Re: [lustre-devel] [PATCH 04/10] staging: lustre: lu_object: move retry logic inside htable_lookup

["Dilger, Andreas" <andreas.dilger@intel.com>]

On Apr 30, 2018, at 21:52, NeilBrown <neilb@suse.com> wrote:
> 
> The current retry logic, to wait when a 'dying' object is found,
> spans multiple functions.  The process is attached to a waitqueue
> and set TASK_UNINTERRUPTIBLE in htable_lookup, and this status
> is passed back through lu_object_find_try() to lu_object_find_at()
> where schedule() is called and the process is removed from the queue.
> 
> This can be simplified by moving all the logic (including
> hashtable locking) inside htable_lookup(), which now never returns
> EAGAIN.
> 
> Note that htable_lookup() is called with the hash bucket lock
> held, and will drop and retake it if it needs to schedule.
> 
> I made this a 'goto' loop rather than a 'while(1)' loop as the
> diff is easier to read.
> 
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
> drivers/staging/lustre/lustre/obdclass/lu_object.c |   73 +++++++-------------
> 1 file changed, 27 insertions(+), 46 deletions(-)
> 
> diff --git a/drivers/staging/lustre/lustre/obdclass/lu_object.c b/drivers/staging/lustre/lustre/obdclass/lu_object.c
> index 2bf089817157..93daa52e2535 100644
> --- a/drivers/staging/lustre/lustre/obdclass/lu_object.c
> +++ b/drivers/staging/lustre/lustre/obdclass/lu_object.c
> @@ -586,16 +586,21 @@ EXPORT_SYMBOL(lu_object_print);
> static struct lu_object *htable_lookup(struct lu_site *s,

It's probably a good idea to add a comment for this function that it may
drop and re-acquire the hash bucket lock internally.

> 				       struct cfs_hash_bd *bd,
> 				       const struct lu_fid *f,
> -				       wait_queue_entry_t *waiter,
> 				       __u64 *version)
> {
> +	struct cfs_hash		*hs = s->ls_obj_hash;
> 	struct lu_site_bkt_data *bkt;
> 	struct lu_object_header *h;
> 	struct hlist_node	*hnode;
> -	__u64  ver = cfs_hash_bd_version_get(bd);
> +	__u64 ver;
> +	wait_queue_entry_t waiter;
> 
> -	if (*version == ver)
> +retry:
> +	ver = cfs_hash_bd_version_get(bd);
> +
> +	if (*version == ver) {
> 		return ERR_PTR(-ENOENT);
> +	}

(style) we don't need the {} around a single-line if statement

> 	*version = ver;
> 	bkt = cfs_hash_bd_extra_get(s->ls_obj_hash, bd);
> @@ -625,11 +630,15 @@ static struct lu_object *htable_lookup(struct lu_site *s,
> 	 * drained), and moreover, lookup has to wait until object is freed.
> 	 */
> 
> -	init_waitqueue_entry(waiter, current);
> -	add_wait_queue(&bkt->lsb_marche_funebre, waiter);
> +	init_waitqueue_entry(&waiter, current);
> +	add_wait_queue(&bkt->lsb_marche_funebre, &waiter);
> 	set_current_state(TASK_UNINTERRUPTIBLE);
> 	lprocfs_counter_incr(s->ls_stats, LU_SS_CACHE_DEATH_RACE);
> -	return ERR_PTR(-EAGAIN);
> +	cfs_hash_bd_unlock(hs, bd, 1);

This looks like it isn't unlocking and locking the hash bucket in the same
manner that it was done in the caller.  Here excl = 1, but in the caller
you changed it to excl = 0?

> +	schedule();
> +	remove_wait_queue(&bkt->lsb_marche_funebre, &waiter);

Is it worthwhile to use your new helper function here to get the wq from "s"?

