On 09/25/15 06:18, Mike Snitzer wrote: > On Thu, Sep 24 2015 at 3:09pm -0400, > Bart Van Assche <bart.vanassche@xxxxxxxxxxx> wrote: >> kernel BUG at drivers/md/dm.c:2811! >> invalid opcode: 0000 [#1] SMP > ... >> CPU: 0 PID: 334 Comm: kworker/0:1H Tainted: G W O 4.3.0-rc2-debug+ #1 >> Hardware name: Dell Inc. PowerEdge R430/03XKDV, BIOS 1.0.2 11/17/2014 >> Workqueue: kblockd blk_mq_run_work_fn >> task: ffff880450604380 ti: ffff880465864000 task.ti: ffff880465864000 >> RIP: 0010:[<ffffffffa03db95d>] [<ffffffffa03db95d>] dm_get+0x1d/0x20 [dm_mod] >> RSP: 0018:ffff880465867c90 EFLAGS: 00010202 >> RAX: 0000000000000018 RBX: ffff88006cc5a520 RCX: 000000010000affd >> RDX: 0000000000000000 RSI: ffffffff81a47140 RDI: ffff88006cc5a520 >> RBP: ffff880465867c90 R08: 0000000000000000 R09: 0000000000000000 >> R10: 0000000000000001 R11: 0000000000000000 R12: ffff8804036c0600 >> R13: ffffc90000ca0040 R14: ffff88040429f620 R15: 0000000000000001 >> FS: 0000000000000000(0000) GS:ffff88047fc00000(0000) knlGS:0000000000000000 >> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> CR2: 0000560a1f749178 CR3: 000000044d3f8000 CR4: 00000000001406f0 >> Stack: >> ffff880465867cb8 ffffffffa03dba40 ffff88006cc5a520 ffff8804036c0600 >> ffff88006cc5a520 ffff880465867cf8 ffffffffa03dbd95 0000000100000001 >> ffff8804034ba4f8 ffff880465867d20 0000000000000000 ffff8804036c0600 >> Call Trace: >> [<ffffffffa03dba40>] dm_start_request+0x60/0x100 [dm_mod] >> [<ffffffffa03dbd95>] dm_mq_queue_rq+0xa5/0x240 [dm_mod] >> [<ffffffff81273275>] __blk_mq_run_hw_queue+0x1c5/0x360 >> [<ffffffff812739d5>] blk_mq_run_work_fn+0x15/0x20 >> [<ffffffff8108d9b8>] process_one_work+0x1d8/0x610 >> [<ffffffff8108d92b>] ? process_one_work+0x14b/0x610 >> [<ffffffff8108df04>] worker_thread+0x114/0x460 >> [<ffffffff8108ddf0>] ? process_one_work+0x610/0x610 >> [<ffffffff810941f8>] kthread+0xf8/0x110 >> [<ffffffff81094100>] ? kthread_create_on_node+0x200/0x200 >> [<ffffffff814f5eaf>] ret_from_fork+0x3f/0x70 >> [<ffffffff81094100>] ? kthread_create_on_node+0x200/0x200 >> Code: 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 55 48 89 e5 f0 ff 87 d0 01 00 00 48 8b 87 70 02 00 00 a8 08 75 02 5d c3 <0f> 0b 90 0f 1f 44 00 00 55 48 89 e5 53 48 89 fb 48 c7 c7 a0 b0 >> RIP [<ffffffffa03db95d>] dm_get+0x1d/0x20 [dm_mod] >> RSP <ffff880465867c90> >> ---[ end trace e0f10c6e2c55ad9a ]--- > > That is dm_get's BUG_ON(test_bit(DMF_FREEING, &md->flags)); > which was introduced back in 2010 via commit 3f77316de0. > In 2012 that dm_get moved from map_request to dm_start_request() with > commit ba1cbad93d. > > dm_get() is called at the end of dm_start_request(). > > __dm_destroy() sets DMF_FREEING when a device is in the process of being > destroyed. And the reason for dm_get's BUG_ON() is detailed in > __dm_destroy's comment: > > /* > * Rare, but there may be I/O requests still going to complete, > * for example. Wait for all references to disappear. > * No one should increment the reference count of the mapped_device, > * after the mapped_device state becomes DMF_FREEING. > */ > > It could be the original intent was that no _new_ requests be > initiated if/when the DM device is destroyed. But AFAICT nothing ever > actually enforced that distinction. > > And clearly in your test dm_start_request() is called after > __dm_destroy() sets DMF_FREEING. > > But of all the dm_get() callers dm_start_request() does strike me as odd > because it is a catch-22. We need to start requests that were issued > even if the device is being destroyed. And __dm_destroy() needs to wait > for them (by waiting for all dm_get references to be dropped). > > It should be noted that bio-based DM doesn't call dm_get() in its IO > submission path. So this is unique to request-based DM (be it blk-mq or > not). > > In short: dm_get's BUG_ON() looks bogus and should likely be removed. > But I've cc'd Junichi to see what he thinks. Since __dm_destroy() depends on monotonic decrease of md->holders, assertion check of !DMF_FREEING in dm_get() is a valid protection from use-after-free. If we are to remove the check, __dm_destroy() should be changed to cope with the situation. I'm curious why there were pending I/Os after DMF_FEEING set. Can this problem be reproducible with non dm-mq setup or older kernels? How did you remove the dm device in your testing? -- Jun'ichi Nomura, NEC Corporation -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
