Re: [PATCH RFC kenrel/rcu] Eliminate BUG_ON() for sync.c

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Thu, Nov 01, 2018 at 04:45:19PM +0100, Oleg Nesterov wrote:
> On 11/01, Paul E. McKenney wrote:
> >
> > Any news on exactly which patch constituted the reworking of this
> > code some time back?
> 
> Again, I never sent a patch, I simply showed the new code (more than 2 years
> ago ;), see below. I need to re-read our discussiong, but iirc your and Peter's
> reviews were mostly positive.

I am glad that I didn't try to apply the various related patches I
found, then.  ;-)

> The new implementation (and the state machine) is simpler, plus the new
> __rcu_sync_enter(). It can be used instead of rcu_sync_enter_start() hack,
> and by freeze_super() which currently need 3 GP's to take 3 percpu rwsems.
> 
> Oleg.
> -------------------------------------------------------------------------------
> 
> #include <linux/rcu_sync.h>
> #include <linux/sched.h>
> 
> #ifdef CONFIG_PROVE_RCU
> #define __INIT_HELD(func)	.held = func,
> #else
> #define __INIT_HELD(func)
> #endif
> 
> static const struct {
> 	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
> 	void (*wait)(void);	// TODO: remove this, see the comment in dtor
> #ifdef CONFIG_PROVE_RCU
> 	int  (*held)(void);
> #endif
> } gp_ops[] = {
> 	[RCU_SYNC] = {
> 		.call = call_rcu,
> 		.wait = rcu_barrier,
> 		__INIT_HELD(rcu_read_lock_held)
> 	},
> 	[RCU_SCHED_SYNC] = {
> 		.call = call_rcu_sched,
> 		.wait = rcu_barrier_sched,

In my -rcu tree, these are now call_rcu and rcu_barrier, courtesy of
RCU flavor consolidation.

> 		__INIT_HELD(rcu_read_lock_sched_held)

This remains the same.

> 	},
> 	[RCU_BH_SYNC] = {
> 		.call = call_rcu_bh,
> 		.wait = rcu_barrier_bh,
> 		__INIT_HELD(rcu_read_lock_bh_held)

And same for these three.

							Thanx, Paul

> 	},
> };
> 
> #define	rss_lock	gp_wait.lock
> 
> enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY };
> 
> #ifdef CONFIG_PROVE_RCU
> void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
> {
> 	RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
> 			 "suspicious rcu_sync_is_idle() usage");
> }
> #endif
> 
> void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
> {
> 	memset(rsp, 0, sizeof(*rsp));
> 	init_waitqueue_head(&rsp->gp_wait);
> 	rsp->gp_type = type;
> }
> 
> static void rcu_sync_func(struct rcu_head *rcu);
> 
> static void rcu_sync_call(struct rcu_sync *rsp)
> {
> 	// TODO: THIS IS SUBOPTIMAL. We want to call it directly
> 	// if rcu_blocking_is_gp() == T, but it has might_sleep().
> 	gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
> }
> 
> static void rcu_sync_func(struct rcu_head *rcu)
> {
> 	struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);
> 	unsigned long flags;
> 
> 	BUG_ON(rsp->gp_state == GP_IDLE);
> 	BUG_ON(rsp->gp_state == GP_PASSED);
> 
> 	spin_lock_irqsave(&rsp->rss_lock, flags);
> 	if (rsp->gp_count) {
> 		/*
> 		 * We're at least a GP after the first __rcu_sync_enter().
> 		 */
> 		rsp->gp_state = GP_PASSED;
> 		wake_up_locked(&rsp->gp_wait);
> 	} else if (rsp->gp_state == GP_REPLAY) {
> 		/*
> 		 * A new rcu_sync_exit() has happened; requeue the callback
> 		 * to catch a later GP.
> 		 */
> 		rsp->gp_state = GP_EXIT;
> 		rcu_sync_call(rsp);
> 	} else {
> 		/*
> 		 * We're at least a GP after the last rcu_sync_exit();
> 		 * eveybody will now have observed the write side critical
> 		 * section. Let 'em rip!.
> 		 *
> 		 * OR. ->gp_state can be still GP_ENTER if __rcu_sync_wait()
> 		 * wasn't called after __rcu_sync_enter(), abort.
> 		 */
> 		rsp->gp_state = GP_IDLE;
> 	}
> 	spin_unlock_irqrestore(&rsp->rss_lock, flags);
> }
> 
> bool __rcu_sync_enter(struct rcu_sync *rsp)
> {
> 	int gp_count, gp_state;
> 
> 	spin_lock_irq(&rsp->rss_lock);
> 	gp_count = rsp->gp_count++;
> 	gp_state = rsp->gp_state;
> 	if (gp_state == GP_IDLE) {
> 		rsp->gp_state = GP_ENTER;
> 		rcu_sync_call(rsp);
> 	}
> 	spin_unlock_irq(&rsp->rss_lock);
> 
> 	BUG_ON(gp_count != 0 && gp_state == GP_IDLE);
> 	BUG_ON(gp_count == 0 && gp_state == GP_PASSED);
> 
> 	return gp_state < GP_PASSED;
> }
> 
> void __rcu_sync_wait(struct rcu_sync *rsp)
> {
> 	BUG_ON(rsp->gp_state == GP_IDLE);
> 	BUG_ON(rsp->gp_count == 0);
> 
> 	wait_event(rsp->gp_wait, rsp->gp_state >= GP_PASSED);
> }
> 
> void rcu_sync_enter(struct rcu_sync *rsp)
> {
> 	if (__rcu_sync_enter(rsp))
> 		__rcu_sync_wait(rsp);
> }
> 
> void rcu_sync_exit(struct rcu_sync *rsp)
> {
> 	BUG_ON(rsp->gp_state == GP_IDLE);
> 	BUG_ON(rsp->gp_count == 0);
> 
> 	spin_lock_irq(&rsp->rss_lock);
> 	if (!--rsp->gp_count) {
> 		if (rsp->gp_state == GP_PASSED) {
> 			rsp->gp_state = GP_EXIT;
> 			rcu_sync_call(rsp);
> 		} else if (rsp->gp_state == GP_EXIT) {
> 			rsp->gp_state = GP_REPLAY;
> 		}
> 	}
> 	spin_unlock_irq(&rsp->rss_lock);
> }
> 
> void rcu_sync_dtor(struct rcu_sync *rsp)
> {
> 	int gp_state;
> 
> 	BUG_ON(rsp->gp_count);
> 	BUG_ON(rsp->gp_state == GP_PASSED);
> 
> 	spin_lock_irq(&rsp->rss_lock);
> 	if (rsp->gp_state == GP_REPLAY)
> 		rsp->gp_state = GP_EXIT;
> 	gp_state = rsp->gp_state;
> 	spin_unlock_irq(&rsp->rss_lock);
> 
> 	// TODO: add another wake_up_locked() into rcu_sync_func(),
> 	// use wait_event + spin_lock_wait, remove gp_ops->wait().
> 	if (gp_state != GP_IDLE) {
> 		gp_ops[rsp->gp_type].wait();
> 		BUG_ON(rsp->gp_state != GP_IDLE);
> 	}
> }
> 
>