On 03/13/2017 03:29 AM, Bharata B Rao wrote: > On Thu, Mar 09, 2017 at 01:34:00PM -0800, Tyrel Datwyler wrote: >> On 03/08/2017 08:37 PM, Bharata B Rao wrote: >>> 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. > >> >> 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. >> >>> >>> 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 <[email protected]> >>> Cc: Nathan Fontenot <[email protected]> >>> --- >>> 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. >> >>> 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 > > Regards, > Bharata. >
