From: Dave Chinner <[email protected]> hash-bl nests spinlocks inside the bit locks. This causes problems for CONFIG_PREEMPT_RT which converts spin locks to sleeping locks, and we're not allowed to sleep while holding a spinning lock.
Further, lockdep does not support bit locks, so we lose lockdep coverage of the inode hash table with the hash-bl conversion. To enable these configs to work, add an external per-chain spinlock to the hlist_bl_head() and add helpers to use this instead of the bit spinlock when preempt_rt or lockdep are enabled. This converts all users of hlist-bl to use the external spinlock in these situations, so we also gain lockdep coverage of things like the dentry cache hash table with this change. Signed-off-by: Dave Chinner <[email protected]> --- include/linux/list_bl.h | 126 ++++++++++++++++++++++++++++--------- include/linux/rculist_bl.h | 13 ++++ 2 files changed, 110 insertions(+), 29 deletions(-) diff --git a/include/linux/list_bl.h b/include/linux/list_bl.h index 8ee2bf5af131..990ad8e24e0b 100644 --- a/include/linux/list_bl.h +++ b/include/linux/list_bl.h @@ -4,14 +4,27 @@ #include <linux/list.h> #include <linux/bit_spinlock.h> +#include <linux/spinlock.h> /* * Special version of lists, where head of the list has a lock in the lowest * bit. This is useful for scalable hash tables without increasing memory * footprint overhead. * - * For modification operations, the 0 bit of hlist_bl_head->first - * pointer must be set. + * Whilst the general use of bit spin locking is considered safe, PREEMPT_RT + * introduces a problem with nesting spin locks inside bit locks: spin locks + * become sleeping locks, and we can't sleep inside spinning locks such as bit + * locks. However, for RTPREEMPT, performance is less of an issue than + * correctness, so we trade off the memory and cache footprint of a spinlock per + * list so the list locks are converted to sleeping locks and work correctly + * with PREEMPT_RT kernels. + * + * An added advantage of this is that we can use the same trick when lockdep is + * enabled (again, performance doesn't matter) and gain lockdep coverage of all + * the hash-bl operations. + * + * For modification operations when using pure bit locking, the 0 bit of + * hlist_bl_head->first pointer must be set. * * With some small modifications, this can easily be adapted to store several * arbitrary bits (not just a single lock bit), if the need arises to store @@ -30,16 +43,21 @@ #define LIST_BL_BUG_ON(x) #endif +#undef LIST_BL_USE_SPINLOCKS +#if defined(CONFIG_PREEMPT_RT) || defined(CONFIG_LOCKDEP) +#define LIST_BL_USE_SPINLOCKS 1 +#endif struct hlist_bl_head { struct hlist_bl_node *first; +#ifdef LIST_BL_USE_SPINLOCKS + spinlock_t lock; +#endif }; struct hlist_bl_node { struct hlist_bl_node *next, **pprev; }; -#define INIT_HLIST_BL_HEAD(ptr) \ - ((ptr)->first = NULL) static inline void INIT_HLIST_BL_NODE(struct hlist_bl_node *h) { @@ -54,6 +72,69 @@ static inline bool hlist_bl_unhashed(const struct hlist_bl_node *h) return !h->pprev; } +#ifdef LIST_BL_USE_SPINLOCKS +#define INIT_HLIST_BL_HEAD(ptr) do { \ + (ptr)->first = NULL; \ + spin_lock_init(&(ptr)->lock); \ +} while (0) + +static inline void hlist_bl_lock(struct hlist_bl_head *b) +{ + spin_lock(&b->lock); +} + +static inline void hlist_bl_unlock(struct hlist_bl_head *b) +{ + spin_unlock(&b->lock); +} + +static inline bool hlist_bl_is_locked(struct hlist_bl_head *b) +{ + return spin_is_locked(&b->lock); +} + +static inline struct hlist_bl_node *hlist_bl_first(struct hlist_bl_head *h) +{ + return h->first; +} + +static inline void hlist_bl_set_first(struct hlist_bl_head *h, + struct hlist_bl_node *n) +{ + h->first = n; +} + +static inline void hlist_bl_set_before(struct hlist_bl_node **pprev, + struct