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    Subject[PATCH v7 01/16] lockdep: Refactor lookup_chain_cache()
    Date
    Currently, lookup_chain_cache() provides both 'lookup' and 'add'
    functionalities in a function. However, each is useful. So this
    patch makes lookup_chain_cache() only do 'lookup' functionality and
    makes add_chain_cahce() only do 'add' functionality. And it's more
    readable than before.

    Signed-off-by: Byungchul Park <byungchul.park@lge.com>
    ---
    kernel/locking/lockdep.c | 132 ++++++++++++++++++++++++++++++-----------------
    1 file changed, 86 insertions(+), 46 deletions(-)

    diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
    index 4d7ffc0..0c6e6b7 100644
    --- a/kernel/locking/lockdep.c
    +++ b/kernel/locking/lockdep.c
    @@ -2110,14 +2110,15 @@ static int check_no_collision(struct task_struct *curr,
    }

    /*
    - * Look up a dependency chain. If the key is not present yet then
    - * add it and return 1 - in this case the new dependency chain is
    - * validated. If the key is already hashed, return 0.
    - * (On return with 1 graph_lock is held.)
    + * Adds a dependency chain into chain hashtable. And must be called with
    + * graph_lock held.
    + *
    + * Return 0 if fail, and graph_lock is released.
    + * Return 1 if succeed, with graph_lock held.
    */
    -static inline int lookup_chain_cache(struct task_struct *curr,
    - struct held_lock *hlock,
    - u64 chain_key)
    +static inline int add_chain_cache(struct task_struct *curr,
    + struct held_lock *hlock,
    + u64 chain_key)
    {
    struct lock_class *class = hlock_class(hlock);
    struct hlist_head *hash_head = chainhashentry(chain_key);
    @@ -2125,49 +2126,18 @@ static inline int lookup_chain_cache(struct task_struct *curr,
    int i, j;

    /*
    + * Allocate a new chain entry from the static array, and add
    + * it to the hash:
    + */
    +
    + /*
    * We might need to take the graph lock, ensure we've got IRQs
    * disabled to make this an IRQ-safe lock.. for recursion reasons
    * lockdep won't complain about its own locking errors.
    */
    if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
    return 0;
    - /*
    - * We can walk it lock-free, because entries only get added
    - * to the hash:
    - */
    - hlist_for_each_entry_rcu(chain, hash_head, entry) {
    - if (chain->chain_key == chain_key) {
    -cache_hit:
    - debug_atomic_inc(chain_lookup_hits);
    - if (!check_no_collision(curr, hlock, chain))
    - return 0;

    - if (very_verbose(class))
    - printk("\nhash chain already cached, key: "
    - "%016Lx tail class: [%p] %s\n",
    - (unsigned long long)chain_key,
    - class->key, class->name);
    - return 0;
    - }
    - }
    - if (very_verbose(class))
    - printk("\nnew hash chain, key: %016Lx tail class: [%p] %s\n",
    - (unsigned long long)chain_key, class->key, class->name);
    - /*
    - * Allocate a new chain entry from the static array, and add
    - * it to the hash:
    - */
    - if (!graph_lock())
    - return 0;
    - /*
    - * We have to walk the chain again locked - to avoid duplicates:
    - */
    - hlist_for_each_entry(chain, hash_head, entry) {
    - if (chain->chain_key == chain_key) {
    - graph_unlock();
    - goto cache_hit;
    - }
    - }
    if (unlikely(nr_lock_chains >= MAX_LOCKDEP_CHAINS)) {
    if (!debug_locks_off_graph_unlock())
    return 0;
    @@ -2219,6 +2189,75 @@ static inline int lookup_chain_cache(struct task_struct *curr,
    return 1;
    }

    +/*
    + * Look up a dependency chain.
    + */
    +static inline struct lock_chain *lookup_chain_cache(u64 chain_key)
    +{
    + struct hlist_head *hash_head = chainhashentry(chain_key);
    + struct lock_chain *chain;
    +
    + /*
    + * We can walk it lock-free, because entries only get added
    + * to the hash:
    + */
    + hlist_for_each_entry_rcu(chain, hash_head, entry) {
    + if (chain->chain_key == chain_key) {
    + debug_atomic_inc(chain_lookup_hits);
    + return chain;
    + }
    + }
    + return NULL;
    +}
    +
    +/*
    + * If the key is not present yet in dependency chain cache then
    + * add it and return 1 - in this case the new dependency chain is
    + * validated. If the key is already hashed, return 0.
    + * (On return with 1 graph_lock is held.)
    + */
    +static inline int lookup_chain_cache_add(struct task_struct *curr,
    + struct held_lock *hlock,
    + u64 chain_key)
    +{
    + struct lock_class *class = hlock_class(hlock);
    + struct lock_chain *chain = lookup_chain_cache(chain_key);
    +
    + if (chain) {
    +cache_hit:
    + if (!check_no_collision(curr, hlock, chain))
    + return 0;
    +
    + if (very_verbose(class))
    + printk("\nhash chain already cached, key: "
    + "%016Lx tail class: [%p] %s\n",
    + (unsigned long long)chain_key,
    + class->key, class->name);
    + return 0;
    + }
    +
    + if (very_verbose(class))
    + printk("\nnew hash chain, key: %016Lx tail class: [%p] %s\n",
    + (unsigned long long)chain_key, class->key, class->name);
    +
    + if (!graph_lock())
    + return 0;
    +
    + /*
    + * We have to walk the chain again locked - to avoid duplicates:
    + */
    + chain = lookup_chain_cache(chain_key);
    + if (chain) {
    + graph_unlock();
    + goto cache_hit;
    + }
    +
    + if (!add_chain_cache(curr, hlock, chain_key))
    + return 0;
    +
    + return 1;
    +}
    +
    static int validate_chain(struct task_struct *curr, struct lockdep_map *lock,
    struct held_lock *hlock, int chain_head, u64 chain_key)
    {
    @@ -2229,11 +2268,11 @@ static int validate_chain(struct task_struct *curr, struct lockdep_map *lock,
    *
    * We look up the chain_key and do the O(N^2) check and update of
    * the dependencies only if this is a new dependency chain.
    - * (If lookup_chain_cache() returns with 1 it acquires
    + * (If lookup_chain_cache_add() return with 1 it acquires
    * graph_lock for us)
    */
    if (!hlock->trylock && hlock->check &&
    - lookup_chain_cache(curr, hlock, chain_key)) {
    + lookup_chain_cache_add(curr, hlock, chain_key)) {
    /*
    * Check whether last held lock:
    *
    @@ -2264,9 +2303,10 @@ static int validate_chain(struct task_struct *curr, struct lockdep_map *lock,
    if (!chain_head && ret != 2)
    if (!check_prevs_add(curr, hlock))
    return 0;
    +
    graph_unlock();
    } else
    - /* after lookup_chain_cache(): */
    + /* after lookup_chain_cache_add(): */
    if (unlikely(!debug_locks))
    return 0;

    --
    1.9.1
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