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Subject: Re: [PATCH] block: fix a crash in do_task_dead()
To:     Oleg Nesterov <oleg@redhat.com>, Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, hch@lst.de, peterz@infradead.org,
        gkohli@codeaurora.org, mingo@redhat.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <1559161526-618-1-git-send-email-cai@lca.pw>
 <20190530111519.GC22536@redhat.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <52fc81e0-b6d4-3c29-8250-9da336aaa62a@kernel.dk>
Date:   Fri, 31 May 2019 15:10:18 -0600
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On 5/30/19 5:15 AM, Oleg Nesterov wrote:
> On 05/29, Qian Cai wrote:
>>
>> The commit 0619317ff8ba ("block: add polled wakeup task helper")
>> replaced wake_up_process() with blk_wake_io_task() in
>> end_swap_bio_read() which triggers a crash when running heavy swapping
>> workloads.
>>
>> [T114538] kernel BUG at kernel/sched/core.c:3462!
>> [T114538] Process oom01 (pid: 114538, stack limit = 0x000000004f40e0c1)
>> [T114538] Call trace:
>> [T114538]  do_task_dead+0xf0/0xf8
>> [T114538]  do_exit+0xd5c/0x10fc
>> [T114538]  do_group_exit+0xf4/0x110
>> [T114538]  get_signal+0x280/0xdd8
>> [T114538]  do_notify_resume+0x720/0x968
>> [T114538]  work_pending+0x8/0x10
>>
>> This is because shortly after set_special_state(TASK_DEAD),
>> end_swap_bio_read() is called from an interrupt handler that revive the
>> task state to TASK_RUNNING causes __schedule() to return and trip the
>> BUG() later.
>>
>> [  C206] Call trace:
>> [  C206]  dump_backtrace+0x0/0x268
>> [  C206]  show_stack+0x20/0x2c
>> [  C206]  dump_stack+0xb4/0x108
>> [  C206]  blk_wake_io_task+0x7c/0x80
>> [  C206]  end_swap_bio_read+0x22c/0x31c
>> [  C206]  bio_endio+0x3d8/0x414
>> [  C206]  dec_pending+0x280/0x378 [dm_mod]
>> [  C206]  clone_endio+0x128/0x2ac [dm_mod]
>> [  C206]  bio_endio+0x3d8/0x414
>> [  C206]  blk_update_request+0x3ac/0x924
>> [  C206]  scsi_end_request+0x54/0x350
>> [  C206]  scsi_io_completion+0xf0/0x6f4
>> [  C206]  scsi_finish_command+0x214/0x228
>> [  C206]  scsi_softirq_done+0x170/0x1a4
>> [  C206]  blk_done_softirq+0x100/0x194
>> [  C206]  __do_softirq+0x350/0x790
>> [  C206]  irq_exit+0x200/0x26c
>> [  C206]  handle_IPI+0x2e8/0x514
>> [  C206]  gic_handle_irq+0x224/0x228
>> [  C206]  el1_irq+0xb8/0x140
>> [  C206]  _raw_spin_unlock_irqrestore+0x3c/0x74
>> [  C206]  do_task_dead+0x88/0xf8
>> [  C206]  do_exit+0xd5c/0x10fc
>> [  C206]  do_group_exit+0xf4/0x110
>> [  C206]  get_signal+0x280/0xdd8
>> [  C206]  do_notify_resume+0x720/0x968
>> [  C206]  work_pending+0x8/0x10
>>
>> Before the offensive commit, wake_up_process() will prevent this from
>> happening by taking the pi_lock and bail out immediately if TASK_DEAD is
>> set.
>>
>> if (!(p->state & TASK_NORMAL))
>> 	goto out;
> 
> I don't understand this code at all but I am just curious, can we do
> something like incomplete patch below ?
> 
> Oleg.
> 
> --- x/mm/page_io.c
> +++ x/mm/page_io.c
> @@ -140,8 +140,10 @@ int swap_readpage(struct page *page, bool synchronous)
>   	unlock_page(page);
>   	WRITE_ONCE(bio->bi_private, NULL);
>   	bio_put(bio);
> -	blk_wake_io_task(waiter);
> -	put_task_struct(waiter);
> +	if (waiter) {
> +		blk_wake_io_task(waiter);
> +		put_task_struct(waiter);
> +	}
>   }
>   
>   int generic_swapfile_activate(struct swap_info_struct *sis,
> @@ -398,11 +400,12 @@ int swap_readpage(struct page *page, boo
>   	 * Keep this task valid during swap readpage because the oom killer may
>   	 * attempt to access it in the page fault retry time check.
>   	 */
> -	get_task_struct(current);
> -	bio->bi_private = current;
>   	bio_set_op_attrs(bio, REQ_OP_READ, 0);
> -	if (synchronous)
> +	if (synchronous) {
>   		bio->bi_opf |= REQ_HIPRI;
> +		get_task_struct(current);
> +		bio->bi_private = current;
> +	}
>   	count_vm_event(PSWPIN);
>   	bio_get(bio);
>   	qc = submit_bio(bio);

I think this would solve it for swap.

-- 
Jens Axboe