arm64: preempt: Fix big-endian when checking preempt count in assembly

Commit 39624469 ("arm64: preempt: Provide our own implementation of
asm/preempt.h") extended the preempt count field in struct thread_info
to 64 bits, so that it consists of a 32-bit count plus a 32-bit flag
indicating whether or not the current task needs rescheduling.

Whilst the asm-offsets definition of TSK_TI_PREEMPT was updated to point
to this new field, the assembly usage was left untouched meaning that a
32-bit load from TSK_TI_PREEMPT on a big-endian machine actually returns
the reschedule flag instead of the count.

Whilst we could fix this by pointing TSK_TI_PREEMPT at the count field,
we're actually better off reworking the two assembly users so that they
operate on the whole 64-bit value in favour of inspecting the thread
flags separately in order to determine whether a reschedule is needed.
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/assembler.h

@@ -722,11 +722,9 @@ USER(\label, ic	ivau, \tmp2)			// invalidate I line PoU
 	.macro		if_will_cond_yield_neon
 #ifdef CONFIG_PREEMPT
 	get_thread_info	x0
-	ldr		w1, [x0, #TSK_TI_PREEMPT]
-	ldr		x0, [x0, #TSK_TI_FLAGS]
-	cmp		w1, #PREEMPT_DISABLE_OFFSET
-	csel		x0, x0, xzr, eq
-	tbnz		x0, #TIF_NEED_RESCHED, .Lyield_\@	// needs rescheduling?
+	ldr		x0, [x0, #TSK_TI_PREEMPT]
+	sub		x0, x0, #PREEMPT_DISABLE_OFFSET
+	cbz		x0, .Lyield_\@
 	/* fall through to endif_yield_neon */
 	.subsection	1
 .Lyield_\@ :

arch/arm64/kernel/entry.S

@@ -619,10 +619,8 @@ el1_irq:
 	irq_handler
 
 #ifdef CONFIG_PREEMPT
-	ldr	w24, [tsk, #TSK_TI_PREEMPT]	// get preempt count
-	cbnz	w24, 1f				// preempt count != 0
-	ldr	x0, [tsk, #TSK_TI_FLAGS]	// get flags
-	tbz	x0, #TIF_NEED_RESCHED, 1f	// needs rescheduling?
+	ldr	x24, [tsk, #TSK_TI_PREEMPT]	// get preempt count
+	cbnz	x24, 1f				// preempt count != 0
 	bl	el1_preempt
 1:
 #endif