[PATCH v6.11-rc1 09/10] dlm: move lkb xarray lookup out of lock

Alexander Aring
2024-07-29T19:36:47Z

This patch moves the xarray lookup functionality for the lkb out of the
ls_lkbxa_lock read lock handling. We can do that as the xarray should be
possible to access lockless in case of reader like xa_load(). We confirm
under ls_lkbxa_lock that the lkb is still part of the data structure and
take a reference when its still part of ls_lkbxa to avoid being freed
after doing the lookup. To do a check if the lkb is still part of the
ls_lkbxa data structure we use a kref_read() as the last put will remove
it from the ls_lkbxa data structure and any reference taken means it is
still part of ls_lkbxa.

A similar approach was done with the DLM rsb rhashtable just with a flag
instead of the refcounter because the refcounter has a slightly
different meaning.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
---
fs/dlm/dlm_internal.h | 1 +
fs/dlm/lock.c | 18 ++++++++++++++----
fs/dlm/memory.c | 9 ++++++++-
3 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/fs/dlm/dlm_internal.h b/fs/dlm/dlm_internal.h
index 0562099e60eb..d534a4bc162b 100644
--- a/fs/dlm/dlm_internal.h
+++ b/fs/dlm/dlm_internal.h
@@ -295,6 +295,7 @@ struct dlm_lkb {
void *lkb_astparam; /* caller's ast arg */
struct dlm_user_args *lkb_ua;
};
+ struct rcu_head rcu;
};

/*
diff --git a/fs/dlm/lock.c b/fs/dlm/lock.c
index 9d3ec359d5e3..865dc70a9dfc 100644
--- a/fs/dlm/lock.c
+++ b/fs/dlm/lock.c
@@ -1527,11 +1527,21 @@ static int find_lkb(struct dlm_ls *ls, uint32_t lkid, struct dlm_lkb **lkb_ret)
{
struct dlm_lkb *lkb;

- read_lock_bh(&ls->ls_lkbxa_lock);
+ rcu_read_lock();
lkb = xa_load(&ls->ls_lkbxa, lkid);
- if (lkb)
- kref_get(&lkb->lkb_ref);
- read_unlock_bh(&ls->ls_lkbxa_lock);
+ if (lkb) {
+ /* check if lkb is still part of lkbxa under lkbxa_lock as
+ * the lkb_ref is tight to the lkbxa data structure, see
+ * __put_lkb().
+ */
+ read_lock_bh(&ls->ls_lkbxa_lock);
+ if (kref_read(&lkb->lkb_ref))
+ kref_get(&lkb->lkb_ref);
+ else
+ lkb = NULL;
+ read_unlock_bh(&ls->ls_lkbxa_lock);
+ }
+ rcu_read_unlock();

*lkb_ret = lkb;
return lkb ? 0 : -ENOENT;
diff --git a/fs/dlm/memory.c b/fs/dlm/memory.c
index 442898cf7185..5c35cc67aca4 100644
--- a/fs/dlm/memory.c
+++ b/fs/dlm/memory.c
@@ -115,8 +115,10 @@ struct dlm_lkb *dlm_allocate_lkb(void)
return kmem_cache_zalloc(lkb_cache, GFP_ATOMIC);
}

-void dlm_free_lkb(struct dlm_lkb *lkb)
+static void __free_lkb_rcu(struct rcu_head *rcu)
{
+ struct dlm_lkb *lkb = container_of(rcu, struct dlm_lkb, rcu);
+
if (test_bit(DLM_DFL_USER_BIT, &lkb->lkb_dflags)) {
struct dlm_user_args *ua;
ua = lkb->lkb_ua;
@@ -129,6 +131,11 @@ void dlm_free_lkb(struct dlm_lkb *lkb)
kmem_cache_free(lkb_cache, lkb);
}

+void dlm_free_lkb(struct dlm_lkb *lkb)
+{
+ call_rcu(&lkb->rcu, __free_lkb_rcu);
+}
+
struct dlm_mhandle *dlm_allocate_mhandle(void)
{
return kmem_cache_alloc(mhandle_cache, GFP_ATOMIC);
--
2.43.0