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    Subject[PATCH v4 11/68] fscache: Implement cache-level access helpers
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    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@redhat.com>
    Reviewed-by: Jeff 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
    ---

    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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