Re: [PATCH v2 6/7] libnvdimm/bus: Fix wait_nvdimm_bus_probe_idle() ABBA deadlock

[Sasha Levin]

On Wed, Jul 17, 2019 at 06:08:21PM -0700, Dan Williams wrote:
> A multithreaded namespace creation/destruction stress test currently
> deadlocks with the following lockup signature:
>
>    INFO: task ndctl:2924 blocked for more than 122 seconds.
>          Tainted: G           OE     5.2.0-rc4+ #3382
>    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>    ndctl           D    0  2924   1176 0x00000000
>    Call Trace:
>     ? __schedule+0x27e/0x780
>     schedule+0x30/0xb0
>     wait_nvdimm_bus_probe_idle+0x8a/0xd0 [libnvdimm]
>     ? finish_wait+0x80/0x80
>     uuid_store+0xe6/0x2e0 [libnvdimm]
>     kernfs_fop_write+0xf0/0x1a0
>     vfs_write+0xb7/0x1b0
>     ksys_write+0x5c/0xd0
>     do_syscall_64+0x60/0x240
>
>     INFO: task ndctl:2923 blocked for more than 122 seconds.
>           Tainted: G           OE     5.2.0-rc4+ #3382
>     "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>     ndctl           D    0  2923   1175 0x00000000
>     Call Trace:
>      ? __schedule+0x27e/0x780
>      ? __mutex_lock+0x489/0x910
>      schedule+0x30/0xb0
>      schedule_preempt_disabled+0x11/0x20
>      __mutex_lock+0x48e/0x910
>      ? nvdimm_namespace_common_probe+0x95/0x4d0 [libnvdimm]
>      ? __lock_acquire+0x23f/0x1710
>      ? nvdimm_namespace_common_probe+0x95/0x4d0 [libnvdimm]
>      nvdimm_namespace_common_probe+0x95/0x4d0 [libnvdimm]
>      __dax_pmem_probe+0x5e/0x210 [dax_pmem_core]
>      ? nvdimm_bus_probe+0x1d0/0x2c0 [libnvdimm]
>      dax_pmem_probe+0xc/0x20 [dax_pmem]
>      nvdimm_bus_probe+0x90/0x2c0 [libnvdimm]
>      really_probe+0xef/0x390
>      driver_probe_device+0xb4/0x100
>
> In this sequence an 'nd_dax' device is being probed and trying to take
> the lock on its backing namespace to validate that the 'nd_dax' device
> indeed has exclusive access to the backing namespace. Meanwhile, another
> thread is trying to update the uuid property of that same backing
> namespace. So one thread is in the probe path trying to acquire the
> lock, and the other thread has acquired the lock and tries to flush the
> probe path.
>
> Fix this deadlock by not holding the namespace device_lock over the
> wait_nvdimm_bus_probe_idle() synchronization step. In turn this requires
> the device_lock to be held on entry to wait_nvdimm_bus_probe_idle() and
> subsequently dropped internally to wait_nvdimm_bus_probe_idle().
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: bf9bccc14c05 ("libnvdimm: pmem label sets and namespace instantiation")
> Cc: Vishal Verma <vishal.l.verma@xxxxxxxxx>
> Tested-by: Jane Chu <jane.chu@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Hi Dan,

The way these patches are split, when we take them to stable this patch
won't apply because it wants "libnvdimm/bus: Prepare the nd_ioctl() path
to be re-entrant".

If you were to send another iteration of this patchset, could you please
re-order the patches so they will apply cleanly to stable? this will
help with reducing mail exchanges later on and possibly a mis-merge into
stable.

If not, this should serve as a reference for future us to double check
the backport.

--
Thanks,
Sasha