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    Subject[PATCH 3.11 064/137] blkcg: don't call into policy draining if root_blkg is already gone
    Date
    3.11.10.15 -stable review patch.  If anyone has any objections, please let me know.

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

    From: Tejun Heo <tj@kernel.org>

    commit 0b462c89e31f7eb6789713437eb551833ee16ff3 upstream.

    While a queue is being destroyed, all the blkgs are destroyed and its
    ->root_blkg pointer is set to NULL. If someone else starts to drain
    while the queue is in this state, the following oops happens.

    NULL pointer dereference at 0000000000000028
    IP: [<ffffffff8144e944>] blk_throtl_drain+0x84/0x230
    PGD e4a1067 PUD b773067 PMD 0
    Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
    Modules linked in: cfq_iosched(-) [last unloaded: cfq_iosched]
    CPU: 1 PID: 537 Comm: bash Not tainted 3.16.0-rc3-work+ #2
    Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
    task: ffff88000e222250 ti: ffff88000efd4000 task.ti: ffff88000efd4000
    RIP: 0010:[<ffffffff8144e944>] [<ffffffff8144e944>] blk_throtl_drain+0x84/0x230
    RSP: 0018:ffff88000efd7bf0 EFLAGS: 00010046
    RAX: 0000000000000000 RBX: ffff880015091450 RCX: 0000000000000001
    RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
    RBP: ffff88000efd7c10 R08: 0000000000000000 R09: 0000000000000001
    R10: ffff88000e222250 R11: 0000000000000000 R12: ffff880015091450
    R13: ffff880015092e00 R14: ffff880015091d70 R15: ffff88001508fc28
    FS: 00007f1332650740(0000) GS:ffff88001fa80000(0000) knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
    CR2: 0000000000000028 CR3: 0000000009446000 CR4: 00000000000006e0
    Stack:
    ffffffff8144e8f6 ffff880015091450 0000000000000000 ffff880015091d80
    ffff88000efd7c28 ffffffff8144ae2f ffff880015091450 ffff88000efd7c58
    ffffffff81427641 ffff880015091450 ffffffff82401f00 ffff880015091450
    Call Trace:
    [<ffffffff8144ae2f>] blkcg_drain_queue+0x1f/0x60
    [<ffffffff81427641>] __blk_drain_queue+0x71/0x180
    [<ffffffff81429b3e>] blk_queue_bypass_start+0x6e/0xb0
    [<ffffffff814498b8>] blkcg_deactivate_policy+0x38/0x120
    [<ffffffff8144ec44>] blk_throtl_exit+0x34/0x50
    [<ffffffff8144aea5>] blkcg_exit_queue+0x35/0x40
    [<ffffffff8142d476>] blk_release_queue+0x26/0xd0
    [<ffffffff81454968>] kobject_cleanup+0x38/0x70
    [<ffffffff81454848>] kobject_put+0x28/0x60
    [<ffffffff81427505>] blk_put_queue+0x15/0x20
    [<ffffffff817d07bb>] scsi_device_dev_release_usercontext+0x16b/0x1c0
    [<ffffffff810bc339>] execute_in_process_context+0x89/0xa0
    [<ffffffff817d064c>] scsi_device_dev_release+0x1c/0x20
    [<ffffffff817930e2>] device_release+0x32/0xa0
    [<ffffffff81454968>] kobject_cleanup+0x38/0x70
    [<ffffffff81454848>] kobject_put+0x28/0x60
    [<ffffffff817934d7>] put_device+0x17/0x20
    [<ffffffff817d11b9>] __scsi_remove_device+0xa9/0xe0
    [<ffffffff817d121b>] scsi_remove_device+0x2b/0x40
    [<ffffffff817d1257>] sdev_store_delete+0x27/0x30
    [<ffffffff81792ca8>] dev_attr_store+0x18/0x30
    [<ffffffff8126f75e>] sysfs_kf_write+0x3e/0x50
    [<ffffffff8126ea87>] kernfs_fop_write+0xe7/0x170
    [<ffffffff811f5e9f>] vfs_write+0xaf/0x1d0
    [<ffffffff811f69bd>] SyS_write+0x4d/0xc0
    [<ffffffff81d24692>] system_call_fastpath+0x16/0x1b

    776687bce42b ("block, blk-mq: draining can't be skipped even if
    bypass_depth was non-zero") made it easier to trigger this bug by
    making blk_queue_bypass_start() drain even when it loses the first
    bypass test to blk_cleanup_queue(); however, the bug has always been
    there even before the commit as blk_queue_bypass_start() could race
    against queue destruction, win the initial bypass test but perform the
    actual draining after blk_cleanup_queue() already destroyed all blkgs.

    Fix it by skippping calling into policy draining if all the blkgs are
    already gone.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Reported-by: Shirish Pargaonkar <spargaonkar@suse.com>
    Reported-by: Sasha Levin <sasha.levin@oracle.com>
    Reported-by: Jet Chen <jet.chen@intel.com>
    Tested-by: Shirish Pargaonkar <spargaonkar@suse.com>
    Signed-off-by: Jens Axboe <axboe@fb.com>
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    block/blk-cgroup.c | 7 +++++++
    1 file changed, 7 insertions(+)

    diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
    index 354efcdad847..61f595f4525f 100644
    --- a/block/blk-cgroup.c
    +++ b/block/blk-cgroup.c
    @@ -856,6 +856,13 @@ void blkcg_drain_queue(struct request_queue *q)
    {
    lockdep_assert_held(q->queue_lock);

    + /*
    + * @q could be exiting and already have destroyed all blkgs as
    + * indicated by NULL root_blkg. If so, don't confuse policies.
    + */
    + if (!q->root_blkg)
    + return;
    +
    blk_throtl_drain(q);
    }

    --
    1.9.1


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