lkml.org 
[lkml]   [2021]   [Dec]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Subject[PATCH v4 12/68] fscache: Implement volume-level access helpers
    From
    Date
    Add a pair of helper functions to manage access to a volume, pinning the
    volume in place for the duration to prevent cache withdrawal from removing
    it:

    bool fscache_begin_volume_access(struct fscache_volume *volume,
    enum fscache_access_trace why);
    void fscache_end_volume_access(struct fscache_volume *volume,
    enum fscache_access_trace why);

    The way the access gate on the volume 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 volume's n_accesses
    count and then recheck the cache liveness, ending the access if it
    ceased to be live.

    (3) When we end the access, we decrement the volume's n_accesses and wake
    up the any waiters if it reaches 0.

    (4) Whilst the cache is caching, the volume's 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, the volume's n_accesses is decremented and we wait for it to
    become 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/163819594158.215744.8285859817391683254.stgit@warthog.procyon.org.uk/ # v1
    Link: https://lore.kernel.org/r/163906894315.143852.5454793807544710479.stgit@warthog.procyon.org.uk/ # v2
    Link: https://lore.kernel.org/r/163967095028.1823006.9173132503876627466.stgit@warthog.procyon.org.uk/ # v3
    ---

    fs/fscache/internal.h | 3 +
    fs/fscache/main.c | 1
    fs/fscache/volume.c | 84 ++++++++++++++++++++++++++++++++++++++++
    include/linux/fscache-cache.h | 4 ++
    include/trace/events/fscache.h | 34 ++++++++++++++++
    5 files changed, 126 insertions(+)

    diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
    index be29816b37ef..91a4ea08ec0b 100644
    --- a/fs/fscache/internal.h
    +++ b/fs/fscache/internal.h
    @@ -130,6 +130,9 @@ struct fscache_volume *fscache_get_volume(struct fscache_volume *volume,
    enum fscache_volume_trace where);
    void fscache_put_volume(struct fscache_volume *volume,
    enum fscache_volume_trace where);
    +bool fscache_begin_volume_access(struct fscache_volume *volume,
    + struct fscache_cookie *cookie,
    + enum fscache_access_trace why);
    void fscache_create_volume(struct fscache_volume *volume, bool wait);


    diff --git a/fs/fscache/main.c b/fs/fscache/main.c
    index 876f4bee5840..6cab5d99ba4c 100644
    --- a/fs/fscache/main.c
    +++ b/fs/fscache/main.c
    @@ -22,6 +22,7 @@ MODULE_PARM_DESC(fscache_debug,
    "FS-Cache debugging mask");

    EXPORT_TRACEPOINT_SYMBOL(fscache_access_cache);
    +EXPORT_TRACEPOINT_SYMBOL(fscache_access_volume);

    struct workqueue_struct *fscache_wq;
    EXPORT_SYMBOL(fscache_wq);
    diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
    index 630894fefd02..20497f0f10bb 100644
    --- a/fs/fscache/volume.c
    +++ b/fs/fscache/volume.c
    @@ -33,6 +33,90 @@ static void fscache_see_volume(struct fscache_volume *volume,
    trace_fscache_volume(volume->debug_id, ref, where);
    }

    +/*
    + * Pin the cache behind a volume so that we can access it.
    + */
    +static void __fscache_begin_volume_access(struct fscache_volume *volume,
    + struct fscache_cookie *cookie,
    + enum fscache_access_trace why)
    +{
    + int n_accesses;
    +
    + n_accesses = atomic_inc_return(&volume->n_accesses);
    + smp_mb__after_atomic();
    + trace_fscache_access_volume(volume->debug_id, cookie ? cookie->debug_id : 0,
    + refcount_read(&volume->ref),
    + n_accesses, why);
    +}
    +
    +/**
    + * fscache_begin_volume_access - Pin a cache so a volume can be accessed
    + * @volume: The volume cookie
    + * @cookie: A datafile cookie for a tracing reference (or NULL)
    + * @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 a volume 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 volume's n_accesses
    + * count and then recheck the cache liveness, ending the access if it
    + * ceased to be live.
    + *
    + * (3) When we end the access, we decrement the volume's n_accesses and wake
    + * up the any waiters if it reaches 0.
    + *
    + * (4) Whilst the cache is caching, the volume's 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, the volume's n_accesses is decremented and we wait for it to
    + * become 0.
    + *
    + * The datafile @cookie and the @why indicator are merely provided for tracing
    + * purposes.
    + */
    +bool fscache_begin_volume_access(struct fscache_volume *volume,
    + struct fscache_cookie *cookie,
    + enum fscache_access_trace why)
    +{
    + if (!fscache_cache_is_live(volume->cache))
    + return false;
    + __fscache_begin_volume_access(volume, cookie, why);
    + if (!fscache_cache_is_live(volume->cache)) {
    + fscache_end_volume_access(volume, cookie, fscache_access_unlive);
    + return false;
    + }
    + return true;
    +}
    +
    +/**
    + * fscache_end_volume_access - Unpin a cache at the end of an access.
    + * @volume: The volume cookie
    + * @cookie: A datafile cookie for a tracing reference (or NULL)
    + * @why: An indication of the circumstances of the access for tracing
    + *
    + * Unpin a cache volume after we've accessed it. The datafile @cookie and the
    + * @why indicator are merely provided for tracing purposes.
    + */
    +void fscache_end_volume_access(struct fscache_volume *volume,
    + struct fscache_cookie *cookie,
    + enum fscache_access_trace why)
    +{
    + int n_accesses;
    +
    + smp_mb__before_atomic();
    + n_accesses = atomic_dec_return(&volume->n_accesses);
    + trace_fscache_access_volume(volume->debug_id, cookie ? cookie->debug_id : 0,
    + refcount_read(&volume->ref),
    + n_accesses, why);
    + if (n_accesses == 0)
    + wake_up_var(&volume->n_accesses);
    +}
    +EXPORT_SYMBOL(fscache_end_volume_access);
    +
    static bool fscache_volume_same(const struct fscache_volume *a,
    const struct fscache_volume *b)
    {
    diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
    index c4355b888c91..fbbd8a2afe12 100644
    --- a/include/linux/fscache-cache.h
    +++ b/include/linux/fscache-cache.h
    @@ -53,6 +53,10 @@ extern struct rw_semaphore fscache_addremove_sem;
    extern struct fscache_cache *fscache_acquire_cache(const char *name);
    extern void fscache_relinquish_cache(struct fscache_cache *cache);

    +extern void fscache_end_volume_access(struct fscache_volume *volume,
    + struct fscache_cookie *cookie,
    + enum fscache_access_trace why);
    +
    extern struct fscache_cookie *fscache_get_cookie(struct fscache_cookie *cookie,
    enum fscache_cookie_trace where);
    extern void fscache_put_cookie(struct fscache_cookie *cookie,
    diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
    index 734966bc49e1..4f40cfa52469 100644
    --- a/include/trace/events/fscache.h
    +++ b/include/trace/events/fscache.h
    @@ -43,6 +43,7 @@ enum fscache_volume_trace {
    fscache_volume_put_relinquish,
    fscache_volume_see_create_work,
    fscache_volume_see_hash_wake,
    + fscache_volume_wait_create_work,
    };

    enum fscache_cookie_trace {
    @@ -245,6 +246,39 @@ TRACE_EVENT(fscache_access_cache,
    __entry->n_accesses)
    );

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


    \
     
     \ /
      Last update: 2021-12-23 00:17    [W:4.831 / U:0.672 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site