From 2a1b4cf2331d92bc009bf94fa02a24604cdaf24c Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Sat, 5 Jul 2014 18:43:21 -0400 Subject: [PATCH] blkcg: don't call into policy draining if root_blkg is already gone 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: [] 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:[] [] 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: [] blkcg_drain_queue+0x1f/0x60 [] __blk_drain_queue+0x71/0x180 [] blk_queue_bypass_start+0x6e/0xb0 [] blkcg_deactivate_policy+0x38/0x120 [] blk_throtl_exit+0x34/0x50 [] blkcg_exit_queue+0x35/0x40 [] blk_release_queue+0x26/0xd0 [] kobject_cleanup+0x38/0x70 [] kobject_put+0x28/0x60 [] blk_put_queue+0x15/0x20 [] scsi_device_dev_release_usercontext+0x16b/0x1c0 [] execute_in_process_context+0x89/0xa0 [] scsi_device_dev_release+0x1c/0x20 [] device_release+0x32/0xa0 [] kobject_cleanup+0x38/0x70 [] kobject_put+0x28/0x60 [] put_device+0x17/0x20 [] __scsi_remove_device+0xa9/0xe0 [] scsi_remove_device+0x2b/0x40 [] sdev_store_delete+0x27/0x30 [] dev_attr_store+0x18/0x30 [] sysfs_kf_write+0x3e/0x50 [] kernfs_fop_write+0xe7/0x170 [] vfs_write+0xaf/0x1d0 [] SyS_write+0x4d/0xc0 [] 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 Reported-by: Shirish Pargaonkar Reported-by: Sasha Levin Reported-by: Jet Chen Cc: stable@vger.kernel.org Tested-by: Shirish Pargaonkar Signed-off-by: Jens Axboe --- block/blk-cgroup.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index b9f4cc494ece..28d227c5ca77 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -872,6 +872,13 @@ 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); }