Re: Hotplug

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Hello,

On Wed 11-04-18 08:11:13, Jens Axboe wrote:
> On 4/11/18 7:58 AM, Jan Kara wrote:
> > On Tue 10-04-18 11:17:46, Jens Axboe wrote:
> >> Been running some tests and I keep running into issues with hotplug.
> >> This looks similar to what Bart posted the other day, but it looks
> >> more deeply rooted than just having to protect the queue in
> >> generic_make_request_checks(). The test run is blktests,
> >> block/001. Current -git doesn't survive it. I've seen at least two
> >> different oopses, pasted below.
> >>
> >> [  102.163442] NULL pointer dereference at 0000000000000010
> >> [  102.163444] PGD 0 P4D 0 
> >> [  102.163447] Oops: 0000 [#1] PREEMPT SMP
> >> [  102.163449] Modules linked in:
> >> [  102.175540] sr 12:0:0:0: [sr2] scsi-1 drive
> >> [  102.180112]  scsi_debug crc_t10dif crct10dif_generic crct10dif_common nvme nvme_core sb_edac xl
> >> [  102.186934] sr 12:0:0:0: Attached scsi CD-ROM sr2
> >> [  102.191896]  sr_mod cdrom btrfs xor zstd_decompress zstd_compress xxhash lzo_compress zlib_defc
> >> [  102.197169] sr 12:0:0:0: Attached scsi generic sg7 type 5
> >> [  102.203475]  igb ahci libahci i2c_algo_bit libata dca [last unloaded: crc_t10dif]
> >> [  102.203484] CPU: 43 PID: 4629 Comm: systemd-udevd Not tainted 4.16.0+ #650
> >> [  102.203487] Hardware name: Dell Inc. PowerEdge T630/0NT78X, BIOS 2.3.4 11/09/2016
> >> [  102.350882] RIP: 0010:sr_block_revalidate_disk+0x23/0x190 [sr_mod]
> >> [  102.358299] RSP: 0018:ffff883ff357bb58 EFLAGS: 00010292
> >> [  102.364734] RAX: ffffffffa00b07d0 RBX: ffff883ff3058000 RCX: ffff883ff357bb66
> >> [  102.373220] RDX: 0000000000000003 RSI: 0000000000007530 RDI: ffff881fea631000
> >> [  102.381705] RBP: 0000000000000000 R08: ffff881fe4d38400 R09: 0000000000000000
> >> [  102.390185] R10: 0000000000000000 R11: 00000000000001b6 R12: 000000000800005d
> >> [  102.398671] R13: 000000000800005d R14: ffff883ffd9b3790 R15: 0000000000000000
> >> [  102.407156] FS:  00007f7dc8e6d8c0(0000) GS:ffff883fff340000(0000) knlGS:0000000000000000
> >> [  102.417138] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >> [  102.424066] CR2: 0000000000000010 CR3: 0000003ffda98005 CR4: 00000000003606e0
> >> [  102.432545] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> >> [  102.441024] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >> [  102.449502] Call Trace:
> >> [  102.452744]  ? __invalidate_device+0x48/0x60
> >> [  102.458022]  check_disk_change+0x4c/0x60
> >> [  102.462900]  sr_block_open+0x16/0xd0 [sr_mod]
> >> [  102.468270]  __blkdev_get+0xb9/0x450
> >> [  102.472774]  ? iget5_locked+0x1c0/0x1e0
> >> [  102.477568]  blkdev_get+0x11e/0x320
> >> [  102.481969]  ? bdget+0x11d/0x150
> >> [  102.486083]  ? _raw_spin_unlock+0xa/0x20
> >> [  102.490968]  ? bd_acquire+0xc0/0xc0
> >> [  102.495368]  do_dentry_open+0x1b0/0x320
> >> [  102.500159]  ? inode_permission+0x24/0xc0
> >> [  102.505140]  path_openat+0x4e6/0x1420
> >> [  102.509741]  ? cpumask_any_but+0x1f/0x40
> >> [  102.514630]  ? flush_tlb_mm_range+0xa0/0x120
> >> [  102.519903]  do_filp_open+0x8c/0xf0
> >> [  102.524305]  ? __seccomp_filter+0x28/0x230
> >> [  102.529389]  ? _raw_spin_unlock+0xa/0x20
> >> [  102.534283]  ? __handle_mm_fault+0x7d6/0x9b0
> >> [  102.539559]  ? list_lru_add+0xa8/0xc0
> >> [  102.544157]  ? _raw_spin_unlock+0xa/0x20
> >> [  102.549047]  ? __alloc_fd+0xaf/0x160
> >> [  102.553549]  ? do_sys_open+0x1a6/0x230
> >> [  102.558244]  do_sys_open+0x1a6/0x230
> >> [  102.562742]  do_syscall_64+0x5a/0x100
> >> [  102.567336]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
> > 
> > Interesting. Thinking out loud: This is cd->device dereference I guess
> > which means disk->private_data was NULL. That gets set in sr_probe()
> > together with disk->fops which are certainly set as they must have led us
> > to the crashing function sr_block_revalidate_disk(). So likely
> > disk->private_data got already cleared. That happens in sr_kref_release()
> > and the fact that that function got called means struct scsi_cd went away -
> > so sr_remove() must have been called as well. That all seems possible like:
> > 
> > CPU1		CPU2
> > sr_probe()
> > 		__blkdev_get()
> > 		  disk = bdev_get_gendisk();
> > <device removed>
> > sr_remove()
> >   del_gendisk()
> >   ...
> >   kref_put(&cd->kref, sr_kref_release);
> >     disk->private_data = NULL;
> >     put_disk(disk);
> >     kfree(cd);
> > 		  if (disk->fops->open) {
> > 		    ret = disk->fops->open(bdev, mode); => sr_block_open
> > 		      check_disk_change(bdev);
> > 		        sr_block_revalidate_disk()
> > 			  CRASH
> > 
> > And I think the problem is in sr_block_revalidate_disk() itself as the
> > scsi_cd() call is not guaranteed that the caller holds reference to 'cd'
> > and thus that 'cd' does not disappear under it. IMHO it needs to use
> > scsi_cd_get() to get struct scsi_cd from gendisk. Am I missing something?
> 
> No I think you are correct, from the revalidate path it should grab/release
> a reference. Looks like sr_block_check_events() needs the same treatment.
> How about the below?
 
Yeah, that looks good to me except one thing:

> diff --git a/drivers/scsi/sr.c b/drivers/scsi/sr.c
> index 0cf25d789d05..3f3cb72e0c0c 100644
> --- a/drivers/scsi/sr.c
> +++ b/drivers/scsi/sr.c
>  static int sr_block_revalidate_disk(struct gendisk *disk)
>  {
> -	struct scsi_cd *cd = scsi_cd(disk);
>  	struct scsi_sense_hdr sshdr;
> +	struct scsi_cd *cd;
> +
> +	cd = scsi_cd_get(disk);
> +	if (!cd)
> +		return -ENXIO;

So if this returns -ENXIO I somewhat wonder about the code in
fs/block_dev.c: revalidate_disk(). Why do we do all the work there and then
return -ENXIO?

								Honza

-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR



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