fscache: Implement cookie-level access helpers

Add a number of helper functions to manage access to a cookie, pinning the
cache object in place for the duration to prevent cache withdrawal from
removing it:

 (1) void fscache_init_access_gate(struct fscache_cookie *cookie);

     This function initialises the access count when a cache binds to a
     cookie.  An extra ref is taken on the access count to prevent wakeups
     while the cache is active.  We're only interested in the wakeup when a
     cookie is being withdrawn and we're waiting for it to quiesce - at
     which point the counter will be decremented before the wait.

     The FSCACHE_COOKIE_NACC_ELEVATED flag is set on the cookie to keep
     track of the extra ref in order to handle a race between
     relinquishment and withdrawal both trying to drop the extra ref.

 (2) bool fscache_begin_cookie_access(struct fscache_cookie *cookie,
				      enum fscache_access_trace why);

     This function attempts to begin access upon a cookie, pinning it in
     place if it's cached.  If successful, it returns true and leaves a the
     access count incremented.

 (3) void fscache_end_cookie_access(struct fscache_cookie *cookie,
				    enum fscache_access_trace why);

     This function drops the access count obtained by (2), permitting
     object withdrawal to take place when it reaches zero.

A tracepoint is provided to track changes to the access counter on a
cookie.

Changes
=======
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>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819595085.215744.1706073049250505427.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906895313.143852.10141619544149102193.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967095980.1823006.1133648159424418877.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021503063.640689.8870918985269528670.stgit@warthog.procyon.org.uk/ # v4
This commit is contained in:
David Howells 2021-10-20 15:53:34 +01:00
parent e6acd3299b
commit a7733fb632
5 changed files with 133 additions and 0 deletions

View File

@ -62,6 +62,104 @@ static void fscache_free_cookie(struct fscache_cookie *cookie)
kmem_cache_free(fscache_cookie_jar, cookie); kmem_cache_free(fscache_cookie_jar, 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.
* We're only interested in this when we withdraw caching services from the
* cookie.
*/
static void fscache_init_access_gate(struct fscache_cookie *cookie)
{
int n_accesses;
n_accesses = atomic_read(&cookie->n_accesses);
trace_fscache_access(cookie->debug_id, refcount_read(&cookie->ref),
n_accesses, fscache_access_cache_pin);
set_bit(FSCACHE_COOKIE_NO_ACCESS_WAKE, &cookie->flags);
}
/**
* fscache_end_cookie_access - Unpin a cache at the end of an access.
* @cookie: A data file cookie
* @why: An indication of the circumstances of the access for tracing
*
* Unpin a cache cookie after we've accessed it and bring a deferred
* relinquishment or withdrawal state into effect.
*
* The @why indicator is provided for tracing purposes.
*/
void fscache_end_cookie_access(struct fscache_cookie *cookie,
enum fscache_access_trace why)
{
int n_accesses;
smp_mb__before_atomic();
n_accesses = atomic_dec_return(&cookie->n_accesses);
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
}
}
EXPORT_SYMBOL(fscache_end_cookie_access);
/*
* Pin the cache behind a cookie so that we can access it.
*/
static void __fscache_begin_cookie_access(struct fscache_cookie *cookie,
enum fscache_access_trace why)
{
int n_accesses;
n_accesses = atomic_inc_return(&cookie->n_accesses);
smp_mb__after_atomic(); /* (Future) read state after is-caching.
* Reread n_accesses after is-caching
*/
trace_fscache_access(cookie->debug_id, refcount_read(&cookie->ref),
n_accesses, why);
}
/**
* fscache_begin_cookie_access - Pin a cache so data can be accessed
* @cookie: A data file cookie
* @why: An indication of the circumstances of the access for tracing
*
* Attempt to pin the cache to prevent it from going away whilst we're
* accessing data and returns true if successful. This works as follows:
*
* (1) If the cookie is not being cached (ie. FSCACHE_COOKIE_IS_CACHING is not
* set), we return false to indicate access was not permitted.
*
* (2) If the cookie is being cached, we increment its n_accesses count and
* then recheck the IS_CACHING flag, ending the access if it got cleared.
*
* (3) When we end the access, we decrement the cookie's n_accesses and wake
* up the any waiters if it reaches 0.
*
* (4) Whilst the cookie is actively being cached, its n_accesses is kept
* artificially incremented to prevent wakeups from happening.
*
* (5) When the cache is taken offline or if the cookie is culled, the flag is
* cleared to prevent new accesses, the cookie's n_accesses is decremented
* and we wait for it to become 0.
*
* The @why indicator are merely provided for tracing purposes.
*/
bool fscache_begin_cookie_access(struct fscache_cookie *cookie,
enum fscache_access_trace why)
{
if (!test_bit(FSCACHE_COOKIE_IS_CACHING, &cookie->flags))
return false;
__fscache_begin_cookie_access(cookie, why);
if (!test_bit(FSCACHE_COOKIE_IS_CACHING, &cookie->flags) ||
!fscache_cache_is_live(cookie->volume->cache)) {
fscache_end_cookie_access(cookie, fscache_access_unlive);
return false;
}
return true;
}
static inline void wake_up_cookie_state(struct fscache_cookie *cookie) static inline void wake_up_cookie_state(struct fscache_cookie *cookie)
{ {
/* Use a barrier to ensure that waiters see the state variable /* Use a barrier to ensure that waiters see the state variable

