From c2a8ec0778b2ca0d360ba9b5cac7fcd5ddfe798f Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 10 Mar 2017 15:31:55 -0800 Subject: [PATCH] srcu: Move to state-based grace-period sequencing The current SRCU grace-period processing might never reach the last portion of srcu_advance_batches(). This is OK given the current implementation, as the first portion, up to the try_check_zero() following the srcu_flip() is sufficient to drive grace periods forward. However, it has the unfortunate side-effect of making it impossible to determine when a given grace period has ended, and it will be necessary to efficiently trace ends of grace periods in order to efficiently handle per-CPU SRCU callback lists. This commit therefore adds states to the SRCU grace-period processing, so that the end of a given SRCU grace period is marked by the transition to the SRCU_STATE_DONE state. Signed-off-by: Paul E. McKenney --- include/linux/srcu.h | 10 ++++- kernel/rcu/srcu.c | 111 ++++++++++++++++++++++++++++----------------------- 2 files changed, 69 insertions(+), 52 deletions(-) diff --git a/include/linux/srcu.h b/include/linux/srcu.h index a598cf3ac70c..f149a685896c 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -48,7 +48,7 @@ struct srcu_struct { unsigned long completed; struct srcu_array __percpu *per_cpu_ref; spinlock_t queue_lock; /* protect ->batch_queue, ->running */ - bool running; + int srcu_state; /* callbacks just queued */ struct rcu_batch batch_queue; /* callbacks try to do the first check_zero */ @@ -62,6 +62,12 @@ struct srcu_struct { #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ }; +/* Values for -> state variable. */ +#define SRCU_STATE_IDLE 0 +#define SRCU_STATE_SCAN1 1 +#define SRCU_STATE_SCAN2 2 +#define SRCU_STATE_DONE 3 + #ifdef CONFIG_DEBUG_LOCK_ALLOC int __init_srcu_struct(struct srcu_struct *sp, const char *name, @@ -89,7 +95,7 @@ void process_srcu(struct work_struct *work); .completed = -300, \ .per_cpu_ref = &name##_srcu_array, \ .queue_lock = __SPIN_LOCK_UNLOCKED(name.queue_lock), \ - .running = false, \ + .srcu_state = SRCU_STATE_IDLE, \ .batch_queue = RCU_BATCH_INIT(name.batch_queue), \ .batch_check0 = RCU_BATCH_INIT(name.batch_check0), \ .batch_check1 = RCU_BATCH_INIT(name.batch_check1), \ diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c index 821ecda873f2..70286f68f3a1 100644 --- a/kernel/rcu/srcu.c +++ b/kernel/rcu/srcu.c @@ -111,7 +111,7 @@ static int init_srcu_struct_fields(struct srcu_struct *sp) { sp->completed = 0; spin_lock_init(&sp->queue_lock); - sp->running = false; + sp->srcu_state = SRCU_STATE_IDLE; rcu_batch_init(&sp->batch_queue); rcu_batch_init(&sp->batch_check0); rcu_batch_init(&sp->batch_check1); @@ -270,7 +270,7 @@ void cleanup_srcu_struct(struct srcu_struct *sp) if (WARN_ON(!rcu_all_batches_empty(sp))) return; /* Leakage unless caller handles error. */ flush_delayed_work(&sp->work); - if (WARN_ON(sp->running)) + if (WARN_ON(READ_ONCE(sp->srcu_state) != SRCU_STATE_IDLE)) return; /* Caller forgot to stop doing call_srcu()? */ free_percpu(sp->per_cpu_ref); sp->per_cpu_ref = NULL; @@ -391,8 +391,8 @@ void call_srcu(struct srcu_struct *sp, struct rcu_head *head, spin_lock_irqsave(&sp->queue_lock, flags); smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */ rcu_batch_queue(&sp->batch_queue, head); - if (!sp->running) { - sp->running = true; + if (READ_ONCE(sp->srcu_state) == SRCU_STATE_IDLE) { + WRITE_ONCE(sp->srcu_state, SRCU_STATE_SCAN1); queue_delayed_work(system_power_efficient_wq, &sp->work, 0); } spin_unlock_irqrestore(&sp->queue_lock, flags); @@ -424,9 +424,9 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount) head->func = wakeme_after_rcu; spin_lock_irq(&sp->queue_lock); smp_mb__after_unlock_lock(); /* Caller's prior accesses before GP. */ - if (!sp->running) { + if (READ_ONCE(sp->srcu_state) == SRCU_STATE_IDLE) { /* steal the processing owner */ - sp->running = true; + WRITE_ONCE(sp->srcu_state, SRCU_STATE_SCAN1); rcu_batch_queue(&sp->batch_check0, head); spin_unlock_irq(&sp->queue_lock); /* give the processing owner to work_struct */ @@ -548,7 +548,9 @@ static void srcu_collect_new(struct srcu_struct *sp) */ static void srcu_advance_batches(struct srcu_struct *sp, int trycount) { - int idx = 1 ^ (sp->completed & 1); + int idx; + + WARN_ON_ONCE(sp->srcu_state == SRCU_STATE_IDLE); /* * Because readers might be delayed for an extended period after @@ -558,48 +560,56 @@ static void srcu_advance_batches(struct srcu_struct *sp, int trycount) * invoking a callback. */ - if (rcu_batch_empty(&sp->batch_check0) && - rcu_batch_empty(&sp->batch_check1)) - return; /* no callbacks need to be advanced */ - - if (!try_check_zero(sp, idx, trycount)) - return; /* failed to advance, will try after SRCU_INTERVAL */ - - /* - * The callbacks in ->batch_check1 have already done with their - * first zero check and flip back when they were enqueued on - * ->batch_check0 in a previous invocation of srcu_advance_batches(). - * (Presumably try_check_zero() returned false during that - * invocation, leaving the callbacks stranded on ->batch_check1.) - * They are therefore ready to invoke, so move them to ->batch_done. - */ - rcu_batch_move(&sp->batch_done, &sp->batch_check1); - - if (rcu_batch_empty(&sp->batch_check0)) - return; /* no callbacks need to be advanced */ - srcu_flip(sp); - - /* - * The callbacks in ->batch_check0 just finished their - * first check zero and flip, so move them to ->batch_check1 - * for future checking on the other idx. - */ - rcu_batch_move(&sp->batch_check1, &sp->batch_check0); - - /* - * SRCU read-side critical sections are normally short, so check - * at least twice in quick succession after a flip. - */ - trycount = trycount < 2 ? 2 : trycount; - if (!try_check_zero(sp, idx^1, trycount)) - return; /* failed to advance, will try after SRCU_INTERVAL */ + if (sp->srcu_state == SRCU_STATE_DONE) + WRITE_ONCE(sp->srcu_state, SRCU_STATE_SCAN1); + + if (sp->srcu_state == SRCU_STATE_SCAN1) { + idx = 1 ^ (sp->completed & 1); + if (!try_check_zero(sp, idx, trycount)) + return; /* readers present, retry after SRCU_INTERVAL */ + + /* + * The callbacks in ->batch_check1 have already done + * with their first zero check and flip back when they were + * enqueued on ->batch_check0 in a previous invocation of + * srcu_advance_batches(). (Presumably try_check_zero() + * returned false during that invocation, leaving the + * callbacks stranded on ->batch_check1.) They are therefore + * ready to invoke, so move them to ->batch_done. + */ + rcu_batch_move(&sp->batch_done, &sp->batch_check1); + srcu_flip(sp); + + /* + * The callbacks in ->batch_check0 just finished their + * first check zero and flip, so move them to ->batch_check1 + * for future checking on the other idx. + */ + rcu_batch_move(&sp->batch_check1, &sp->batch_check0); + + WRITE_ONCE(sp->srcu_state, SRCU_STATE_SCAN2); + } - /* - * The callbacks in ->batch_check1 have now waited for all - * pre-existing readers using both idx values. They are therefore - * ready to invoke, so move them to ->batch_done. - */ - rcu_batch_move(&sp->batch_done, &sp->batch_check1); + if (sp->srcu_state == SRCU_STATE_SCAN2) { + + /* + * SRCU read-side critical sections are normally short, + * so check at least twice in quick succession after a flip. + */ + idx = 1 ^ (sp->completed & 1); + trycount = trycount < 2 ? 2 : trycount; + if (!try_check_zero(sp, idx, trycount)) + return; /* readers present, retry after SRCU_INTERVAL */ + + /* + * The callbacks in ->batch_check1 have now waited for + * all pre-existing readers using both idx values. They are + * therefore ready to invoke, so move them to ->batch_done. + */ + rcu_batch_move(&sp->batch_done, &sp->batch_check1); + + WRITE_ONCE(sp->srcu_state, SRCU_STATE_DONE); + } } /* @@ -633,8 +643,9 @@ static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay) if (rcu_all_batches_empty(sp)) { spin_lock_irq(&sp->queue_lock); - if (rcu_all_batches_empty(sp)) { - sp->running = false; + if (rcu_all_batches_empty(sp) && + READ_ONCE(sp->srcu_state) == SRCU_STATE_DONE) { + WRITE_ONCE(sp->srcu_state, SRCU_STATE_IDLE); pending = false; } spin_unlock_irq(&sp->queue_lock); -- 2.11.0