On Thu, Sep 24 2015 at 8:37pm -0400, Junichi Nomura <j-nomura@xxxxxxxxxxxxx> wrote: > 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. Good point. dm_remove() _should_ protect us from ever destroying a device that is still in use. Unless there is some bug that caused something like dm_lock_for_deletion() to regress -- which seems unlikely. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