> +	cfs_hash_bd_lock(hs, bd, 1);
> +	goto retry;
> }
> 
> /**
> @@ -693,13 +702,14 @@ static struct lu_object *lu_object_new(const struct lu_env *env,
> }
> 
> /**
> - * Core logic of lu_object_find*() functions.
> + * Much like lu_object_find(), but top level device of object is specifically
> + * \a dev rather than top level device of the site. This interface allows
> + * objects of different "stacking" to be created within the same site.
>  */
> -static struct lu_object *lu_object_find_try(const struct lu_env *env,
> -					    struct lu_device *dev,
> -					    const struct lu_fid *f,
> -					    const struct lu_object_conf *conf,
> -					    wait_queue_entry_t *waiter)
> +struct lu_object *lu_object_find_at(const struct lu_env *env,
> +				    struct lu_device *dev,
> +				    const struct lu_fid *f,
> +				    const struct lu_object_conf *conf)
> {
> 	struct lu_object      *o;
> 	struct lu_object      *shadow;
> @@ -725,17 +735,16 @@ static struct lu_object *lu_object_find_try(const struct lu_env *env,
> 	 * It is unnecessary to perform lookup-alloc-lookup-insert, instead,
> 	 * just alloc and insert directly.
> 	 *
> -	 * If dying object is found during index search, add @waiter to the
> -	 * site wait-queue and return ERR_PTR(-EAGAIN).
> 	 */
> 	if (conf && conf->loc_flags & LOC_F_NEW)
> 		return lu_object_new(env, dev, f, conf);
> 
> 	s  = dev->ld_site;
> 	hs = s->ls_obj_hash;
> -	cfs_hash_bd_get_and_lock(hs, (void *)f, &bd, 1);
> -	o = htable_lookup(s, &bd, f, waiter, &version);
> -	cfs_hash_bd_unlock(hs, &bd, 1);
> +	cfs_hash_bd_get_and_lock(hs, (void *)f, &bd, 0);
> +	o = htable_lookup(s, &bd, f, &version);
> +	cfs_hash_bd_unlock(hs, &bd, 0);

Here you changed the locking to a non-exclusive (read) lock instead of an
exclusive (write) lock?  Why.

> +
> 	if (!IS_ERR(o) || PTR_ERR(o) != -ENOENT)
> 		return o;
> 
> @@ -751,7 +760,7 @@ static struct lu_object *lu_object_find_try(const struct lu_env *env,
> 
> 	cfs_hash_bd_lock(hs, &bd, 1);
> 
> -	shadow = htable_lookup(s, &bd, f, waiter, &version);
> +	shadow = htable_lookup(s, &bd, f, &version);
> 	if (likely(PTR_ERR(shadow) == -ENOENT)) {
> 		cfs_hash_bd_add_locked(hs, &bd, &o->lo_header->loh_hash);
> 		cfs_hash_bd_unlock(hs, &bd, 1);
> @@ -766,34 +775,6 @@ static struct lu_object *lu_object_find_try(const struct lu_env *env,
> 	lu_object_free(env, o);
> 	return shadow;
> }
> -
> -/**
> - * Much like lu_object_find(), but top level device of object is specifically
> - * \a dev rather than top level device of the site. This interface allows
> - * objects of different "stacking" to be created within the same site.
> - */
> -struct lu_object *lu_object_find_at(const struct lu_env *env,
> -				    struct lu_device *dev,
> -				    const struct lu_fid *f,
> -				    const struct lu_object_conf *conf)
> -{
> -	wait_queue_head_t	*wq;
> -	struct lu_object	*obj;
> -	wait_queue_entry_t	   wait;
> -
> -	while (1) {
> -		obj = lu_object_find_try(env, dev, f, conf, &wait);
> -		if (obj != ERR_PTR(-EAGAIN))
> -			return obj;
> -		/*
> -		 * lu_object_find_try() already added waiter into the
> -		 * wait queue.
> -		 */
> -		schedule();
> -		wq = lu_site_wq_from_fid(dev->ld_site, (void *)f);
> -		remove_wait_queue(wq, &wait);
> -	}
> -}
> EXPORT_SYMBOL(lu_object_find_at);
> 
> /**
> 
> 
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Cheers, Andreas
--
Andreas Dilger
Lustre Principal Architect
Intel Corporation