Message-ID: <20120515224137.GA15386@google.com>
Date:	Tue, 15 May 2012 15:41:37 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, agk@...hat.com
Subject: Re: [Bcache v13 07/16] Closures

Hello, Kent.

On Wed, May 09, 2012 at 11:09:46PM -0400, Kent Overstreet wrote:
> Closures are asynchronous refcounty things based on workqueues, used
> extensively in bcache.
> 
> Signed-off-by: Kent Overstreet <koverstreet@...gle.com>

Overall, I can't find myself liking this closure thing much.  It seems
to be trying to roll too much into itself.  To me, it almost feels
like some combination of semaphore and async execution with hierarchy
and some other stuff thrown in for good measure.  Kernel moved away
from semaphores for pretty good reasons.

I haven't seen the actual usages yet so my opinion might do a
back-flip but for now I would much prefer something simpler -
e.g. work item triggering mechanism which might include hierarchy
support, some wrappers around wait_queue_head_t and so on.

Another concern that the terms and concepts introduced are foreign.
When something gets executed asynchronously, we make that explicit.
Such explicit bouncing tends to cause some tension especially in IO
code paths which often end up doing stuff which might require sleeping
without process context but it usually is manageable with sensible
split between what gets done with and without process context.  Maybe
having a mechanism to blur the distinction helps such situations
enough to justify introduction of new constructs but I feel somewhat
skeptical at this point.

So, those are my high level concerns.  I generally feel reserved about
it.  Let's see whether that changes with the actual review of the
usages.

> +struct closure;
> +typedef void (closure_fn) (struct closure *);
> +
> +typedef struct llist_head	closure_list_t;

Please just use llist_head directly.

> +#define CLOSURE_REMAINING_MASK	(~(~0 << 20))
> +#define CLOSURE_GUARD_MASK					\
> +	((1 << 20)|(1 << 22)|(1 << 24)|(1 << 26)|(1 << 28)|(1 << 30))
> +
> +	/*
> +	 * CLOSURE_RUNNING: Set when a closure is running (i.e. by
> +	 * closure_init() and when closure_put() runs then next function), and
> +	 * must be cleared before remaining hits 0. Primarily to help guard
> +	 * against incorrect usage and accidently transferring references.
> +	 * continue_at() and closure_return() clear it for you, if you're doing
> +	 * something unusual you can use closure_set_dead() which also helps
> +	 * annotate where references are being transferred.
> +	 *
> +	 * CLOSURE_BLOCKING: Causes closure_wait_event() to block, instead of
> +	 * waiting asynchronously
> +	 *
> +	 * CLOSURE_STACK: Sanity check - remaining should never hit 0 on a
> +	 * closure with this flag set
> +	 *
> +	 * CLOSURE_WAITING: Set iff the closure is on a waitlist. Must be set by
> +	 * the thread that owns the closure, and cleared by the thread that's
> +	 * waking up the closure.
> +	 *
> +	 * CLOSURE_SLEEPING: Must be set before a thread uses a closure to sleep
> +	 * - indicates that cl->task is valid and closure_put() may wake it up.
> +	 * Only set or cleared by the thread that owns the closure.
> +	 *
> +	 * CLOSURE_TIMER: Analagous to CLOSURE_WAITING, indicates that a closure
> +	 * has an outstanding timer. Must be set by the thread that owns the
> +	 * closure, and cleared by the timer function when the timer goes off.
> +	 */
> +
> +#define	CLOSURE_RUNNING		(1 << 21)
> +#define	CLOSURE_BLOCKING	(1 << 23)
> +#define CLOSURE_STACK		(1 << 25)
> +#define	CLOSURE_WAITING		(1 << 27)
> +#define	CLOSURE_SLEEPING	(1 << 29)
> +#define	CLOSURE_TIMER		(1 << 31)
> +	atomic_t		remaining;

So, these bits are to be set alonside refcnt in lower 20 bits and
there are guard bits between flag bits to detect cases where flag
modification over/underflow caused by atomic_add/sub(), right?
Hmmmm... I *hope* it were something dumber, especially given that the
flags seem to be mostly for debugging and it would be nice which ones
are for debugging and which ones are actually necessary for operation
with better overall explanation.

Also, I personally don't like embedding constant definitions inside
struct definition and wish these were defined outside as enums but
that's just my preference.

> +#ifdef CONFIG_DEBUG_CLOSURES
> +#define CLOSURE_MAGIC_DEAD	0xc1054e0dead
> +#define CLOSURE_MAGIC_ALIVE	0xc1054e0a11e
> +
> +	unsigned long		magic;
> +	struct list_head	all;
> +	unsigned long		ip;
> +	unsigned long		waiting_on;
> +#endif
> +};

Maybe using debugobj is better for debugging object lifetime issues?

