From: Yandong Zhao <yandong77...@gmail.com>
Dear Dave,
The scenario for this bug is:
The A process is sched out when the CPU0 executes the function
raw_cpu_read(kernel_neon_busy) and just gets the address of
kernel_neon_busy without reading.
The B process starts running kernel_neon_begin() on CPU0, and the variable
kernel_neon_busy on CPU0 becomes true. At this time, the A process is
executed on CPU1 and the kernel_neon_busy value is CPU0 (true),so BUG_ON()!
crash64> kernel_neon_busy
PER-CPU DATA TYPE:
bool kernel_neon_busy;
PER-CPU ADDRESSES:
[0]: ffffffc07fee30a0
[1]: ffffffc07fef90a0
[2]: ffffffc07ff0f0a0
[3]: ffffffc07ff250a0
CPU0 CPU1
| |
A task have get addr ffffffc07fee30a0 |
and sched out |
| |
B task kernel_neon_begin() |
[ffffffc07fee30a0]=1 |
| |
| A task sched in and read
| [ffffffc07fee30a0]==1,so BUG_ON.
| |
B task kernel_neon_end() |
[ffffffc07fee30a0]=0 |
| |
Signed-off-by: Yandong Zhao <yandong77...@gmail.com>
---
arch/arm64/include/asm/simd.h | 16 ++++------------
1 file changed, 4 insertions(+), 12 deletions(-)
diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
index fa8b3fe..6580dcd 100644
--- a/arch/arm64/include/asm/simd.h
+++ b/arch/arm64/include/asm/simd.h
@@ -29,20 +29,12 @@
static __must_check inline bool may_use_simd(void)
{
/*
- * The raw_cpu_read() is racy if called with preemption enabled.
- * This is not a bug: kernel_neon_busy is only set when
- * preemption is disabled, so we cannot migrate to another CPU
- * while it is set, nor can we migrate to a CPU where it is set.
- * So, if we find it clear on some CPU then we're guaranteed to
- * find it clear on any CPU we could migrate to.
- *
- * If we are in between kernel_neon_begin()...kernel_neon_end(),
- * the flag will be set, but preemption is also disabled, so we
- * can't migrate to another CPU and spuriously see it become
- * false.
+ * Operations for contexts where we do not want to do any checks for
+ * preemptions. Unless strictly necessary, always use this_cpu_*()
+ * instead.
*/
return !in_irq() && !irqs_disabled() && !in_nmi() &&
- !raw_cpu_read(kernel_neon_busy);
+ !this_cpu_read(kernel_neon_busy);
}
#else /* ! CONFIG_KERNEL_MODE_NEON */
--
1.9.1
