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rcu/sync: Simplify the state machine

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With this patch rcu_sync has a single state variable and the transition rules
become really simple:

	GP_IDLE   - owned by the first rcu_sync_enter() which moves it to

	GP_ENTER  - owned by rcu-callback which moves it to

	GP_PASSED - owned by the last rcu_sync_exit() which moves it to

	GP_EXIT   - and this is the only "nontrivial" state.

		rcu-callback moves it back to GP_IDLE unless another enter()
		comes before a GP pass.

		If rcu-callback is invoked before the next rcu_sync_exit() it
		must see gp_count incremented by that enter() and set GP_PASSED.

		Otherwise, if the next rcu_sync_exit() wins the race, it will
		move it to

	GP_REPLAY - owned by rcu-callback which moves it to GP_EXIT

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
[ paulmck: While here, apply READ_ONCE() and WRITE_ONCE() to ->gp_state. ]
[ paulmck: Tweaks to make htmldocs happy. (Reported by kbuild test robot.) ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Oleg Nesterov
2019-04-25 18:50:55 +02:00
committed by Paul E. McKenney
parent 3f2947b781
commit 89da3b94bb
2 changed files with 110 additions and 87 deletions
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4
include/linux/rcu_sync.h
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@@ -19,7 +19,6 @@ struct rcu_sync {
int gp_count;
wait_queue_head_t gp_wait;
int cb_state;
struct rcu_head cb_head;
};
@@ -36,7 +35,7 @@ static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
!rcu_read_lock_bh_held() &&
!rcu_read_lock_sched_held(),
"suspicious rcu_sync_is_idle() usage");
return !rsp->gp_state; /* GP_IDLE */
return !READ_ONCE(rsp->gp_state); /* GP_IDLE */
}
extern void rcu_sync_init(struct rcu_sync *);
@@ -49,7 +48,6 @@ extern void rcu_sync_dtor(struct rcu_sync *);
.gp_state = 0, \
.gp_count = 0, \
.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait), \
.cb_state = 0, \
}
#define DEFINE_RCU_SYNC(name) \