + // rcu_seq_start() in srcu_gp_start() above and rcu_seq_end() + // below keeps get_state_synchronize_srcu() and + // poll_state_synchronize_srcu() working, all under ->lock + // and ->srcu_atomic_gp_flag, which excludes concurrent + // sequence-number updates. + raw_spin_lock_irq_rcu_node(sup); + rcu_seq_end(&sup->srcu_gp_seq); + raw_spin_unlock_irq_rcu_node(sup); + WARN_ON_ONCE(!poll_state_synchronize_srcu(ssp, srcu_state)); + atomic_set_release(&sup->srcu_atomic_gp_flag, 0); + preempt_enable(); + non_block_end(); + return; + } + // Wait for it to complete, helping it along. while (!poll_state_synchronize_srcu(ssp, srcu_state)) { cpu_relax(); -- 2.43.0
