Fix mismerge of the arm64 / timer-core interrupt handling changes

Commit c43267e ("Merge tag 'arm64-upstream' of git://...") had a
conflict in the irq entry/exit code due to commit c5538d0 ("entry:
Split kernel mode logic from irqentry_{enter,exit}()") having moved the
core code in irqentry_enter/exit() from kernel/entry/common.c into
helper inline functions in include/linux/irq-entry-common.h.

On the other side of the merge, the timer-core code had introduced
deferred hrtimer rearming infrastructure in commit 0e98eb1 ("entry:
Prepare for deferred hrtimer rearming"), adding two calls to
hrtimer_rearm_deferred() in irqentry_enter().

When merging the two, moving the two calls to the new location wasn't a
problem, but afterwards I had made the mistake of looking what had
happened in linux-next.  And linux-next had a very different merge
resolution in commit 04f02dc3ea74 ("Merge tag 'entry-for-arm64-26-04-08'
into sched/hrtick"), which had unified the two calls into one single
call-site in irqentry_exit_to_kernel_mode_preempt().

And that merge resolution looked cleverer than the straightforward one I
had done, so I re-did my merge the way it had been done in linux-next.

But it turns out nobody apparently tests linux-next, and the merge in
linux-next was just wrong.

The difference is that hrtimer_rearm_deferred() doesn't get called at
all for the case when state.exit_rcu is true, and the boot will
typically fail due to timers not triggering correctly.

So this undoes the "clever" merge, and does the straightforward one
instead.

Fixes: c43267e ("Merge tag 'arm64-upstream' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux"
Reported-and-tested-by: Alexei Starovoitov <[email protected]>
Link: https://lore.kernel.org/all/CAADnVQJ=MoiX4=guPWhL9vtnAELkpNx=GNm8RA1-aV424UFz2A@mail.gmail.com/
Link: https://lore.kernel.org/all/CAHk-=wg8+BER4VyFKG3rnPi2gXxbf-jbHS=EU+xhFqGVQfbutw@mail.gmail.com/
Signed-off-by: Linus Torvalds <[email protected]>

Author: torvalds

include/linux/irq-entry-common.h

@@ -474,8 +474,6 @@ static inline void irqentry_exit_to_kernel_mode_preempt(struct pt_regs *regs,
 
 	if (IS_ENABLED(CONFIG_PREEMPTION))
 		irqentry_exit_cond_resched();
-
-	hrtimer_rearm_deferred();
 }
 
 /**
@@ -501,6 +499,7 @@ irqentry_exit_to_kernel_mode_after_preempt(struct pt_regs *regs, irqentry_state_
 		 */
 		if (state.exit_rcu) {
 			instrumentation_begin();
+			hrtimer_rearm_deferred();
 			/* Tell the tracer that IRET will enable interrupts */
 			trace_hardirqs_on_prepare();
 			lockdep_hardirqs_on_prepare();
@@ -511,6 +510,7 @@ irqentry_exit_to_kernel_mode_after_preempt(struct pt_regs *regs, irqentry_state_
 		}
 
 		instrumentation_begin();
+		hrtimer_rearm_deferred();
 		/* Covers both tracing and lockdep */
 		trace_hardirqs_on();
 		instrumentation_end();