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SubjectRe: [PATCH 2/2] sched: do not call workqueue sleep hook if task is already dead
On Wed, Sep 21, 2016 at 8:43 AM, Roman Pen
<roman.penyaev@profitbricks.com> wrote:
> If panic_on_oops is not set and oops happens inside workqueue kthread,
> kernel kills this kthread. Current patch fixes recursive GPF which
> happens in that case with the following stack:

Oleg, can you take a look at this?

--Andy

>
> [<ffffffff81397f75>] dump_stack+0x68/0x93
> [<ffffffff8106954b>] ? do_exit+0x7ab/0xc10
> [<ffffffff8108fd73>] __schedule_bug+0x83/0xe0
> [<ffffffff81716d5a>] __schedule+0x7ea/0xba0
> [<ffffffff810c864f>] ? vprintk_default+0x1f/0x30
> [<ffffffff8116a63c>] ? printk+0x48/0x50
> [<ffffffff81717150>] schedule+0x40/0x90
> [<ffffffff8106976a>] do_exit+0x9ca/0xc10
> [<ffffffff810c8e3d>] ? kmsg_dump+0x11d/0x190
> [<ffffffff810c8d37>] ? kmsg_dump+0x17/0x190
> [<ffffffff81021ee9>] oops_end+0x99/0xd0
> [<ffffffff81052da5>] no_context+0x185/0x3e0
> [<ffffffff81053083>] __bad_area_nosemaphore+0x83/0x1c0
> [<ffffffff810c820e>] ? vprintk_emit+0x25e/0x530
> [<ffffffff810531d4>] bad_area_nosemaphore+0x14/0x20
> [<ffffffff8105355c>] __do_page_fault+0xac/0x570
> [<ffffffff810c66fe>] ? console_trylock+0x1e/0xe0
> [<ffffffff81002036>] ? trace_hardirqs_off_thunk+0x1a/0x1c
> [<ffffffff81053a2c>] do_page_fault+0xc/0x10
> [<ffffffff8171f812>] page_fault+0x22/0x30
> [<ffffffff81089bc3>] ? kthread_data+0x33/0x40
> [<ffffffff8108427e>] ? wq_worker_sleeping+0xe/0x80
> [<ffffffff817169eb>] __schedule+0x47b/0xba0
> [<ffffffff81717150>] schedule+0x40/0x90
> [<ffffffff8106957d>] do_exit+0x7dd/0xc10
> [<ffffffff81021ee9>] oops_end+0x99/0xd0
>
> The root cause is that zeroed task->vfork_done member is accessed from
> wq_worker_sleeping() hook. The zeroing out happens on the following
> path:
>
> oops_end()
> do_exit()
> exit_mm()
> mm_release()
> complete_vfork_done()
>
> In order to fix a bug dead tasks must be ignored.
>
> Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: linux-kernel@vger.kernel.org
> ---
> kernel/sched/core.c | 16 +++++++++++++++-
> 1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2c303e7..50772e5 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3380,8 +3380,22 @@ static void __sched notrace __schedule(bool preempt)
> * If a worker went to sleep, notify and ask workqueue
> * whether it wants to wake up a task to maintain
> * concurrency.
> + *
> + * Also the following stack is possible:
> + * oops_end()
> + * do_exit()
> + * schedule()
> + *
> + * If panic_on_oops is not set and oops happens on
> + * a workqueue execution path, thread will be killed.
> + * That is definitly sad, but not to make the situation
> + * even worse we have to ignore dead tasks in order not
> + * to step on zeroed out members (e.g. t->vfork_done is
> + * already NULL on that path, since we were called by
> + * do_exit()))
> */
> - if (prev->flags & PF_WQ_WORKER) {
> + if (prev->flags & PF_WQ_WORKER &&
> + prev->state != TASK_DEAD) {
> struct task_struct *to_wakeup;
>
> to_wakeup = wq_worker_sleeping(prev);
> --
> 2.9.3
>



--
Andy Lutomirski
AMA Capital Management, LLC

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