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Subject[PATCH v3 17/68] fscache: Implement simple cookie state machine
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Date
Implement a very simple cookie state machine to handle lookup,
invalidation, withdrawal, relinquishment and, to be added later, commit on
LRU discard.

Three cache methods are provided: ->lookup_cookie() to look up and, if
necessary, create a data storage object; ->withdraw_cookie() to free the
resources associated with that object and potentially delete it; and
->prepare_to_write(), to do prepare for changes to the cached data to be
modified locally.

Changes
=======
ver #3:
- Fix a race between LRU discard and relinquishment whereby the former
would override the latter and thus the latter would never happen[1].

ver #2:
- Don't hold n_accesses elevated whilst cache is bound to a cookie, but
rather add a flag that prevents the state machine from being queued when
n_accesses reaches 0.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/599331.1639410068@warthog.procyon.org.uk/ [1]
Link: https://lore.kernel.org/r/163819599657.215744.15799615296912341745.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906903925.143852.1805855338154353867.stgit@warthog.procyon.org.uk/ # v2
---

fs/fscache/cookie.c | 313 +++++++++++++++++++++++++++++++++++-----
include/linux/fscache-cache.h | 27 +++
include/trace/events/fscache.h | 4 +
3 files changed, 300 insertions(+), 44 deletions(-)

diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index 04d2127bd354..336046de08ee 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -15,7 +15,8 @@

struct kmem_cache *fscache_cookie_jar;

-static void fscache_drop_cookie(struct fscache_cookie *cookie);
+static void fscache_cookie_worker(struct work_struct *work);
+static void fscache_unhash_cookie(struct fscache_cookie *cookie);

#define fscache_cookie_hash_shift 15
static struct hlist_bl_head fscache_cookie_hash[1 << fscache_cookie_hash_shift];
@@ -62,6 +63,19 @@ static void fscache_free_cookie(struct fscache_cookie *cookie)
kmem_cache_free(fscache_cookie_jar, cookie);
}

+static void __fscache_queue_cookie(struct fscache_cookie *cookie)
+{
+ if (!queue_work(fscache_wq, &cookie->work))
+ fscache_put_cookie(cookie, fscache_cookie_put_over_queued);
+}
+
+static void fscache_queue_cookie(struct fscache_cookie *cookie,
+ enum fscache_cookie_trace where)
+{
+ fscache_get_cookie(cookie, where);
+ __fscache_queue_cookie(cookie);
+}
+
/*
* Initialise the access gate on a cookie by setting a flag to prevent the
* state machine from being queued when the access counter transitions to 0.
@@ -98,9 +112,8 @@ void fscache_end_cookie_access(struct fscache_cookie *cookie,
trace_fscache_access(cookie->debug_id, refcount_read(&cookie->ref),
n_accesses, why);
if (n_accesses == 0 &&
- !test_bit(FSCACHE_COOKIE_NO_ACCESS_WAKE, &cookie->flags)) {
- // PLACEHOLDER: Need to poke the state machine
- }
+ !test_bit(FSCACHE_COOKIE_NO_ACCESS_WAKE, &cookie->flags))
+ fscache_queue_cookie(cookie, fscache_cookie_get_end_access);
}
EXPORT_SYMBOL(fscache_end_cookie_access);

@@ -171,35 +184,58 @@ static inline void wake_up_cookie_state(struct fscache_cookie *cookie)
wake_up_var(&cookie->state);
}

+/*
+ * Change the state a cookie is at and wake up anyone waiting for that. Impose
+ * an ordering between the stuff stored in the cookie and the state member.
+ * Paired with fscache_cookie_state().
+ */
static void __fscache_set_cookie_state(struct fscache_cookie *cookie,
enum fscache_cookie_state state)
{
- cookie->state = state;
+ smp_store_release(&cookie->state, state);
}

