linux-kernel - [PATCH 4.19 077/321] arm64: preempt: Fix big-endian when checking preempt count in assembly

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Date:   Tue,  3 Dec 2019 23:32:23 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "kernelci.org bot" <bot@...nelci.org>,
        Kevin Hilman <khilman@...libre.com>,
        Will Deacon <will.deacon@....com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.19 077/321] arm64: preempt: Fix big-endian when checking preempt count in assembly

From: Will Deacon <will.deacon@....com>

[ Upstream commit 7faa313f05cad184e8b17750f0cbe5216ac6debb ]

Commit 396244692232 ("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@...aro.org>
Reported-by: "kernelci.org bot" <bot@...nelci.org>
Tested-by: Kevin Hilman <khilman@...libre.com>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/arm64/include/asm/assembler.h | 8 +++-----
 arch/arm64/kernel/entry.S          | 6 ++----
 2 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index 5a97ac8531682..0c100506a29aa 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -683,11 +683,9 @@ 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_\@ :
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 5f800384cb9a8..bb68323530458 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -622,10 +622,8 @@ 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
-- 
2.20.1