Commit 23e12e28 authored by David Howells's avatar David Howells
Browse files

fscache: Implement cache-level access helpers 



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: default avatarDavid Howells <dhowells@redhat.com>
Reviewed-by: default avatarJeff Layton <jlayton@kernel.org>
cc: linux-cachefs@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
Link: https://lore.kernel.org/r/164021499995.640689.18286203753480287850.stgit@warthog.procyon.org.uk/ # v4
parent 7f3283ab

fs/fscache/cache.c

+62 −0
Original line number Diff line number Diff line
@@ -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";

fs/fscache/internal.h

+2 −0
Original line number Diff line number Diff line
@@ -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);

fs/fscache/main.c

+2 −0
Original line number Diff line number Diff line
@@ -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);

include/trace/events/fscache.h

+41 −0
Original line number Diff line number Diff line
@@ -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),