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Subject[PATCH 2/6] libnvdimm/bus: Prevent duplicate device_unregister() calls
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A multithreaded namespace creation/destruction stress test currently
fails with signatures like the following:

sysfs group 'power' not found for kobject 'dax1.1'
RIP: 0010:sysfs_remove_group+0x76/0x80
Call Trace:
device_del+0x73/0x370
device_unregister+0x16/0x50
nd_async_device_unregister+0x1e/0x30 [libnvdimm]
async_run_entry_fn+0x39/0x160
process_one_work+0x23c/0x5e0
worker_thread+0x3c/0x390

BUG: kernel NULL pointer dereference, address: 0000000000000020
RIP: 0010:klist_put+0x1b/0x6c
Call Trace:
klist_del+0xe/0x10
device_del+0x8a/0x2c9
? __switch_to_asm+0x34/0x70
? __switch_to_asm+0x40/0x70
device_unregister+0x44/0x4f
nd_async_device_unregister+0x22/0x2d [libnvdimm]
async_run_entry_fn+0x47/0x15a
process_one_work+0x1a2/0x2eb
worker_thread+0x1b8/0x26e

Use the kill_device() helper to atomically resolve the race of multiple
threads issuing kill, device_unregister(), requests.

Reported-by: Jane Chu <jane.chu@oracle.com>
Reported-by: Erwin Tsaur <erwin.tsaur@oracle.com>
Fixes: 4d88a97aa9e8 ("libnvdimm, nvdimm: dimm driver and base libnvdimm device-driver...")
Cc: <stable@vger.kernel.org>
Link: https://github.com/pmem/ndctl/issues/96
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
drivers/nvdimm/bus.c | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)

diff --git a/drivers/nvdimm/bus.c b/drivers/nvdimm/bus.c
index 2dca3034fee0..42713b210f51 100644
--- a/drivers/nvdimm/bus.c
+++ b/drivers/nvdimm/bus.c
@@ -547,13 +547,38 @@ EXPORT_SYMBOL(nd_device_register);

void nd_device_unregister(struct device *dev, enum nd_async_mode mode)
{
+ bool killed;
+
switch (mode) {
case ND_ASYNC:
+ /*
+ * In the async case this is being triggered with the
+ * device lock held and the unregistration work needs to
+ * be moved out of line iff this is thread has won the
+ * race to schedule the deletion.
+ */
+ if (!kill_device(dev))
+ return;
+
get_device(dev);
async_schedule_domain(nd_async_device_unregister, dev,
&nd_async_domain);
break;
case ND_SYNC:
+ /*
+ * In the sync case the device is being unregistered due
+ * to a state change of the parent. Claim the kill state
+ * to synchronize against other unregistration requests,
+ * or otherwise let the async path handle it if the
+ * unregistration was already queued.
+ */
+ device_lock(dev);
+ killed = kill_device(dev);
+ device_unlock(dev);
+
+ if (!killed)
+ return;
+
nd_synchronize();
device_unregister(dev);
break;
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