[PATCH] md/raid5: avoid R5_Overlap races while breaking stripe batches

["Chen Cheng" <chencheng@fnnas.com>]

From: Chen Cheng <chencheng@fnnas.com>

KCSAN report a race in break_stripe_batch_list() vs. raid5_make_request()
on sh->dev[i].flags (plain word write vs. atomic bit op)..

and .. one possible scenario is:

CPU1                            CPU2
break_stripe_batch_list(sh1)
-> handle sh2
-> lock(sh2)
-> sh2->batch_head = NULL
-> unlock(sh2)
-> test_and_clear_bit(R5_Overlap, sh2->dev[i].flags)
-> wake_up_bit(sh2->dev[i].flags)
                                raid5_make_request()
                                -> add_all_stripe_bios(sh2)
                                -> lock(sh2)
                                -> stripe_bio_overlaps(sh2) returns true
				   batch_head is NULL, so new bio overlap
				   exist bio on sh2 -> true
                                -> set_bit(R5_Overlap, sh2->dev[i].flags)
                                -> unlock(sh2)
                                -> wait_on_bit(sh2->dev[i].flags)
-> sh2->dev[i].flags = sh1->dev[i].flags & ~R5_Overlap

No wait_up_bit(), CPU2 could be wait_on_bit() forever...

Fix by :
- Expand the protect zone.
- Use batch_head's device flag's snaphot when no held head_sh->stripe_lock.
- Move sh/head_sh->batch_head = NULL to the end of protected zone , and ,
  any concurrent add_all_stripe_bios() grabs sh->stripe_lock now either:
	- see batch_head != null, and , is rejected by stripe_bio_overlaps()
	  under the lock (no R5_Overlap wait ) , or ,
	- sees batch_head == NULL, only after dev[i].flags has already been
	  set and the prior R5_Overlap waiters worken.

KCSAN report:
================================================
  BUG: KCSAN: data-race in break_stripe_batch_list / raid5_make_request

  write (marked) to 0xffff8e89c8117548 of 8 bytes by task 4042 on cpu 0:
    raid5_make_request+0xea0/0x2930
    md_handle_request+0x4a2/0xa40
    md_submit_bio+0x109/0x1a0
    __submit_bio+0x2ec/0x390
    submit_bio_noacct_nocheck+0x457/0x710
    submit_bio_noacct+0x2a7/0xc20
    submit_bio+0x56/0x250
    blkdev_direct_IO+0x54c/0xda0
    blkdev_write_iter+0x38f/0x570
    aio_write+0x22b/0x490
    io_submit_one+0xa51/0xf70
    __x64_sys_io_submit+0xf7/0x220
    x64_sys_call+0x1907/0x1c60
    do_syscall_64+0x130/0x570
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

  read to 0xffff8e89c8117548 of 8 bytes by task 4010 on cpu 5:
    break_stripe_batch_list+0x249/0x480
    handle_stripe_clean_event+0x720/0x9b0
    handle_stripe+0x32fb/0x4500
    handle_active_stripes.isra.0+0x6e0/0xa50
    raid5d+0x7e0/0xba0
    md_thread+0x15a/0x2d0
    kthread+0x1e3/0x220
    ret_from_fork+0x37a/0x410
    ret_from_fork_asm+0x1a/0x30

  value changed: 0x0000000000000019 -> 0x0000000000000099 --> R5_Overlap

Fixes: fb642b92c267b

Signed-off-by: Chen Cheng <chencheng@fnnas.com>
---
 drivers/md/raid5.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index a376560be92e..5521051a9425 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -4939,30 +4939,30 @@ static void break_stripe_batch_list(struct stripe_head *head_sh,
 			      state & (1 << STRIPE_INSYNC));
 
 		sh->check_state = head_sh->check_state;
 		sh->reconstruct_state = head_sh->reconstruct_state;
 		spin_lock_irq(&sh->stripe_lock);
-		sh->batch_head = NULL;
-		spin_unlock_irq(&sh->stripe_lock);
 		for (i = 0; i < sh->disks; i++) {
 			if (test_and_clear_bit(R5_Overlap, &sh->dev[i].flags))
 				wake_up_bit(&sh->dev[i].flags, R5_Overlap);
-			sh->dev[i].flags = head_sh->dev[i].flags &
+			sh->dev[i].flags = READ_ONCE(head_sh->dev[i].flags) &
 				(~((1 << R5_WriteError) | (1 << R5_Overlap)));
 		}
+		sh->batch_head = NULL;
+		spin_unlock_irq(&sh->stripe_lock);
 
 		state = READ_ONCE(sh->state);
 		if (handle_flags == 0 || (state & handle_flags))
 			set_bit(STRIPE_HANDLE, &sh->state);
 		raid5_release_stripe(sh);
 	}
 	spin_lock_irq(&head_sh->stripe_lock);
-	head_sh->batch_head = NULL;
-	spin_unlock_irq(&head_sh->stripe_lock);
 	for (i = 0; i < head_sh->disks; i++)
 		if (test_and_clear_bit(R5_Overlap, &head_sh->dev[i].flags))
 			wake_up_bit(&head_sh->dev[i].flags, R5_Overlap);
+	head_sh->batch_head = NULL;
+	spin_unlock_irq(&head_sh->stripe_lock);
 
 	state = READ_ONCE(head_sh->state);
 	if (state & handle_flags)
 		set_bit(STRIPE_HANDLE, &head_sh->state);
 }
-- 
2.54.0