> +#define __CL_TYPE(cl, _t)						\
> +	  __builtin_types_compatible_p(typeof(cl), struct _t)		\
> +		? TYPE_ ## _t :						\
> +
> +#define __closure_type(cl)						\
> +(									\
> +	__CL_TYPE(cl, closure)						\
> +	__CL_TYPE(cl, closure_with_waitlist)				\
> +	__CL_TYPE(cl, closure_with_timer)				\
> +	__CL_TYPE(cl, closure_with_waitlist_and_timer)			\
> +	invalid_closure_type()						\
> +)

Again, I with this were dumber and more conventional.  Not everything
has to be smart and it's not like this can cover all cases anyway.
What about INITIALIZERs?  I'd suggest just following what everyone
else does.

> +#define closure_init_type(cl, parent, running, memset)		\
> +do {								\
> +	struct closure *_cl = __to_internal_closure(cl);	\
> +	_cl->type = __closure_type(*(cl));			\
> +	closure_set_ip(_cl);					\
> +	do_closure_init(_cl, parent, running);			\
> +} while (0)

Why is @memset unused?  And is it actually worth having a separate
initializer to avoid memset?

> +/**
> + * closure_sleep() - asynchronous sleep

Heh, I would much prefer delayed queue (or something along that line).
Sleep has a lot of connotation attached to it and asynchronous sleep
is way too confusing.

> +#define closure_flush(cl)				\
> +	__closure_flush(__to_internal_closure(cl), &(cl)->timer)
> +
> +#define closure_flush_sync(cl)				\
> +	__closure_flush_sync(__to_internal_closure(cl), &(cl)->timer)

Is this distinction really necessary?

> +/**
> + * closure_wake_up() - wake up all closures on a wait list.
> + */
> +static inline void closure_wake_up(closure_list_t *list)
> +{
> +	smp_mb();

Why is this mb() instead of wmb()?  Which barrier is it paired with?
In general, please always annotate barrier pairs when using barriers.
Nothing causes more head scratches than memory barriers without proper
explanation.

> +	__closure_wake_up(list);
> +}
> +
> +/*
> + * Wait on an event, synchronously or asynchronously - analagous to wait_event()
> + * but for closures.
> + *
> + * The loop is oddly structured so as to avoid a race; we must check the
> + * condition again after we've added ourself to the waitlist. We know if we were
> + * already on the waitlist because closure_wait() returns false; thus, we only
> + * schedule or break if closure_wait() returns false. If it returns true, we
> + * just loop again - rechecking the condition.
> + *
> + * The __closure_wake_up() is necessary because we may race with the event
> + * becoming true; i.e. we see event false -> wait -> recheck condition, but the
> + * thread that made the event true may have called closure_wake_up() before we
> + * added ourself to the wait list.
> + *
> + * We have to call closure_sync() at the end instead of just
> + * __closure_end_sleep() because a different thread might've called
> + * closure_wake_up() before us and gotten preempted before they dropped the
> + * refcount on our closure. If this was a stack allocated closure, that would be
> + * bad.
> + */
> +#define __closure_wait_event(list, cl, condition, _block)		\
> +({									\
> +	__label__ out;							\
> +	bool block = _block;						\

What if @condition contains @block?  Local vars in macros better have
long and ugly prefixes.

> +	typeof(condition) ret;						\
> +									\
> +	while (!(ret = (condition))) {					\
> +		if (block)						\
> +			__closure_start_sleep(cl);			\
> +		if (!closure_wait(list, cl)) {				\
> +			if (!block)					\
> +				goto out;				\
> +			schedule();					\
> +		}							\
> +	}								\
> +	__closure_wake_up(list);					\
> +	if (block)							\
> +		closure_sync(cl);					\
> +out:									\

I suppose __label__ out decl makes this local somehow.  I'd *much*
prefer if something this unusual wasn't used tho.

> +static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
> +				  struct workqueue_struct *wq)
> +{
> +	cl->fn = fn;
> +	cl->wq = wq;
> +	/* between atomic_dec() in closure_put() */
> +	smp_mb__before_atomic_dec();

Please be more explicit.  I can't follow.

> +#define continue_at(_cl, _fn, _wq, ...)					\
> +do {									\
> +	BUG_ON(!(_cl) || object_is_on_stack(_cl));			\
> +	closure_set_ip(_cl);						\
> +	set_closure_fn(_cl, _fn, _wq);					\
> +	closure_sub(_cl, CLOSURE_RUNNING + 1);				\
> +	return __VA_ARGS__;						\
> +} while (0)
> +
> +#define closure_return(_cl)	continue_at((_cl), NULL, NULL)

Umm... so, anything which wraps something which are baked into
people's reflexes is a bad idea.  It might seem to increase
writability or whatnot somewhat and could feel fun but the accumulated
overhead of all other people going WTF when trying to read the code
far outweights any benefit which can be gained that way.  So, please
don't bury return in a macro.  It's just a bad idea.

> +void closure_queue(struct closure *cl)
> +{
> +	struct workqueue_struct *wq = cl->wq;
> +	if (wq) {
> +		cl->work.data = (atomic_long_t) WORK_DATA_INIT();
> +		INIT_LIST_HEAD(&cl->work.entry);
> +		BUG_ON(!queue_work(wq, &cl->work));
> +	} else
> +		cl->fn(cl);

Umm... so the code is taking advantage of how fields of work_struct is
laid and using internal initializer to partially initialize work item?
Am I reading it correctly?  If so, please don't do this.  It's way too
fragile.

