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On Mon, Apr 18, 2022 at 12:43:43PM -0700, Linus Torvalds wrote:
> Similarly, tsk_is_pi_blocked() will disable the plug flush.

Bah, I'm forever confused by that clause... I did ask for a comment at
some point but that never seems to have happened.

  https://lore.kernel.org/all/20190816160626.12742-1-bigeasy@linutronix.de/T/#md983b64256814c046cec9ec875f4a975029b0daa

Seems to indicate it is about scheduling while holding a lock and the
unplug then leading to a deadlock.