[FFmpeg-devel] [PATCH 2/3] swscale/swscale: Fix races when using unaligned strides/data

[Andreas Rheinhardt]

In this case the current code tries to warn once; to do so, it uses
ordinary static ints to store whether the warning has already been
emitted. This is both a data race (and therefore undefined behaviour)
as well as a race condition, because it is really possible for multiple
threads to be the one thread to emit the warning. This is actually
common since the introduction of the new multithreaded scaling API.

This commit fixes this by using atomic integers for the state;
furthermore, these are not static anymore, but rather contained
in the user-facing SwsContext (i.e. the parent SwsContext in case
of slice-threading).

Given that these atomic variables are not intended for synchronization
at all (but only for atomicity, i.e. only to output the warning once),
the atomic operations use memory_order_relaxed.

This affected nv12, nv21, yuv420, yuv420p10, yuv422, yuv422p10 and
yuv444 filter-overlay FATE-tests.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
---
Of course, one could also use static atomic integers.

 libswscale/swscale.c          | 12 ++++++------
 libswscale/swscale_internal.h |  4 ++++
 libswscale/utils.c            |  2 ++
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/libswscale/swscale.c b/libswscale/swscale.c
index b9c9647fcb..040172752f 100644
--- a/libswscale/swscale.c
+++ b/libswscale/swscale.c
@@ -312,12 +312,12 @@ static int swscale(SwsContext *c, const uint8_t *src[],
 
     if (dstStride[0]&15 || dstStride[1]&15 ||
         dstStride[2]&15 || dstStride[3]&15) {
-        static int warnedAlready = 0; // FIXME maybe move this into the context
-        if (flags & SWS_PRINT_INFO && !warnedAlready) {
+        SwsContext *const ctx = c->parent ? c->parent : c;
+        if (flags & SWS_PRINT_INFO &&
+            !atomic_exchange_explicit(&ctx->stride_unaligned_warned, 1, memory_order_relaxed)) {
             av_log(c, AV_LOG_WARNING,
                    "Warning: dstStride is not aligned!\n"
                    "         ->cannot do aligned memory accesses anymore\n");
-            warnedAlready = 1;
         }
     }
 
@@ -326,11 +326,11 @@ static int swscale(SwsContext *c, const uint8_t *src[],
         || dstStride[0]&15 || dstStride[1]&15 || dstStride[2]&15 || dstStride[3]&15
         || srcStride[0]&15 || srcStride[1]&15 || srcStride[2]&15 || srcStride[3]&15
     ) {
-        static int warnedAlready=0;
+        SwsContext *const ctx = c->parent ? c->parent : c;
         int cpu_flags = av_get_cpu_flags();
-        if (HAVE_MMXEXT && (cpu_flags & AV_CPU_FLAG_SSE2) && !warnedAlready){
+        if (HAVE_MMXEXT && (cpu_flags & AV_CPU_FLAG_SSE2) &&
+            !atomic_exchange_explicit(&ctx->stride_unaligned_warned,1, memory_order_relaxed)) {
             av_log(c, AV_LOG_WARNING, "Warning: data is not aligned! This can lead to a speed loss\n");
-            warnedAlready=1;
         }
     }
 
diff --git a/libswscale/swscale_internal.h b/libswscale/swscale_internal.h
index 0d60dd2e6f..e6e7b934b6 100644
--- a/libswscale/swscale_internal.h
+++ b/libswscale/swscale_internal.h
@@ -21,6 +21,8 @@
 #ifndef SWSCALE_SWSCALE_INTERNAL_H
 #define SWSCALE_SWSCALE_INTERNAL_H
 
+#include <stdatomic.h>
+
 #include "config.h"
 #include "version.h"
 
@@ -672,6 +674,8 @@ typedef struct SwsContext {
     unsigned int xyz_scratch_allocated;
 
     unsigned int dst_slice_align;
+    atomic_int   stride_unaligned_warned;
+    atomic_int   data_unaligned_warned;
 } SwsContext;
 //FIXME check init (where 0)
 
diff --git a/libswscale/utils.c b/libswscale/utils.c
index 84a29c4dc7..fcfba971c6 100644
--- a/libswscale/utils.c
+++ b/libswscale/utils.c
@@ -1112,6 +1112,8 @@ SwsContext *sws_alloc_context(void)
     if (c) {
         c->av_class = &ff_sws_context_class;
         av_opt_set_defaults(c);
+        atomic_init(&c->stride_unaligned_warned, 0);
+        atomic_init(&c->data_unaligned_warned,   0);
     }
 
     return c;
-- 
2.30.2