[PATCH v4 11/68] fscache: Implement cache-level access helpers
David Howells
dhowells at redhat.com
Wed Dec 22 15:16:39 PST 2021
Add a pair of functions to pin/unpin a cache that we're wanting to do a
high-level access to (such as creating or removing a volume):
bool fscache_begin_cache_access(struct fscache_cache *cache,
enum fscache_access_trace why);
void fscache_end_cache_access(struct fscache_cache *cache,
enum fscache_access_trace why);
The way the access gate works/will work is:
(1) If the cache tests as not live (state is not FSCACHE_CACHE_IS_ACTIVE),
then we return false to indicate access was not permitted.
(2) If the cache tests as live, then we increment the n_accesses count and
then recheck the liveness, ending the access if it ceased to be live.
(3) When we end the access, we decrement n_accesses and wake up the any
waiters if it reaches 0.
(4) Whilst the cache is caching, n_accesses is kept artificially
incremented to prevent wakeups from happening.
(5) When the cache is taken offline, the state is changed to prevent new
accesses, n_accesses is decremented and we wait for n_accesses to
become 0.
Note that some of this is implemented in a later patch.
Signed-off-by: David Howells <dhowells at redhat.com>
Reviewed-by: Jeff Layton <jlayton at kernel.org>
cc: linux-cachefs at redhat.com
Link: https://lore.kernel.org/r/163819593239.215744.7537428720603638088.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906893368.143852.14164004598465617981.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967093977.1823006.6967886507023056409.stgit@warthog.procyon.org.uk/ # v3
---
fs/fscache/cache.c | 62 ++++++++++++++++++++++++++++++++++++++++
fs/fscache/internal.h | 2 +
fs/fscache/main.c | 2 +
include/trace/events/fscache.h | 41 ++++++++++++++++++++++++++
4 files changed, 107 insertions(+)
diff --git a/fs/fscache/cache.c b/fs/fscache/cache.c
index 8db77bb9f8e2..e867cff53a70 100644
--- a/fs/fscache/cache.c
+++ b/fs/fscache/cache.c
@@ -216,6 +216,68 @@ void fscache_relinquish_cache(struct fscache_cache *cache)
}
EXPORT_SYMBOL(fscache_relinquish_cache);
+/**
+ * fscache_begin_cache_access - Pin a cache so it can be accessed
+ * @cache: The cache-level 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 it and returns true if successful. This works as follows:
+ *
+ * (1) If the cache tests as not live (state is not FSCACHE_CACHE_IS_ACTIVE),
+ * then we return false to indicate access was not permitted.
+ *
+ * (2) If the cache tests as live, then we increment the n_accesses count and
+ * then recheck the liveness, ending the access if it ceased to be live.
+ *
+ * (3) When we end the access, we decrement n_accesses and wake up the any
+ * waiters if it reaches 0.
+ *
+ * (4) Whilst the cache is caching, n_accesses is kept artificially
+ * incremented to prevent wakeups from happening.
+ *
+ * (5) When the cache is taken offline, the state is changed to prevent new
+ * accesses, n_accesses is decremented and we wait for n_accesses to
+ * become 0.
+ */
+bool fscache_begin_cache_access(struct fscache_cache *cache, enum fscache_access_trace why)
+{
+ int n_accesses;
+
+ if (!fscache_cache_is_live(cache))
+ return false;
+
+ n_accesses = atomic_inc_return(&cache->n_accesses);
+ smp_mb__after_atomic(); /* Reread live flag after n_accesses */
+ trace_fscache_access_cache(cache->debug_id, refcount_read(&cache->ref),
+ n_accesses, why);
+ if (!fscache_cache_is_live(cache)) {
+ fscache_end_cache_access(cache, fscache_access_unlive);
+ return false;
+ }
+ return true;
+}
+
+/**
+ * fscache_end_cache_access - Unpin a cache at the end of an access.
+ * @cache: The cache-level cookie
+ * @why: An indication of the circumstances of the access for tracing
+ *
+ * Unpin a cache after we've accessed it. The @why indicator is merely
+ * provided for tracing purposes.
