hrtimer: Use the mask to clear TIF_HRTIMER_REARM from the exit work

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hrtimer_rearm_deferred_user_irq() removes the rearm bit from the local
copy of the TIF work with:

	*tif_work &= ~TIF_HRTIMER_REARM;

TIF_HRTIMER_REARM is the bit number (12), not the mask. That clears
TIF_NOTIFY_SIGNAL (bit 2) and TIF_MEMDIE (bit 3) from the copy and
leaves bit 12 set.

So the function never returns true, and the copy handed to
exit_to_user_mode_loop() lacks TIF_NOTIFY_SIGNAL. If that was the only
work for the loop, the task returns to user space with the task work
still pending. hrtimer_interrupt() sets TIF_HRTIMER_REARM every time,
so the next tick repeats that. Task work queued with TWA_SIGNAL from a
hrtimer callback stays pending until the task does a syscall, takes an
interrupt without deferred rearm or has to reschedule.

A task which polls the io_uring completion ring in user space sees a
timeout completion after 30ms (median) instead of 70us. 2 CPU QEMU
guest, HZ=1000.

Use the mask.

Fixes: 15dd3a9488 ("hrtimer: Push reprogramming timers into the interrupt return path")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: LLM
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260919144027.70250-1-kmehltretter@gmail.com

...

This commit is contained in:

Karl Mehltretter 2026-09-19 16:40:27 +02:00 • committed by Thomas Gleixner

parent 72d3fcf802

commit 28fa9af353

1 changed files with 1 additions and 1 deletions

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include/linux/hrtimer_rearm.h

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