Re: [PATCH] blkcg: handle dying request_queue when associating a blkg

From: Bart Van Assche
Date: Wed Dec 12 2018 - 18:54:58 EST

On Tue, 2018-12-11 at 23:06 -0500, Dennis Zhou wrote:
+AD4 Hi Bart,
+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 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 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