Re: [PATCH] block: fix a crash in do_task_dead()

[Oleg Nesterov <oleg@redhat.com>]

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);