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Subject[PATCH 1/6] staging: lustre: make key_set_version an atomic_t
As a first step to simplifying the locking in lu_object.c,
change key_set_version to an atomic_t. This will mean
we don't need to hold a spinlock when incrementing.

It is clear that keys_fill() (and so lu_context_refill())
cannot race with itself as it holds no locks for the main
part of the work. So updates to lc_version, and testing of
that value cannot need locking either.
So remove the locking from keys_fill() and lu_context_refill()
as there is no longer anything to protect. The locking around
lu_keys_initing_cnt is preserved for now.

Also, don't increment when deregistering or quiescing a key.
key_set_version is *only* use to avoid filling new key values
if there have been no changes to the set of key.
Deregistering a key does not mean that we need to try filling
any new value, so the increment is pointless.

Finally, remove the refill loop in keys_fill(). If a key
is registered or revived while keys_fill() is running it must be safe
to ignore it just as we would if it was registered immediately
after keys_fill() ran. The important thing is that the
keys_set_version stored in ctx->lc_version must be sampled
*before* those unseen keys were added.
So sample keys_set_version early.

Signed-off-by: NeilBrown <neilb@suse.com>
---
drivers/staging/lustre/lustre/obdclass/lu_object.c | 26 ++++----------------
1 file changed, 5 insertions(+), 21 deletions(-)

diff --git a/drivers/staging/lustre/lustre/obdclass/lu_object.c b/drivers/staging/lustre/lustre/obdclass/lu_object.c
index f14e3509f059..b38adf75e8e4 100644
--- a/drivers/staging/lustre/lustre/obdclass/lu_object.c
+++ b/drivers/staging/lustre/lustre/obdclass/lu_object.c
@@ -1329,7 +1329,7 @@ static atomic_t lu_key_initing_cnt = ATOMIC_INIT(0);
* lu_context_refill(). No locking is provided, as initialization and shutdown
* are supposed to be externally serialized.
*/
-static unsigned int key_set_version;
+static atomic_t key_set_version = ATOMIC_INIT(0);

/**
* Register new key.
@@ -1352,7 +1352,7 @@ int lu_context_key_register(struct lu_context_key *key)
lu_keys[i] = key;
lu_ref_init(&key->lct_reference);
result = 0;
- ++key_set_version;
+ atomic_inc(&key_set_version);
break;
}
}
@@ -1390,7 +1390,6 @@ void lu_context_key_degister(struct lu_context_key *key)
lu_context_key_quiesce(key);

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

/**
@@ -1550,7 +1549,6 @@ void lu_context_key_quiesce(struct lu_context_key *key)
list_for_each_entry(ctx, &lu_context_remembered, lc_remember)
key_fini(ctx, key->lct_index);

- ++key_set_version;
write_unlock(&lu_keys_guard);
}
}
@@ -1559,7 +1557,7 @@ void lu_context_key_revive(struct lu_context_key *key)
{
write_lock(&lu_keys_guard);
key->lct_tags &= ~LCT_QUIESCENT;
- ++key_set_version;
+ atomic_inc(&key_set_version);
write_unlock(&lu_keys_guard);
}

@@ -1579,7 +1577,6 @@ static void keys_fini(struct lu_context *ctx)

static int keys_fill(struct lu_context *ctx)
{
- unsigned int pre_version;
unsigned int i;

/*
@@ -1593,10 +1590,9 @@ static int keys_fill(struct lu_context *ctx)
*/
read_lock(&lu_keys_guard);
atomic_inc(&lu_key_initing_cnt);
- pre_version = key_set_version;
read_unlock(&lu_keys_guard);
+ ctx->lc_version = atomic_read(&key_set_version);

-refill:
LINVRNT(ctx->lc_value);
for (i = 0; i < ARRAY_SIZE(lu_keys); ++i) {
struct lu_context_key *key;
@@ -1639,15 +1635,7 @@ static int keys_fill(struct lu_context *ctx)
}
}

- read_lock(&lu_keys_guard);
- if (pre_version != key_set_version) {
- pre_version = key_set_version;
- read_unlock(&lu_keys_guard);
- goto refill;
- }
- ctx->lc_version = key_set_version;
atomic_dec(&lu_key_initing_cnt);
- read_unlock(&lu_keys_guard);
return 0;
}

@@ -1756,13 +1744,9 @@ EXPORT_SYMBOL(lu_context_exit);
*/
int lu_context_refill(struct lu_context *ctx)
{
- read_lock(&lu_keys_guard);
- if (likely(ctx->lc_version == key_set_version)) {
- read_unlock(&lu_keys_guard);
+ if (likely(ctx->lc_version == atomic_read(&key_set_version)))
return 0;
- }

- read_unlock(&lu_keys_guard);
return keys_fill(ctx);
}


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