rseq: Avoid pointless evaluation in __rseq_notify_resume()

rseq: Avoid pointless evaluation in __rseq_notify_resume()

The RSEQ critical section mechanism only clears the event mask when a
critical section is registered, otherwise it is stale and collects
bits.

That means once a critical section is installed the first invocation of
that code when TIF_NOTIFY_RESUME is set will abort the critical section,
even when the TIF bit was not raised by the rseq preempt/migrate/signal
helpers.

This also has a performance implication because TIF_NOTIFY_RESUME is a
multiplexing TIF bit, which is utilized by quite some infrastructure. That
means every invocation of __rseq_notify_resume() goes unconditionally
through the heavy lifting of user space access and consistency checks even
if there is no reason to do so.

Keeping the stale event mask around when exiting to user space also
prevents it from being utilized by the upcoming time slice extension
mechanism.

Avoid this by reading and clearing the event mask before doing the user
space critical section access with interrupts or preemption disabled, which
ensures that the read and clear operation is CPU local atomic versus
scheduling and the membarrier IPI.

This is correct as after re-enabling interrupts/preemption any relevant
event will set the bit again and raise TIF_NOTIFY_RESUME, which makes the
user space exit code take another round of TIF bit clearing.

If the event mask was non-zero, invoke the slow path. On debug kernels the
slow path is invoked unconditionally and the result of the event mask
evaluation is handed in.

Add a exit path check after the TIF bit loop, which validates on debug
kernels that the event mask is zero before exiting to user space.

While at it reword the convoluted comment why the pt_regs pointer can be
NULL under certain circumstances.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084306.022571576@linutronix.de