> +#define CL_FIELD(type, field)					\
> +	case TYPE_ ## type:					\
> +	return &container_of(cl, struct type, cl)->field
> +
> +static closure_list_t *closure_waitlist(struct closure *cl)
> +{
> +	switch (cl->type) {
> +		CL_FIELD(closure_with_waitlist, wait);
> +		CL_FIELD(closure_with_waitlist_and_timer, wait);
> +	}
> +	return NULL;
> +}
> +
> +static struct timer_list *closure_timer(struct closure *cl)
> +{
> +	switch (cl->type) {
> +		CL_FIELD(closure_with_timer, timer);
> +		CL_FIELD(closure_with_waitlist_and_timer, timer);
> +	}
> +	return NULL;
> +}

C code trumps macros.  There are only four of them.  Let's just open
code.

> +static void closure_put_after_sub(struct closure *cl, int r)
> +{
> +	BUG_ON(r & CLOSURE_GUARD_MASK);
> +	/* CLOSURE_BLOCK is the only flag that's allowed when r hits 0 */
> +	BUG_ON((r & CLOSURE_REMAINING_MASK) == 0 &&
> +	       (r & ~CLOSURE_BLOCKING));
> +
> +	/* Must deliver precisely one wakeup */
> +	if ((r & CLOSURE_REMAINING_MASK) == 1 &&
> +	    (r & CLOSURE_SLEEPING)) {
> +		smp_mb__after_atomic_dec();

Why?  wake_up_process() includes smp_wmb() and atomic_sub_return() has
full smp_mb().

> +		wake_up_process(cl->task);
> +	}
> +
> +	if ((r & CLOSURE_REMAINING_MASK) == 0) {
> +		smp_mb__after_atomic_dec();

Ditto.

> +		if (cl->fn) {
> +			/* CLOSURE_BLOCKING might be set - clear it */
> +			atomic_set(&cl->remaining,
> +				   CLOSURE_REMAINING_INITIALIZER);
> +			closure_queue(cl);
> +		} else {
> +			struct closure *parent = cl->parent;
> +			closure_list_t *wait = closure_waitlist(cl);
> +
> +			closure_debug_destroy(cl);
> +
> +			smp_wmb();

Why?  What is it paired with?  The only thing between the previous
smp_mb() and here is closure_debug_destroy().

> +			/* mb between last use of closure and unlocking it */
> +			atomic_set(&cl->remaining, -1);
> +
> +			if (wait)
> +				closure_wake_up(wait);
> +
> +			if (parent)
> +				closure_put(parent);
> +		}
> +	}
> +}
...
> +/*
> + * Broken up because closure_put() has to do the xchg() and grab the wait list
> + * before unlocking the closure, but the wakeup has to come after unlocking the
> + * closure.
> + */

Doesn't seem to be used by closure_put().  Stale comment?

> +static void closure_wake_up_after_xchg(struct llist_node *list)
> +{
> +	struct closure *cl;
> +	struct llist_node *reverse = NULL;
> +
> +	while (list) {
> +		struct llist_node *t = list;
> +		list = llist_next(list);
> +
> +		t->next = reverse;
> +		reverse = t;
> +	}
> +
> +	while (reverse) {
> +		cl = container_of(reverse, struct closure, list);
> +		reverse = llist_next(reverse);
> +
> +		set_waiting(cl, 0);
> +		closure_sub(cl, CLOSURE_WAITING + 1);

Why is it done in reverse?  To keep FIFO order?  If so, what
difference does that make and why no explanation?

> +/**
> + * closure_sync() - sleep until a closure a closure has nothing left to wait on

                                   ^^^^^^^^^^^^^^^^^^^
> + *
> + * Sleeps until the refcount hits 1 - the thread that's running the closure owns
> + * the last refcount.
> + */
> +void closure_sync(struct closure *cl)
> +{
> +	while (1) {
> +		__closure_start_sleep(cl);
> +		closure_set_ret_ip(cl);
> +
> +		if ((atomic_read(&cl->remaining) &
> +		     CLOSURE_REMAINING_MASK) == 1)
> +			break;
> +
> +		schedule();
> +	}
> +
> +	__closure_end_sleep(cl);
> +}
> +EXPORT_SYMBOL_GPL(closure_sync);
...
> +void __closure_flush(struct closure *cl, struct timer_list *timer)
> +{
> +	if (del_timer(timer))
> +		closure_sub(cl, CLOSURE_TIMER + 1);
> +}
> +EXPORT_SYMBOL_GPL(__closure_flush);
> +
> +void __closure_flush_sync(struct closure *cl, struct timer_list *timer)
> +{
> +	if (del_timer_sync(timer))
> +		closure_sub(cl, CLOSURE_TIMER + 1);
> +}
> +EXPORT_SYMBOL_GPL(__closure_flush_sync);

I'm kinda lost why this distinction is necessary.  Can you please
explain a bit?

Thanks.

-- 
tejun
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