Re: [PATCH] ext4: make concurrent DEFAULT shutdown waits killable

[sashiko-bot@kernel.org]

> From: Qing Zhang <diasyzhang@tencent.com>
> 
> Concurrent DEFAULT shutdown callers can all pass the shutdown flag
> check before the first caller finishes bdev_freeze().  They then wait
> uninterruptibly on bd_fsfreeze_mutex and may perform redundant
> freeze/thaw operations, triggering hung-task reports if the initial
> freeze takes long enough.
> 
> Serialize DEFAULT shutdown attempts with a per-superblock mutex
> acquired killably.  Re-check the shutdown flag after acquiring it to
> skip the freeze after a successful attempt, or retry if the previous
> attempt failed.
> 
> Leave LOGFLUSH and NOLOGFLUSH unchanged.  The ->shutdown callback uses
> NOLOGFLUSH with s_umount held, so it must not acquire a mutex held
> across a freeze.  This does not shorten the initial freeze or prevent
> hung-task reports for the active freezer and other freeze-blocked
> tasks.
> 
> Fixes: 783d94854499 ("ext4: add EXT4_IOC_GOINGDOWN ioctl")
> Reported-by: syzbot+d4ecb9a1e5b3dcf8baec@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=d4ecb9a1e5b3dcf8baec
> Signed-off-by: Qing Zhang <diasyzhang@tencent.com>

Sashiko has reviewed this patch and found no issues.

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