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@ -59,6 +59,9 @@ extern struct kmem_cache *fscache_cookie_jar;
extern const struct seq_operations fscache_cookies_seq_ops; extern const struct seq_operations fscache_cookies_seq_ops;
extern void fscache_print_cookie(struct fscache_cookie *cookie, char prefix); extern void fscache_print_cookie(struct fscache_cookie *cookie, char prefix);
extern bool fscache_begin_cookie_access(struct fscache_cookie *cookie,
enum fscache_access_trace why);
static inline void fscache_see_cookie(struct fscache_cookie *cookie, static inline void fscache_see_cookie(struct fscache_cookie *cookie,
enum fscache_cookie_trace where) enum fscache_cookie_trace where)
{ {

View File

@ -23,6 +23,7 @@ MODULE_PARM_DESC(fscache_debug,
EXPORT_TRACEPOINT_SYMBOL(fscache_access_cache); EXPORT_TRACEPOINT_SYMBOL(fscache_access_cache);
EXPORT_TRACEPOINT_SYMBOL(fscache_access_volume); EXPORT_TRACEPOINT_SYMBOL(fscache_access_volume);
EXPORT_TRACEPOINT_SYMBOL(fscache_access);
struct workqueue_struct *fscache_wq; struct workqueue_struct *fscache_wq;
EXPORT_SYMBOL(fscache_wq); EXPORT_SYMBOL(fscache_wq);

View File

@ -61,6 +61,8 @@ extern struct fscache_cookie *fscache_get_cookie(struct fscache_cookie *cookie,
enum fscache_cookie_trace where); enum fscache_cookie_trace where);
extern void fscache_put_cookie(struct fscache_cookie *cookie, extern void fscache_put_cookie(struct fscache_cookie *cookie,
enum fscache_cookie_trace where); 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, extern void fscache_set_cookie_state(struct fscache_cookie *cookie,
enum fscache_cookie_state state); enum fscache_cookie_state state);

View File

@ -279,6 +279,35 @@ TRACE_EVENT(fscache_access_volume,
__entry->n_accesses) __entry->n_accesses)
); );
TRACE_EVENT(fscache_access,
TP_PROTO(unsigned int cookie_debug_id,
int ref,
int n_accesses,
enum fscache_access_trace why),
TP_ARGS(cookie_debug_id, ref, n_accesses, why),
TP_STRUCT__entry(
__field(unsigned int, cookie )
__field(int, ref )
__field(int, n_accesses )
__field(enum fscache_access_trace, why )
),
TP_fast_assign(
__entry->cookie = cookie_debug_id;
__entry->ref = ref;
__entry->n_accesses = n_accesses;
__entry->why = why;
),
TP_printk("c=%08x %s r=%d a=%d",
__entry->cookie,
__print_symbolic(__entry->why, fscache_access_traces),
__entry->ref,
__entry->n_accesses)
);
TRACE_EVENT(fscache_acquire, TRACE_EVENT(fscache_acquire,
TP_PROTO(struct fscache_cookie *cookie), TP_PROTO(struct fscache_cookie *cookie),