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    Subject[PATCH v4 09/14] epoll: support polling from userspace for ep_remove()
    Date
    On ep_remove() simply mark a user item with EPOLLREMOVE if the item was
    ready (i.e. has some bits set). That will prevent further user index
    entry creation on item ->bit reuse.

    Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: linux-fsdevel@vger.kernel.org
    Cc: linux-kernel@vger.kernel.org
    ---
    fs/eventpoll.c | 34 ++++++++++++++++++++++++++++------
    1 file changed, 28 insertions(+), 6 deletions(-)

    diff --git a/fs/eventpoll.c b/fs/eventpoll.c
    index 4f541f85c7e5..edf7ba28bce0 100644
    --- a/fs/eventpoll.c
    +++ b/fs/eventpoll.c
    @@ -879,12 +879,30 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
    return res;
    }

    -static void epi_rcu_free(struct rcu_head *head)
    +static void epi_rcu_free_cb(struct rcu_head *head)
    {
    struct epitem *epi = container_of(head, struct epitem, rcu);
    kmem_cache_free(epi_cache, epi);
    }

    +static void uepi_rcu_free_cb(struct rcu_head *head)
    +{
    + struct epitem *epi = container_of(head, struct epitem, rcu);
    + kmem_cache_free(uepi_cache, uep_item_from_epi(epi));
    +}
    +
    +static void epi_rcu_free(struct eventpoll *ep, struct epitem *epi)
    +{
    + /*
    + * Check if `ep` is polled by user here, in this function, not
    + * in the callback, in order not to control lifetime of the 'ep'.
    + */
    + if (ep_polled_by_user(ep))
    + call_rcu(&epi->rcu, uepi_rcu_free_cb);
    + else
    + call_rcu(&epi->rcu, epi_rcu_free_cb);
    +}
    +
    static inline int ep_get_bit(struct eventpoll *ep)
    {
    bool was_set;
    @@ -1156,10 +1174,14 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)

    rb_erase_cached(&epi->rbn, &ep->rbr);

    - write_lock_irq(&ep->lock);
    - if (ep_is_linked(epi))
    - list_del_init(&epi->rdllink);
    - write_unlock_irq(&ep->lock);
    + if (ep_polled_by_user(ep)) {
    + ep_remove_user_item(epi);
    + } else {
    + write_lock_irq(&ep->lock);
    + if (ep_is_linked(epi))
    + list_del_init(&epi->rdllink);
    + write_unlock_irq(&ep->lock);
    + }

    wakeup_source_unregister(ep_wakeup_source(epi));
    /*
    @@ -1169,7 +1191,7 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
    * ep->mtx. The rcu read side, reverse_path_check_proc(), does not make
    * use of the rbn field.
    */
    - call_rcu(&epi->rcu, epi_rcu_free);
    + epi_rcu_free(ep, epi);

    atomic_long_dec(&ep->user->epoll_watches);

    --
    2.21.0
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