-/*
- * Change the state a cookie is at and wake up anyone waiting for that - but
- * only if the cookie isn't already marked as being in a cleanup state.
- */
-void fscache_set_cookie_state(struct fscache_cookie *cookie,
- enum fscache_cookie_state state)
+static void fscache_set_cookie_state(struct fscache_cookie *cookie,
+ enum fscache_cookie_state state)
{
- bool changed = false;
-
spin_lock(&cookie->lock);
- switch (cookie->state) {
- case FSCACHE_COOKIE_STATE_RELINQUISHING:
- break;
- default:
- __fscache_set_cookie_state(cookie, state);
- changed = true;
- break;
- }
+ __fscache_set_cookie_state(cookie, state);
spin_unlock(&cookie->lock);
- if (changed)
- wake_up_cookie_state(cookie);
+ wake_up_cookie_state(cookie);
+}
+
+/**
+ * fscache_cookie_lookup_negative - Note negative lookup
+ * @cookie: The cookie that was being looked up
+ *
+ * Note that some part of the metadata path in the cache doesn't exist and so
+ * we can release any waiting readers in the certain knowledge that there's
+ * nothing for them to actually read.
+ *
+ * This function uses no locking and must only be called from the state machine.
+ */
+void fscache_cookie_lookup_negative(struct fscache_cookie *cookie)
+{
+ set_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags);
+ fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_CREATING);
+}
+EXPORT_SYMBOL(fscache_cookie_lookup_negative);
+
+/**
+ * fscache_caching_failed - Report that a failure stopped caching on a cookie
+ * @cookie: The cookie that was affected
+ *
+ * Tell fscache that caching on a cookie needs to be stopped due to some sort
+ * of failure.
+ *
+ * This function uses no locking and must only be called from the state machine.
+ */
+void fscache_caching_failed(struct fscache_cookie *cookie)
+{
+ clear_bit(FSCACHE_COOKIE_IS_CACHING, &cookie->flags);
+ fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_FAILED);
}
-EXPORT_SYMBOL(fscache_set_cookie_state);
+EXPORT_SYMBOL(fscache_caching_failed);

/*
* Set the index key in a cookie. The cookie struct has space for a 16-byte
@@ -291,10 +327,10 @@ static struct fscache_cookie *fscache_alloc_cookie(

refcount_set(&cookie->ref, 1);
cookie->debug_id = atomic_inc_return(&fscache_cookie_debug_id);
- cookie->state = FSCACHE_COOKIE_STATE_QUIESCENT;
spin_lock_init(&cookie->lock);
INIT_LIST_HEAD(&cookie->commit_link);
- INIT_WORK(&cookie->work, NULL /* PLACEHOLDER */);
+ INIT_WORK(&cookie->work, fscache_cookie_worker);
+ __fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_QUIESCENT);

write_lock(&fscache_cookies_lock);
list_add_tail(&cookie->proc_link, &fscache_cookies);
@@ -417,6 +453,192 @@ struct fscache_cookie *__fscache_acquire_cookie(
}
EXPORT_SYMBOL(__fscache_acquire_cookie);

