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    Subject[PATCH 3.16 345/366] fix io_destroy()/aio_complete() race
    3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Al Viro <viro@zeniv.linux.org.uk>

    commit 4faa99965e027cc057c5145ce45fa772caa04e8d upstream.

    If io_destroy() gets to cancelling everything that can be cancelled and
    gets to kiocb_cancel() calling the function driver has left in ->ki_cancel,
    it becomes vulnerable to a race with IO completion. At that point req
    is already taken off the list and aio_complete() does *NOT* spin until
    we (in free_ioctx_users()) releases ->ctx_lock. As the result, it proceeds
    to kiocb_free(), freing req just it gets passed to ->ki_cancel().

    Fix is simple - remove from the list after the call of kiocb_cancel(). All
    instances of ->ki_cancel() already have to cope with the being called with
    iocb still on list - that's what happens in io_cancel(2).

    Fixes: 0460fef2a921 "aio: use cancellation list lazily"
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    [bwh: Backported to 3.16: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    fs/aio.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    --- a/fs/aio.c
    +++ b/fs/aio.c
    @@ -565,9 +565,8 @@ static void free_ioctx_users(struct perc
    while (!list_empty(&ctx->active_reqs)) {
    req = list_first_entry(&ctx->active_reqs,
    struct kiocb, ki_list);
    -
    - list_del_init(&req->ki_list);
    kiocb_cancel(req);
    + list_del_init(&req->ki_list);
    }

    spin_unlock_irq(&ctx->ctx_lock);
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