From ce2f2d668e56f0aaca3550a28118eaba5ce861c5 Mon Sep 17 00:00:00 2001
From: James Zern <jzern@google.com>
Date: Thu, 27 Oct 2022 13:43:32 -0700
Subject: [PATCH] SharpYuvConvert: fix a race on SharpYuvGetCPUInfo

Rather than make a copy, requiring an additional lock/unlock only to set
the pointer to itself, pass the address of SharpYuvGetCPUInfo and use it
as a sentinel to avoid updating the pointer.

Change-Id: I22fb467f1659c16805c0d3bc7aaeba6a1bb16dbb
---
 sharpyuv/sharpyuv.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/sharpyuv/sharpyuv.c b/sharpyuv/sharpyuv.c
index 6585041e..38153cce 100644
--- a/sharpyuv/sharpyuv.c
+++ b/sharpyuv/sharpyuv.c
@@ -439,7 +439,11 @@ void SharpYuvInit(VP8CPUInfo cpu_info_func) {
   static volatile VP8CPUInfo sharpyuv_last_cpuinfo_used =
       (VP8CPUInfo)&sharpyuv_last_cpuinfo_used;
   LOCK_ACCESS;
-  SharpYuvGetCPUInfo = cpu_info_func;
+  // Only update SharpYuvGetCPUInfo when called from external code to avoid a
+  // race on reading the value in SharpYuvConvert().
+  if (cpu_info_func != (VP8CPUInfo)&SharpYuvGetCPUInfo) {
+    SharpYuvGetCPUInfo = cpu_info_func;
+  }
   if (sharpyuv_last_cpuinfo_used == SharpYuvGetCPUInfo) {
     UNLOCK_ACCESS_AND_RETURN;
   }
@@ -484,7 +488,8 @@ int SharpYuvConvert(const void* r_ptr, const void* g_ptr,
     // Stride should be even for uint16_t buffers.
     return 0;
   }
-  SharpYuvInit(SharpYuvGetCPUInfo);
+  // The address of the function pointer is used to avoid a read race.
+  SharpYuvInit((VP8CPUInfo)&SharpYuvGetCPUInfo);
 
   // Add scaling factor to go from rgb_bit_depth to yuv_bit_depth, to the
   // rgb->yuv conversion matrix.