+/*
+ * Prepare a cache object to be written to.
+ */
+static void fscache_prepare_to_write(struct fscache_cookie *cookie)
+{
+ cookie->volume->cache->ops->prepare_to_write(cookie);
+}
+
+/*
+ * Look up a cookie in the cache.
+ */
+static void fscache_perform_lookup(struct fscache_cookie *cookie)
+{
+ enum fscache_access_trace trace = fscache_access_lookup_cookie_end_failed;
+ bool need_withdraw = false;
+
+ _enter("");
+
+ if (!cookie->volume->cache_priv) {
+ fscache_create_volume(cookie->volume, true);
+ if (!cookie->volume->cache_priv) {
+ fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_QUIESCENT);
+ goto out;
+ }
+ }
+
+ if (!cookie->volume->cache->ops->lookup_cookie(cookie)) {
+ if (cookie->state != FSCACHE_COOKIE_STATE_FAILED)
+ fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_QUIESCENT);
+ need_withdraw = true;
+ _leave(" [fail]");
+ goto out;
+ }
+
+ fscache_see_cookie(cookie, fscache_cookie_see_active);
+ fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_ACTIVE);
+ trace = fscache_access_lookup_cookie_end;
+
+out:
+ fscache_end_cookie_access(cookie, trace);
+ if (need_withdraw)
+ fscache_withdraw_cookie(cookie);
+ fscache_end_volume_access(cookie->volume, cookie, trace);
+}
+
+/*
+ * Perform work upon the cookie, such as committing its cache state,
+ * relinquishing it or withdrawing the backing cache. We're protected from the
+ * cache going away under us as object withdrawal must come through this
+ * non-reentrant work item.
+ */
+static void fscache_cookie_state_machine(struct fscache_cookie *cookie)
+{
+ enum fscache_cookie_state state;
+ bool wake = false;
+
+ _enter("c=%x", cookie->debug_id);
+
+again:
+ spin_lock(&cookie->lock);
+again_locked:
+ state = cookie->state;
+ switch (state) {
+ case FSCACHE_COOKIE_STATE_QUIESCENT:
+ /* The QUIESCENT state is jumped to the LOOKING_UP state by
+ * fscache_use_cookie().
+ */
+
+ if (atomic_read(&cookie->n_accesses) == 0 &&
+ test_bit(FSCACHE_COOKIE_DO_RELINQUISH, &cookie->flags)) {
+ __fscache_set_cookie_state(cookie,
+ FSCACHE_COOKIE_STATE_RELINQUISHING);
+ wake = true;
+ goto again_locked;
+ }
+ break;
+
+ case FSCACHE_COOKIE_STATE_LOOKING_UP:
+ spin_unlock(&cookie->lock);
+ fscache_init_access_gate(cookie);
+ fscache_perform_lookup(cookie);
+ goto again;
+
+ case FSCACHE_COOKIE_STATE_ACTIVE:
+ if (test_and_clear_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags)) {
+ spin_unlock(&cookie->lock);
+ fscache_prepare_to_write(cookie);
+ spin_lock(&cookie->lock);
+ }
+ fallthrough;
+
+ case FSCACHE_COOKIE_STATE_FAILED:
+ if (atomic_read(&cookie->n_accesses) != 0)
+ break;
+ if (test_bit(FSCACHE_COOKIE_DO_RELINQUISH, &cookie->flags)) {
+ __fscache_set_cookie_state(cookie,
+ FSCACHE_COOKIE_STATE_RELINQUISHING);
+ wake = true;
+ goto again_locked;
+ }
+ if (test_bit(FSCACHE_COOKIE_DO_WITHDRAW, &cookie->flags)) {
+ __fscache_set_cookie_state(cookie,
+ FSCACHE_COOKIE_STATE_WITHDRAWING);
+ wake = true;
+ goto again_locked;
+ }
+ break;
+
+ case FSCACHE_COOKIE_STATE_RELINQUISHING:
+ case FSCACHE_COOKIE_STATE_WITHDRAWING:
+ if (cookie->cache_priv) {
+ spin_unlock(&cookie->lock);
+ cookie->volume->cache->ops->withdraw_cookie(cookie);
+ spin_lock(&cookie->lock);
+ }
+
+ switch (state) {
+ case FSCACHE_COOKIE_STATE_RELINQUISHING:
+ fscache_see_cookie(cookie, fscache_cookie_see_relinquish);
+ fscache_unhash_cookie(cookie);
+ __fscache_set_cookie_state(cookie,
+ FSCACHE_COOKIE_STATE_DROPPED);
+ wake = true;
+ goto out;
+ case FSCACHE_COOKIE_STATE_WITHDRAWING:
+ fscache_see_cookie(cookie, fscache_cookie_see_withdraw);
+ break;
+ default:
+ BUG();
+ }
+
+ clear_bit(FSCACHE_COOKIE_NEEDS_UPDATE, &cookie->flags);
+ clear_bit(FSCACHE_COOKIE_DO_WITHDRAW, &cookie->flags);
+ clear_bit(FSCACHE_COOKIE_DO_LRU_DISCARD, &cookie->flags);
+ clear_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags);
+ set_bit(FSCACHE_COOKIE_NO_DATA_TO_READ, &cookie->flags);
+ __fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_QUIESCENT);
+ wake = true;
+ goto again_locked;
+
+ case FSCACHE_COOKIE_STATE_DROPPED:
+ break;
+
+ default:
+ WARN_ONCE(1, "Cookie %x in unexpected state %u\n",
+ cookie->debug_id, state);
+ break;
+ }
+
+out:
+ spin_unlock(&cookie->lock);
+ if (wake)
+ wake_up_cookie_state(cookie);
+ _leave("");
+}
+
+static void fscache_cookie_worker(struct work_struct *work)
+{
+ struct fscache_cookie *cookie = container_of(work, struct fscache_cookie, work);
+
+ fscache_see_cookie(cookie, fscache_cookie_see_work);
+ fscache_cookie_state_machine(cookie);
+ fscache_put_cookie(cookie, fscache_cookie_put_work);
+}
+
+/*
+ * Wait for the object to become inactive. The cookie's work item will be
+ * scheduled when someone transitions n_accesses to 0 - but if someone's
+ * already done that, schedule it anyway.
+ */
+static void __fscache_withdraw_cookie(struct fscache_cookie *cookie)
+{
+ int n_accesses;
+ bool unpinned;
+
+ unpinned = test_and_clear_bit(FSCACHE_COOKIE_NO_ACCESS_WAKE, &cookie->flags);
+
+ /* Need to read the access count after unpinning */
+ n_accesses = atomic_read(&cookie->n_accesses);
+ if (unpinned)
+ trace_fscache_access(cookie->debug_id, refcount_read(&cookie->ref),
+ n_accesses, fscache_access_cache_unpin);
+ if (n_accesses == 0)
+ fscache_queue_cookie(cookie, fscache_cookie_get_end_access);
+}
+
/*
* Remove a cookie from the hash table.
*/
@@ -432,21 +654,27 @@ static void fscache_unhash_cookie(struct fscache_cookie *cookie)
hlist_bl_del(&cookie->hash_link);
clear_bit(FSCACHE_COOKIE_IS_HASHED, &cookie->flags);
hlist_bl_unlock(h);
+ fscache_stat(&fscache_n_relinquishes_dropped);
}

