On 20 Jun 2024, at 11:46, Chuck Lever wrote:
> On Thu, Jun 20, 2024 at 11:30:54AM -0400, Benjamin Coddington wrote:
>> On 20 Jun 2024, at 10:34, Chuck Lever wrote:
>>
>>> On Thu, Jun 20, 2024 at 09:51:46AM -0400, Benjamin Coddington wrote:
>>>> On 19 Jun 2024, at 13:39, cel@xxxxxxxxxx wrote:
>>>>
>>>>> From: Chuck Lever <chuck.lever@xxxxxxxxxx>
>>>>>
>>>>> During generic/069 runs with pNFS SCSI layouts, the NFS client emits
>>>>> the following in the system journal:
>>>>>
>>>>> kernel: pNFS: failed to open device /dev/disk/by-id/dm-uuid-mpath-0x6001405e3366f045b7949eb8e4540b51 (-2)
>>>>> kernel: pNFS: using block device sdb (reservation key 0x666b60901e7b26b3)
>>>>> kernel: pNFS: failed to open device /dev/disk/by-id/dm-uuid-mpath-0x6001405e3366f045b7949eb8e4540b51 (-2)
>>>>> kernel: pNFS: using block device sdb (reservation key 0x666b60901e7b26b3)
>>>>> kernel: sd 6:0:0:1: reservation conflict
>>>>> kernel: sd 6:0:0:1: [sdb] tag#16 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_OK cmd_age=0s
>>>>> kernel: sd 6:0:0:1: [sdb] tag#16 CDB: Write(10) 2a 00 00 00 00 50 00 00 08 00
>>>>> kernel: reservation conflict error, dev sdb, sector 80 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 2
>>>>> kernel: sd 6:0:0:1: reservation conflict
>>>>> kernel: sd 6:0:0:1: reservation conflict
>>>>> kernel: sd 6:0:0:1: [sdb] tag#18 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_OK cmd_age=0s
>>>>> kernel: sd 6:0:0:1: [sdb] tag#17 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_OK cmd_age=0s
>>>>> kernel: sd 6:0:0:1: [sdb] tag#18 CDB: Write(10) 2a 00 00 00 00 60 00 00 08 00
>>>>> kernel: sd 6:0:0:1: [sdb] tag#17 CDB: Write(10) 2a 00 00 00 00 58 00 00 08 00
>>>>> kernel: reservation conflict error, dev sdb, sector 96 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
>>>>> kernel: reservation conflict error, dev sdb, sector 88 op 0x1:(WRITE) flags 0x0 phys_seg 1 prio class 0
>>>>> systemd[1]: fstests-generic-069.scope: Deactivated successfully.
>>>>> systemd[1]: fstests-generic-069.scope: Consumed 5.092s CPU time.
>>>>> systemd[1]: media-test.mount: Deactivated successfully.
>>>>> systemd[1]: media-scratch.mount: Deactivated successfully.
>>>>> kernel: sd 6:0:0:1: reservation conflict
>>>>> kernel: failed to unregister PR key.
>>>>>
>>>>> This appears to be due to a race. bl_alloc_lseg() calls this:
>>>>>
>>>>> 561 static struct nfs4_deviceid_node *
>>>>> 562 bl_find_get_deviceid(struct nfs_server *server,
>>>>> 563 const struct nfs4_deviceid *id, const struct cred *cred,
>>>>> 564 gfp_t gfp_mask)
>>>>> 565 {
>>>>> 566 struct nfs4_deviceid_node *node;
>>>>> 567 unsigned long start, end;
>>>>> 568
>>>>> 569 retry:
>>>>> 570 node = nfs4_find_get_deviceid(server, id, cred, gfp_mask);
>>>>> 571 if (!node)
>>>>> 572 return ERR_PTR(-ENODEV);
>>>>>
>>>>> nfs4_find_get_deviceid() does a lookup without the spin lock first.
>>>>> If it can't find a matching deviceid, it creates a new device_info
>>>>> (which calls bl_alloc_deviceid_node, and that registers the device's
>>>>> PR key).
>>>>>
>>>>> Then it takes the nfs4_deviceid_lock and looks up the deviceid again.
>>>>> If it finds it this time, bl_find_get_deviceid() frees the spare
>>>>> (new) device_info, which unregisters the PR key for the same device.
>>>>>
>>>>> Any subsequent I/O from this client on that device gets EBADE.
>>>>>
>>>>> The umount later unregisters the device's PR key again.
>>>>>
>>>>> To prevent this problem, register the PR key after the deviceid_node
>>>>> lookup.
>>>>
>>>> Hi Chuck - nice catch, but I'm not seeing how we don't have the same problem
>>>> after this patch, instead it just seems like it moves the race. What
>>>> prevents another process waiting to take the nfs4_deviceid_lock from
>>>> unregistering the same device? I think we need another way to signal
>>>> bl_free_device that we don't want to unregister for the case where the new
>>>> device isn't added to nfs4_deviceid_cache.
>>>
>>> That's a (related but) somewhat different issue. I haven't seen
>>> that in practice so far.
>>>
>>>
>>>> No good ideas yet - maybe we can use a flag set within the
>>>> nfs4_deviceid_lock?
>>>
>>> Well this smells like a use for a reference count on the block
>>> device, but fs/nfs doesn't control the definition of that data
>>> structure.
>>
>> I think we need two things to fix this race:
>> - a way to determine if the current thread is the one
>> that added the device to the to the cache, if so do the register
>> - a way to determine if we're freeing because we lost the race to the
>> cache, if so don't un-register.
>
> My patch is supposed to deal with all of that already. Can you show
> me specifically what is not getting handled by my proposed change?
Well - I may be missing something, but it looks like with this patch you can
still have:
Thread
A B
nfs4_find_get_deviceid
new{a} = nfs4_get_device_info
locks nfs4_deviceid_lock
nfs4_find_get_deviceid
new{b} = nfs4_get_device_info
spins on nfs4_deviceid_lock
adds new{a} to the cache
unlocks nfs4_deviceid_lock
pr_register
locks nfs4_deviceid_lock
finds new{a}
pr_UNregisters new{b}
In this case, you end up with an unregistered device.
Also, you can have more than one thread doing the initial pr_register, but I
think as we've already discussed that's no big deal - it should be rare and
I don't think it returns an error.
Ben