hlist_bl_node *n) +{ + WRITE_ONCE(*pprev, n); +} + +static inline bool hlist_bl_empty(const struct hlist_bl_head *h) +{ + return !READ_ONCE(h->first); +} + +#else /* !LIST_BL_USE_SPINLOCKS */ + +#define INIT_HLIST_BL_HEAD(ptr) \ + ((ptr)->first = NULL) + +static inline void hlist_bl_lock(struct hlist_bl_head *b) +{ + bit_spin_lock(0, (unsigned long *)b); +} + +static inline void hlist_bl_unlock(struct hlist_bl_head *b) +{ + __bit_spin_unlock(0, (unsigned long *)b); +} + +static inline bool hlist_bl_is_locked(struct hlist_bl_head *b) +{ + return bit_spin_is_locked(0, (unsigned long *)b); +} + static inline struct hlist_bl_node *hlist_bl_first(struct hlist_bl_head *h) { return (struct hlist_bl_node *) @@ -69,11 +150,21 @@ static inline void hlist_bl_set_first(struct hlist_bl_head *h, h->first = (struct hlist_bl_node *)((unsigned long)n | LIST_BL_LOCKMASK); } +static inline void hlist_bl_set_before(struct hlist_bl_node **pprev, + struct hlist_bl_node *n) +{ + WRITE_ONCE(*pprev, + (struct hlist_bl_node *) + ((uintptr_t)n | ((uintptr_t)*pprev & LIST_BL_LOCKMASK))); +} + static inline bool hlist_bl_empty(const struct hlist_bl_head *h) { return !((unsigned long)READ_ONCE(h->first) & ~LIST_BL_LOCKMASK); } +#endif /* LIST_BL_USE_SPINLOCKS */ + static inline void hlist_bl_add_head(struct hlist_bl_node *n, struct hlist_bl_head *h) { @@ -94,11 +185,7 @@ static inline void hlist_bl_add_before(struct hlist_bl_node *n, n->pprev = pprev; n->next = next; next->pprev = &n->next; - - /* pprev may be `first`, so be careful not to lose the lock bit */ - WRITE_ONCE(*pprev, - (struct hlist_bl_node *) - ((uintptr_t)n | ((uintptr_t)*pprev & LIST_BL_LOCKMASK))); + hlist_bl_set_before(pprev, n); } static inline void hlist_bl_add_behind(struct hlist_bl_node *n, @@ -119,11 +206,7 @@ static inline void __hlist_bl_del(struct hlist_bl_node *n) LIST_BL_BUG_ON((unsigned long)n & LIST_BL_LOCKMASK); - /* pprev may be `first`, so be careful not to lose the lock bit */ - WRITE_ONCE(*pprev, - (struct hlist_bl_node *) - ((unsigned long)next | - ((unsigned long)*pprev & LIST_BL_LOCKMASK))); + hlist_bl_set_before(pprev, next); if (next) next->pprev = pprev; } @@ -165,21 +248,6 @@ static inline bool hlist_bl_fake(struct hlist_bl_node *n) return n->pprev == &n->next; } -static inline void hlist_bl_lock(struct hlist_bl_head *b) -{ - bit_spin_lock(0, (unsigned long *)b); -} - -static inline void hlist_bl_unlock(struct hlist_bl_head *b) -{ - __bit_spin_unlock(0, (unsigned long *)b); -} - -static inline bool hlist_bl_is_locked(struct hlist_bl_head *b) -{ - return bit_spin_is_locked(0, (unsigned long *)b); -} - /** * hlist_bl_for_each_entry - iterate over list of given type * @tpos: the type * to use as a loop cursor. diff --git a/include/linux/rculist_bl.h b/include/linux/rculist_bl.h index 0b952d06eb0b..2d5eb5153121 100644 --- a/include/linux/rculist_bl.h +++ b/include/linux/rculist_bl.h @@ -8,6 +8,18 @@ #include <linux/list_bl.h> #include <linux/rcupdate.h> +#ifdef LIST_BL_USE_SPINLOCKS +static inline void hlist_bl_set_first_rcu(struct hlist_bl_head *h, + struct hlist_bl_node *n) +{ + rcu_assign_pointer(h->first, n); +} + +static inline struct hlist_bl_node *hlist_bl_first_rcu(struct hlist_bl_head *h) +{ + return rcu_dereference_check(h->first, hlist_bl_is_locked(h)); +} +#else /* !LIST_BL_USE_SPINLOCKS */ static inline void hlist_bl_set_first_rcu(struct hlist_bl_head *h, struct hlist_bl_node *n) { @@ -23,6 +35,7 @@ static inline struct hlist_bl_node *hlist_bl_first_rcu(struct hlist_bl_head *h) return (struct hlist_bl_node *) ((unsigned long)rcu_dereference_check(h->first, hlist_bl_is_locked(h)) & ~LIST_BL_LOCKMASK); } +#endif /* LIST_BL_USE_SPINLOCKS */ /** * hlist_bl_del_rcu - deletes entry from hash list without re-initialization -- 2.42.0
