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Subject[PATCH 4/6] staging: lustre: use wait_event_var() in lu_context_key_degister()
lu_context_key_degister() has an open coded loop which calls
schedule() without setting a new task state. This is generally
a bad idea - it could easily just spin.

Instead, use wait_event_var() to wait for ->lct_used to be zero,
and arrange to get a wakeup when that happens.
Previously ->lct_used would only fall down to 1. Now we decrement
it an extra time so that wake_up, which only happens when the
count reaches zero, will only happen when lu_context_key_degister()
is actually waiting for it.

Note that this patch removes key_fini() from protection by
lu_keys_guard. key_fini() calls are not always protected
by a lock, and there seems to be no need here. Nothing else
can be acting on the given key in that context at this point,
so no race is possible.

Signed-off-by: NeilBrown <neilb@suse.com>
---
drivers/staging/lustre/lustre/obdclass/lu_object.c | 18 +++++++-----------
1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/drivers/staging/lustre/lustre/obdclass/lu_object.c b/drivers/staging/lustre/lustre/obdclass/lu_object.c
index 7bcf5ee47e04..f1c35a71c413 100644
--- a/drivers/staging/lustre/lustre/obdclass/lu_object.c
+++ b/drivers/staging/lustre/lustre/obdclass/lu_object.c
@@ -1372,7 +1372,8 @@ static void key_fini(struct lu_context *ctx, int index)

key->lct_fini(ctx, key, ctx->lc_value[index]);
lu_ref_del(&key->lct_reference, "ctx", ctx);
- atomic_dec(&key->lct_used);
+ if (atomic_dec_and_test(&key->lct_used))
+ wake_up_var(&key->lct_used);

if ((ctx->lc_tags & LCT_NOREF) == 0)
module_put(key->lct_owner);
@@ -1390,28 +1391,23 @@ void lu_context_key_degister(struct lu_context_key *key)

lu_context_key_quiesce(key);

- write_lock(&lu_keys_guard);
key_fini(&lu_shrink_env.le_ctx, key->lct_index);

/**
* Wait until all transient contexts referencing this key have
* run lu_context_key::lct_fini() method.
*/
- while (atomic_read(&key->lct_used) > 1) {
- write_unlock(&lu_keys_guard);
- CDEBUG(D_INFO, "%s: \"%s\" %p, %d\n",
- __func__, module_name(key->lct_owner),
- key, atomic_read(&key->lct_used));
- schedule();
- write_lock(&lu_keys_guard);
- }
+ atomic_dec(&key->lct_used);
+ wait_var_event(&key->lct_used, atomic_read(&key->lct_used) == 0);
+
+ write_lock(&lu_keys_guard);
if (lu_keys[key->lct_index]) {
lu_keys[key->lct_index] = NULL;
lu_ref_fini(&key->lct_reference);
}
write_unlock(&lu_keys_guard);

- LASSERTF(atomic_read(&key->lct_used) == 1,
+ LASSERTF(atomic_read(&key->lct_used) == 0,
"key has instances: %d\n",
atomic_read(&key->lct_used));
}

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