[PATCH] mm/page_io: fix a crash in do_task_dead()

[Qian Cai <cai@lca.pw>]

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;

Fixes: 0619317ff8ba ("block: add polled wakeup task helper")
Signed-off-by: Qian Cai <cai@lca.pw>
---
 mm/page_io.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 2e8019d0e048..dc2d3e037ccf 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -140,7 +140,8 @@ static void end_swap_bio_read(struct bio *bio)
 	unlock_page(page);
 	WRITE_ONCE(bio->bi_private, NULL);
 	bio_put(bio);
-	blk_wake_io_task(waiter);
+	/* end_swap_bio_read() could be called from an interrupt handler. */
+	wake_up_process(waiter);
 	put_task_struct(waiter);
 }
 
-- 
1.8.3.1