+ */
+void fscache_end_cache_access(struct fscache_cache *cache, enum fscache_access_trace why)
+{
+ int n_accesses;
+
+ smp_mb__before_atomic();
+ n_accesses = atomic_dec_return(&cache->n_accesses);
+ trace_fscache_access_cache(cache->debug_id, refcount_read(&cache->ref),
+ n_accesses, why);
+ if (n_accesses == 0)
+ wake_up_var(&cache->n_accesses);
+}
+
#ifdef CONFIG_PROC_FS
static const char fscache_cache_states[NR__FSCACHE_CACHE_STATE] = "-PAEW";
diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index 71c897757d44..be29816b37ef 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -23,6 +23,8 @@
#ifdef CONFIG_PROC_FS
extern const struct seq_operations fscache_caches_seq_ops;
#endif
+bool fscache_begin_cache_access(struct fscache_cache *cache, enum fscache_access_trace why);
+void fscache_end_cache_access(struct fscache_cache *cache, enum fscache_access_trace why);
struct fscache_cache *fscache_lookup_cache(const char *name, bool is_cache);
void fscache_put_cache(struct fscache_cache *cache, enum fscache_cache_trace where);
diff --git a/fs/fscache/main.c b/fs/fscache/main.c
index ae493e9ca1c9..876f4bee5840 100644
--- a/fs/fscache/main.c
+++ b/fs/fscache/main.c
@@ -21,6 +21,8 @@ module_param_named(debug, fscache_debug, uint,
MODULE_PARM_DESC(fscache_debug,
"FS-Cache debugging mask");
+EXPORT_TRACEPOINT_SYMBOL(fscache_access_cache);
+
struct workqueue_struct *fscache_wq;
EXPORT_SYMBOL(fscache_wq);
diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
index 9286e1c4b2ac..734966bc49e1 100644
--- a/include/trace/events/fscache.h
+++ b/include/trace/events/fscache.h
@@ -62,6 +62,12 @@ enum fscache_cookie_trace {
fscache_cookie_see_work,
};
+enum fscache_access_trace {
+ fscache_access_cache_pin,
+ fscache_access_cache_unpin,
+ fscache_access_unlive,
+};
+
#endif
/*
@@ -107,6 +113,11 @@ enum fscache_cookie_trace {
EM(fscache_cookie_see_withdraw, "- x-wth") \
E_(fscache_cookie_see_work, "- work ")
+#define fscache_access_traces \
+ EM(fscache_access_cache_pin, "PIN cache ") \
+ EM(fscache_access_cache_unpin, "UNPIN cache ") \
+ E_(fscache_access_unlive, "END unlive ")
+
/*
* Export enum symbols via userspace.
*/
@@ -118,6 +129,7 @@ enum fscache_cookie_trace {
fscache_cache_traces;
fscache_volume_traces;
fscache_cookie_traces;
+fscache_access_traces;
/*
* Now redefine the EM() and E_() macros to map the enums to the strings that
@@ -204,6 +216,35 @@ TRACE_EVENT(fscache_cookie,
__entry->ref)
);
+TRACE_EVENT(fscache_access_cache,
+ TP_PROTO(unsigned int cache_debug_id,
+ int ref,
+ int n_accesses,
+ enum fscache_access_trace why),
+
+ TP_ARGS(cache_debug_id, ref, n_accesses, why),
+
+ TP_STRUCT__entry(
+ __field(unsigned int, cache )
+ __field(int, ref )
+ __field(int, n_accesses )
+ __field(enum fscache_access_trace, why )
+ ),
+
+ TP_fast_assign(
+ __entry->cache = cache_debug_id;
+ __entry->ref = ref;
+ __entry->n_accesses = n_accesses;
+ __entry->why = why;
+ ),
+
+ TP_printk("C=%08x %s r=%d a=%d",
+ __entry->cache,
+ __print_symbolic(__entry->why, fscache_access_traces),
+ __entry->ref,
+ __entry->n_accesses)
+ );
+
TRACE_EVENT(fscache_acquire,
TP_PROTO(struct fscache_cookie *cookie),
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