On Tue, 2018-12-11 at 23:06 -0500, Dennis Zhou wrote: +AD4 Hi Bart, +AD4 +AD4 On Tue, Dec 11, 2018 at 03:16:13PM -0800, Bart Van Assche wrote: +AD4 +AD4 On Tue, 2018-12-11 at 18:03 -0500, Dennis Zhou wrote: +AD4 +AD4 +AD4 diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c +AD4 +AD4 +AD4 index 6bd0619a7d6e..c30661ddc873 100644 +AD4 +AD4 +AD4 --- a/block/blk-cgroup.c +AD4 +AD4 +AD4 +-+-+- b/block/blk-cgroup.c +AD4 +AD4 +AD4 +AEAAQA -202,6 +-202,12 +AEAAQA static struct blkcg+AF8-gq +ACo-blkg+AF8-create(struct blkcg +ACo-blkcg, +AD4 +AD4 +AD4 WARN+AF8-ON+AF8-ONCE(+ACE-rcu+AF8-read+AF8-lock+AF8-held())+ADs +AD4 +AD4 +AD4 lockdep+AF8-assert+AF8-held(+ACY-q-+AD4-queue+AF8-lock)+ADs +AD4 +AD4 +AD4 +AD4 +AD4 +AD4 +- /+ACo request+AF8-queue is dying, do not create/recreate a blkg +ACo-/ +AD4 +AD4 +AD4 +- if (blk+AF8-queue+AF8-dying(q)) +AHs +AD4 +AD4 +AD4 +- ret +AD0 -ENODEV+ADs +AD4 +AD4 +AD4 +- goto err+AF8-free+AF8-blkg+ADs +AD4 +AD4 +AD4 +- +AH0 +AD4 +AD4 +AD4 +- +AD4 +AD4 +AD4 /+ACo blkg holds a reference to blkcg +ACo-/ +AD4 +AD4 +AD4 if (+ACE-css+AF8-tryget+AF8-online(+ACY-blkcg-+AD4-css)) +AHs +AD4 +AD4 +AD4 ret +AD0 -ENODEV+ADs +AD4 +AD4 +AD4 +AD4 What prevents that the queue state changes after blk+AF8-queue+AF8-dying() has returned +AD4 +AD4 and before blkg+AF8-create() returns? Are you sure you don't need to protect this +AD4 +AD4 code with a blk+AF8-queue+AF8-enter() / blk+AF8-queue+AF8-exit() pair? +AD4 +AD4 +AD4 +AD4 Hmmm. So I think the idea is that we rely on normal shutdown as I don't +AD4 think there is anything wrong with creating a blkg on a dying +AD4 request+AF8-queue. When we are doing association, the request+AF8-queue should +AD4 be pinned by the open call. What we are racing against is when the +AD4 request+AF8-queue is shutting down, it goes around and destroys the blkgs. +AD4 For clarity, QUEUE+AF8-FLAG+AF8-DYING is set in blk+AF8-cleanup+AF8-queue() before +AD4 calling blk+AF8-exit+AF8-queue() which eventually calls blkcg+AF8-exit+AF8-queue(). +AD4 +AD4 The use of blk+AF8-queue+AF8-dying() is to determine whether blkg shutdown has +AD4 already started as if we create one after it has started, we may +AD4 incorrectly orphan a blkg and leak it. Both blkg creation and +AD4 destruction require holding the queue+AF8-lock, so if the QUEUE+AF8-FLAG+AF8-DYING +AD4 flag is set after we've checked it, it means blkg destruction hasn't +AD4 started because it has to wait on the queue+AF8-lock. If QUEUE+AF8-FLAG+AF8-DYING is +AD4 set, then we have no guarantee of knowing what phase blkg destruction is +AD4 in leading to a potential leak. Hi Dennis, To answer my own question: since all queue flag manipulations are protected by the queue lock and since blkg+AF8-create() is called with the queue lock held the above code does not need any further protection. Hence feel free to add the following: Reviewed-by: Bart Van Assche +ADw-bvanassche+AEA-acm.org+AD4
