linux-kernel - Re: [PATCH v2 1/2] kthread: allocate kthread structure using kmalloc

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Date:   Mon, 24 Oct 2016 21:37:32 +0200
From:   Roman Penyaev <roman.penyaev@...fitbricks.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] kthread: allocate kthread structure using kmalloc

On Mon, Oct 24, 2016 at 7:08 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Oct 24, 2016 at 9:08 AM, Roman Pen
> <roman.penyaev@...fitbricks.com> wrote:
>> This patch avoids allocation of kthread structure on a stack, and simply
>> uses kmalloc.  Allocation on a stack became a huge problem (with memory
>> corruption and all other not nice consequences) after the commit 2deb4be28
>> by Andy Lutomirski, which rewinds the stack on oops, thus ooopsed kthread
>> steps on a garbage memory while completion of task->vfork_done structure
>> on the following path:
>
> This is IMO a *huge* improvement.
>
> Shouldn't the patch also remove the try_get_task_stack() /
> put_task_stack() hackery in kthread.c, though?

Do you mean that commit from Oleg https://patchwork.kernel.org/patch/9335295/ ?

Hm, I missed that to_live_kthread() function completely. So, yes, we can remove
put/get_task_stack(),  but only replacing on get/put_kthread.  So still need to
be careful with the refs.

Oleg, could you please take a look on the hunk below, just a quick thought.
(get_kthread is simple atomic kthread->refs++).

--
Roman

--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -64,9 +64,20 @@ static inline struct kthread *to_kthread(struct
task_struct *k)
 static struct kthread *to_live_kthread(struct task_struct *k)
 {
        struct completion *vfork = ACCESS_ONCE(k->vfork_done);
-       if (likely(vfork) && try_get_task_stack(k))
-               return __to_kthread(vfork);
-       return NULL;
+       struct kthread *kthread = NULL;
+
+       BUG_ON(!(k->flags & PF_KTHREAD));
+       if (likely(vfork)) {
+               task_lock(k);
+               vfork = ACCESS_ONCE(k->vfork_done);
+               if (likely(vfork)) {
+                       kthread = __to_kthread(vfork);
+                       get_kthread(kthread);
+               }
+               task_unlock(k);
+       }
+
+       return kthread;
 }