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Date: Thu, 1 Nov 2018 10:03:48 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: peterz@...radead.org, linux-kernel@...r.kernel.org,
josh@...htriplett.org, rostedt@...dmis.org,
mathieu.desnoyers@...icios.com, jiangshanlai@...il.com
Subject: Re: [PATCH RFC kenrel/rcu] Eliminate BUG_ON() for sync.c
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);
> }
> }
>
>
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