-/*
- * Finalise a cookie after all its resources have been disposed of.
- */
-static void fscache_drop_cookie(struct fscache_cookie *cookie)
+static void fscache_drop_withdraw_cookie(struct fscache_cookie *cookie)
{
- spin_lock(&cookie->lock);
- __fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_DROPPED);
- spin_unlock(&cookie->lock);
- wake_up_cookie_state(cookie);
+ __fscache_withdraw_cookie(cookie);
+}

- fscache_unhash_cookie(cookie);
- fscache_stat(&fscache_n_relinquishes_dropped);
+/**
+ * fscache_withdraw_cookie - Mark a cookie for withdrawal
+ * @cookie: The cookie to be withdrawn.
+ *
+ * Allow the cache backend to withdraw the backing for a cookie for its own
+ * reasons, even if that cookie is in active use.
+ */
+void fscache_withdraw_cookie(struct fscache_cookie *cookie)
+{
+ set_bit(FSCACHE_COOKIE_DO_WITHDRAW, &cookie->flags);
+ fscache_drop_withdraw_cookie(cookie);
}
+EXPORT_SYMBOL(fscache_withdraw_cookie);

/*
* Allow the netfs to release a cookie back to the cache.
@@ -473,12 +701,13 @@ void __fscache_relinquish_cookie(struct fscache_cookie *cookie, bool retire)
ASSERTCMP(atomic_read(&cookie->volume->n_cookies), >, 0);
atomic_dec(&cookie->volume->n_cookies);

