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    Subject[PATCH v2 1/2] epoll: introduce POLLFREE to flush ->signalfd_wqh before kfree()
    This patch is intentionally incomplete to simplify the review.
    It ignores ep_unregister_pollwait() which plays with the same wqh.
    See the next change.

    epoll assumes that the EPOLL_CTL_ADD'ed file controls everything
    f_op->poll() needs. In particular it assumes that the wait queue
    can't go away until eventpoll_release(). This is not true in case
    of signalfd, the task which does EPOLL_CTL_ADD uses its ->sighand
    which is not connected to the file.

    This patch adds the special event, POLLFREE, currently only for
    epoll. It expects that init_poll_funcptr()'ed hook should do the
    necessary cleanup. Perhaps it should be defined as EPOLLFREE in
    eventpoll.

    __cleanup_sighand() is changed to do wake_up_poll(POLLFREE) if
    ->signalfd_wqh is not empty, we add the new signalfd_cleanup()
    helper.

    ep_poll_callback(POLLFREE) simply does list_del_init(task_list).
    This make this poll entry inconsistent, but we don't care. If you
    share epoll fd which contains our sigfd with another process you
    should blame yourself. signalfd is "really special". I simply do
    not know how we can define the "right" semantics if it used with
    epoll.

    The main problem is, epoll calls signalfd_poll() once to establish
    the connection with the wait queue, after that signalfd_poll(NULL)
    returns the different/inconsistent results depending on who does
    EPOLL_CTL_MOD/signalfd_read/etc. IOW: apart from sigmask, signalfd
    has nothing to do with the file, it works with the current thread.

    In short: this patch is the hack which tries to fix the symptoms.
    It also assumes that nobody can take tasklist_lock under epoll
    locks, this seems to be true.

    Note:

    - we do not have wake_up_all_poll() but wake_up_poll()
    is fine, poll/epoll doesn't use WQ_FLAG_EXCLUSIVE.

    - signalfd_cleanup() uses POLLHUP along with POLLFREE,
    we need a couple of simple changes in eventpoll.c to
    make sure it can't be "lost".

    Reported-by: Maxime Bizon <mbizon@freebox.fr>
    Cc: <stable@kernel.org>
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    ---
    fs/eventpoll.c | 4 ++++
    fs/signalfd.c | 11 +++++++++++
    include/asm-generic/poll.h | 2 ++
    include/linux/signalfd.h | 5 ++++-
    kernel/fork.c | 5 ++++-
    5 files changed, 25 insertions(+), 2 deletions(-)

    diff --git a/fs/eventpoll.c b/fs/eventpoll.c
    index aabdfc3..34bbfc6 100644
    --- a/fs/eventpoll.c
    +++ b/fs/eventpoll.c
    @@ -842,6 +842,10 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k
    struct epitem *epi = ep_item_from_wait(wait);
    struct eventpoll *ep = epi->ep;

    + /* the caller holds eppoll_entry->whead->lock */
    + if ((unsigned long)key & POLLFREE)
    + list_del_init(&wait->task_list);
    +
    spin_lock_irqsave(&ep->lock, flags);

    /*
    diff --git a/fs/signalfd.c b/fs/signalfd.c
    index 492465b..79c1eea 100644
    --- a/fs/signalfd.c
    +++ b/fs/signalfd.c
    @@ -30,6 +30,17 @@
    #include <linux/signalfd.h>
    #include <linux/syscalls.h>

    +void signalfd_cleanup(struct sighand_struct *sighand)
    +{
    + wait_queue_head_t *wqh = &sighand->signalfd_wqh;
    +
    + if (likely(!waitqueue_active(wqh)))
    + return;
    +
    + /* wait_queue_t->func(POLLFREE) should do remove_wait_queue() */
    + wake_up_poll(wqh, POLLHUP | POLLFREE);
    +}
    +
    struct signalfd_ctx {
    sigset_t sigmask;
    };
    diff --git a/include/asm-generic/poll.h b/include/asm-generic/poll.h
    index 44bce83..9ce7f44 100644
    --- a/include/asm-generic/poll.h
    +++ b/include/asm-generic/poll.h
    @@ -28,6 +28,8 @@
    #define POLLRDHUP 0x2000
    #endif

    +#define POLLFREE 0x4000 /* currently only for epoll */
    +
    struct pollfd {
    int fd;
    short events;
    diff --git a/include/linux/signalfd.h b/include/linux/signalfd.h
    index 3ff4961..247399b 100644
    --- a/include/linux/signalfd.h
    +++ b/include/linux/signalfd.h
    @@ -61,13 +61,16 @@ static inline void signalfd_notify(struct task_struct *tsk, int sig)
    wake_up(&tsk->sighand->signalfd_wqh);
    }

    +extern void signalfd_cleanup(struct sighand_struct *sighand);
    +
    #else /* CONFIG_SIGNALFD */

    static inline void signalfd_notify(struct task_struct *tsk, int sig) { }

    +static inline void signalfd_cleanup(struct sighand_struct *sighand) { }
    +
    #endif /* CONFIG_SIGNALFD */

    #endif /* __KERNEL__ */

    #endif /* _LINUX_SIGNALFD_H */
    -
    diff --git a/kernel/fork.c b/kernel/fork.c
    index b77fd55..e2cd3e2 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -66,6 +66,7 @@
    #include <linux/user-return-notifier.h>
    #include <linux/oom.h>
    #include <linux/khugepaged.h>
    +#include <linux/signalfd.h>

    #include <asm/pgtable.h>
    #include <asm/pgalloc.h>
    @@ -935,8 +936,10 @@ static int copy_sighand(unsigned long clone_flags, struct task_struct *tsk)

    void __cleanup_sighand(struct sighand_struct *sighand)
    {
    - if (atomic_dec_and_test(&sighand->count))
    + if (atomic_dec_and_test(&sighand->count)) {
    + signalfd_cleanup(sighand);
    kmem_cache_free(sighand_cachep, sighand);
    + }
    }


    --
    1.5.5.1



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