I get the error when removing a CPU that has been hotplugged after boot.

On 03/14/2017 03:42 PM, Tyrel Datwyler wrote:

On 03/13/2017 03:29 AM, Bharata B Rao wrote:

quoted

On Thu, Mar 09, 2017 at 01:34:00PM -0800, Tyrel Datwyler wrote:

quoted

On 03/08/2017 08:37 PM, Bharata B Rao wrote:

quoted

The following warning is seen when a CPU is hot unplugged on a PowerKVM
guest:

Is this the case with cpus present at boot? What about cpus hotplugged
after boot?

I have observed this for CPUs that are hotplugged.

If removing a cpu present at boot works, but removing one that has been
hotplugged after boot reproduces the problem it is more likely the case
that we failed to take a reference during hotplug or released a
reference we shouldn't have. I'd have to go look at the hot add path.

quoted

quoted

My suspicion is that the refcount was wrong to begin with. See my
comments below. The use of the of_node_put() calls is correct as in each
case we incremented the ref count earlier in the same function.

quoted

refcount_t: underflow; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 53 at lib/refcount.c:128 refcount_sub_and_test+0xd8/0xf0
Modules linked in:
CPU: 0 PID: 53 Comm: kworker/u510:1 Not tainted 4.11.0-rc1 #3
Workqueue: pseries hotplug workque pseries_hp_work_fn
task: c0000000fb475000 task.stack: c0000000fb81c000
NIP: c0000000006f0808 LR: c0000000006f0804 CTR: c0000000007b98c0
REGS: c0000000fb81f710 TRAP: 0700   Not tainted  (4.11.0-rc1)
MSR: 800000000282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>
  CR: 48002222  XER: 20000000
CFAR: c000000000c438e0 SOFTE: 1
GPR00: c0000000006f0804 c0000000fb81f990 c000000001573b00 0000000000000026
GPR04: 0000000000000000 000000000000016c 667265652e0d0a73 652d61667465722d
GPR08: 0000000000000007 0000000000000007 0000000000000001 0000000000000006
GPR12: 0000000000002200 c00000000ff40000 c00000000010c578 c0000001f11b9f40
GPR16: c0000001fe0312a8 c0000001fe031078 c0000001fe031020 0000000000000001
GPR20: 0000000000000000 0000000000000000 c000000001454808 fffffffffffffef7
GPR24: 0000000000000000 c0000001f1677648 0000000000000000 0000000000000000
GPR28: 0000000010000008 c000000000e4d3d8 0000000000000000 c0000001eaae07d8
NIP [c0000000006f0808] refcount_sub_and_test+0xd8/0xf0
LR [c0000000006f0804] refcount_sub_and_test+0xd4/0xf0
Call Trace:
[c0000000fb81f990] [c0000000006f0804] refcount_sub_and_test+0xd4/0xf0 (unreliable)
[c0000000fb81f9f0] [c0000000006d04b4] kobject_put+0x44/0x2a0
[c0000000fb81fa70] [c0000000009d5284] of_node_put+0x34/0x50
[c0000000fb81faa0] [c0000000000aceb8] dlpar_cpu_remove_by_index+0x108/0x130
[c0000000fb81fb30] [c0000000000ae128] dlpar_cpu+0x78/0x550
[c0000000fb81fbe0] [c0000000000a7b40] handle_dlpar_errorlog+0xc0/0x160
[c0000000fb81fc50] [c0000000000a7c74] pseries_hp_work_fn+0x94/0xa0
[c0000000fb81fc80] [c000000000102cec] process_one_work+0x23c/0x540
[c0000000fb81fd20] [c00000000010309c] worker_thread+0xac/0x620
[c0000000fb81fdc0] [c00000000010c6c4] kthread+0x154/0x1a0
[c0000000fb81fe30] [c00000000000bbe0] ret_from_kernel_thread+0x5c/0x7c

Fix this by ensuring that of_node_put() is called only from the
error path in dlpar_cpu_remove_by_index(). In the normal path,
of_node_put() happens as part of dlpar_detach_node().

Signed-off-by: Bharata B Rao <redacted>
Cc: Nathan Fontenot <redacted>
---
Changes in v1:
- Fixed the refcount problem in the userspace driven unplug path
  in addition to in-kernel unplug path. (Sachin Sant)

v0: https://patchwork.ozlabs.org/patch/736547/

 arch/powerpc/platforms/pseries/hotplug-cpu.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index 7bc0e91..c5ed510 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c

@@ -619,7 +619,8 @@ static int dlpar_cpu_remove_by_index(u32 drc_index)
 	}

 	rc = dlpar_cpu_remove(dn, drc_index);
-	of_node_put(dn);
+	if (rc)
+		of_node_put(dn);

I think there is another issue at play here because this is wrong.
Regardless of whether the dlpar_cpu_remove() succeeds or fails we still
need of_node_put() for both cases because we incremented the ref count
earlier in this function with a call to cpu_drc_index_to_dn() call. That
call doesn't, but shoul, document that it returns a device_node with
incremented refcount.

quoted

 	return rc;
 }

@@ -856,9 +857,12 @@ static ssize_t dlpar_cpu_release(const char *buf, size_t count)
 	}

 	rc = dlpar_cpu_remove(dn, drc_index);
-	of_node_put(dn);
-
-	return rc ? rc : count;
+	if (rc) {
+		of_node_put(dn);
+		return rc;
+	} else {
+		return count;
+	}

Same comment as above. The call earlier in the function to
of_find_node_by_path() returned a device_node struct with its ref count
incremented. So, regardless of whether dlpar_cpu_remove() succeeds or
fails we need decrement the ref count with of_node_put().

Looking closer at the call paths for attach and detach one will notice
that __of_attach_node_sysfs() does not take a device_node reference with
of_node_get(), but __of_detach_node_sysfs() does a of_node_put(). In the
old days we use to keep the device tree in /proc. Now it lives in sysfs
and is symlinked to /proc for userspace ABI reasons. Further, pseries
was the only platform in those days that did any sort of dynamic OF
operations. So, in those dark days we were responsible for calling
of_node_put in dlpar_detach_node() to decrement the of_node_init()
reference. Looking at the comments in __of_detach_node_sysfs() it seems
that they expect to decrement that reference there now.

void __of_detach_node_sysfs(struct device_node *np)
{

...snip...

        /* finally remove the kobj_init ref */
        of_node_put(np);
}

So you suggest that adding of_node_get() to __of_attach_node_sysfs()
is the right fix ?

If I understand that this only creates for hot-added cpus then no. Also
for this to be the correct fix I would expect to see this recreate for
all hot-remove operations such as memory and pci devices as well.

-Tyrel

quoted

Regards,
Bharata.

-- 
Michael W. Bringmann
Linux Technology Center
IBM Corporation
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