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SubjectRe: [PATCH] mm-add-vfree_atomic-fix
On Tue, Dec 13, 2016 at 2:12 AM, Michal Hocko <mhocko@kernel.org> wrote:
> [CC Andy]
>
> I've noticed the same
> http://lkml.kernel.org/r/20161209142820.GA4334@dhcp22.suse.cz
> and also concluded same as you
>
> On Mon 12-12-16 17:46:21, Andrey Ryabinin wrote:
>> DEBUG_PREEMPT complains about using this_cpu_ptr() in preemptible:
>> BUG: using smp_processor_id() in preemptible [00000000] code: iperf-300s-cs-l/277
>> caller is debug_smp_processor_id+0x17/0x19
>> CPU: 1 PID: 277 Comm: iperf-300s-cs-l Not tainted 4.9.0-rc8-00140-gcc639db #2
>> ffffc900003f3cf0 ffffffff8123ae6f 0000000000000001 ffffffff818181da
>> ffffc900003f3d20 ffffffff81252f41 0000000000012de0 00000000fffffdff
>> ffff880009328f40 ffff88000592c400 ffffc900003f3d30 ffffffff81252f6a
>> Call Trace:
>> [<ffffffff8123ae6f>] dump_stack+0x9a/0xd0
>> [<ffffffff81252f41>] check_preemption_disabled+0xdd/0xef
>> [<ffffffff81252f6a>] debug_smp_processor_id+0x17/0x19
>> [<ffffffff811796df>] __vfree_deferred+0x16/0x4c
>> [<ffffffff8117b584>] vfree_atomic+0x22/0x24
>> [<ffffffff81094f5d>] free_thread_stack+0xc2/0x106
>> [<ffffffff810951be>] put_task_stack+0x4c/0x62
>> [<ffffffff81095f81>] copy_process+0x7e0/0x16e8
>> [<ffffffff8109702d>] _do_fork+0xbb/0x2d3
>> [<ffffffff810465e8>] ? __do_page_fault+0x2e1/0x384
>> [<ffffffff8112633f>] ? trace_hardirqs_off_caller+0x12/0x24
>> [<ffffffff810972cb>] SyS_clone+0x19/0x1b
>> [<ffffffff81003800>] do_syscall_64+0x143/0x173
>> [<ffffffff81507289>] entry_SYSCALL64_slow_path+0x25/0x25
>>
>> Use raw_cpu_ptr() instead of this_cpu_ptr() to hide this warning.
>> It's fine because llist_add() implementation is lock-less, so it works even
>> if we adding to the list of some other cpu. schedule_work() is also preempt-safe.
>>
>> Reported-by: kernel test robot <ying.huang@linux.intel.com>
>> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
>
> Acked-by: Michal Hocko <mhocko@suse.com>

But not quite acked by me. What happened to the vfree code that
causes vfree_deferred to be called in a preemptable context? That
sounds like a bug.

(This code doesn't exist in Linus' tree. What tree does this apply to.)

>
>> ---
>> mm/vmalloc.c | 9 ++++++++-
>> 1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 43f0608..d8813963 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -1498,7 +1498,14 @@ static void __vunmap(const void *addr, int deallocate_pages)
>>
>> static inline void __vfree_deferred(const void *addr)
>> {
>> - struct vfree_deferred *p = this_cpu_ptr(&vfree_deferred);
>> + /*
>> + * Use raw_cpu_ptr() because this can be called from preemptible
>> + * context. Preemption is absolutely fine here, because llist_add()
>> + * implementation is lockless, so it works even if we adding to list
>> + * of the other cpu.
>> + * schedule_work() should be fine with this too.
>> + */
>> + struct vfree_deferred *p = raw_cpu_ptr(&vfree_deferred);
>>
>> if (llist_add((struct llist_node *)addr, &p->list))
>> schedule_work(&p->wq);
>> --
>> 2.7.3
>
> --
> Michal Hocko
> SUSE Labs



--
Andy Lutomirski
AMA Capital Management, LLC

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