- set_bit(FSCACHE_COOKIE_DO_RELINQUISH, &cookie->flags);
-
- if (test_bit(FSCACHE_COOKIE_HAS_BEEN_CACHED, &cookie->flags))
- ; // PLACEHOLDER: Do something here if the cookie was cached
- else
- fscache_drop_cookie(cookie);
+ if (test_bit(FSCACHE_COOKIE_HAS_BEEN_CACHED, &cookie->flags)) {
+ set_bit(FSCACHE_COOKIE_DO_RELINQUISH, &cookie->flags);
+ fscache_drop_withdraw_cookie(cookie);
+ } else {
+ fscache_set_cookie_state(cookie, FSCACHE_COOKIE_STATE_DROPPED);
+ fscache_unhash_cookie(cookie);
+ }
fscache_put_cookie(cookie, fscache_cookie_put_relinquish);
}
EXPORT_SYMBOL(__fscache_relinquish_cookie);
diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 936ef731bbc7..ae6a75976450 100644
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -57,6 +57,15 @@ struct fscache_cache_ops {

/* Free the cache's data attached to a volume */
void (*free_volume)(struct fscache_volume *volume);
+
+ /* Look up a cookie in the cache */
+ bool (*lookup_cookie)(struct fscache_cookie *cookie);
+
+ /* Withdraw an object without any cookie access counts held */
+ void (*withdraw_cookie)(struct fscache_cookie *cookie);
+
+ /* Prepare to write to a live cache object */
+ void (*prepare_to_write)(struct fscache_cookie *cookie);
};

extern struct workqueue_struct *fscache_wq;
@@ -72,6 +81,7 @@ extern int fscache_add_cache(struct fscache_cache *cache,
void *cache_priv);
extern void fscache_withdraw_cache(struct fscache_cache *cache);
extern void fscache_withdraw_volume(struct fscache_volume *volume);
+extern void fscache_withdraw_cookie(struct fscache_cookie *cookie);

extern void fscache_io_error(struct fscache_cache *cache);

@@ -85,8 +95,21 @@ extern void fscache_put_cookie(struct fscache_cookie *cookie,
enum fscache_cookie_trace where);
extern void fscache_end_cookie_access(struct fscache_cookie *cookie,
enum fscache_access_trace why);
-extern void fscache_set_cookie_state(struct fscache_cookie *cookie,
- enum fscache_cookie_state state);
+extern void fscache_cookie_lookup_negative(struct fscache_cookie *cookie);
+extern void fscache_caching_failed(struct fscache_cookie *cookie);
+
+/**
+ * fscache_cookie_state - Read the state of a cookie
+ * @cookie: The cookie to query
+ *
+ * Get the state of a cookie, imposing an ordering between the cookie contents
+ * and the state value. Paired with fscache_set_cookie_state().
+ */
+static inline
+enum fscache_cookie_state fscache_cookie_state(struct fscache_cookie *cookie)
+{
+ return smp_load_acquire(&cookie->state);
+}

/**
* fscache_get_key - Get a pointer to the cookie key
diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
index 1d576bd8112e..030c97bb9c8b 100644
--- a/include/trace/events/fscache.h
+++ b/include/trace/events/fscache.h
@@ -68,6 +68,8 @@ enum fscache_access_trace {
fscache_access_acquire_volume_end,
fscache_access_cache_pin,
fscache_access_cache_unpin,
+ fscache_access_lookup_cookie_end,
+ fscache_access_lookup_cookie_end_failed,
fscache_access_relinquish_volume,
fscache_access_relinquish_volume_end,
fscache_access_unlive,
@@ -124,6 +126,8 @@ enum fscache_access_trace {
EM(fscache_access_acquire_volume_end, "END acq_vol") \
EM(fscache_access_cache_pin, "PIN cache ") \
EM(fscache_access_cache_unpin, "UNPIN cache ") \
+ EM(fscache_access_lookup_cookie_end, "END lookup ") \
+ EM(fscache_access_lookup_cookie_end_failed,"END lookupf") \
EM(fscache_access_relinquish_volume, "BEGIN rlq_vol") \
EM(fscache_access_relinquish_volume_end,"END rlq_vol") \
E_(fscache_access_unlive, "END unlive ")

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