Re: [PATCH 10/11] list_bl: don't use bit locks for PREEMPT_RT or lockdep

Kent Overstreet
2023-12-07T04:17:04Z

On Wed, Dec 06, 2023 at 05:05:39PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> 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 <dchinner@redhat.com>

Sleepable bit locks can be done with wait_on_bit(), is that worth
considering for PREEMPT_RT? Or are the other features of real locks
important there?

(not a request for the current patchset, just perhaps a note for future
work)

Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>

> ---
> 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
>