avcodec/fft_template, fft_init_table: Make ff_fft_init() thread-safe

Commit 1af61568 put initializing
the ff_fft_offsets_lut (which is typically used if FFT_FIXED_32)
behind an ff_thread_once() to make ff_fft_init() thread-safe; yet
there is a second place where said table may be initialized which
is not guarded by this AVOnce: ff_fft_init_mips(). MIPS uses this LUT
even for ordinary floating point FFTs, so that ff_fft_init() is not
thread-safe (on MIPS) for both 32bit fixed-point as well as
floating-point FFTs; e.g. ff_mdct_init() inherits this flaw and
therefore initializing e.g. the AAC decoders is not thread-safe (on
MIPS) despite them having FF_CODEC_CAP_INIT_CLEANUP set.

This commit fixes this by moving the AVOnce to fft_init_table.c and
using it to guard all initializations of ff_fft_offsets_lut.

(It is not that bad in practice, because every entry of
ff_fft_offsets_lut is never read during initialization and is only once
ever written to (namely to its final value); but even these are
conflicting actions which are (by definition) data races and lead to
undefined behaviour.)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/fft_init_table.c

@@ -51,6 +51,8 @@
  * @file
  * definitions and initialization of LUT table for FFT
  */
+#include "libavutil/thread.h"
+
 #include "libavcodec/fft_table.h"
 
 const int32_t ff_w_tab_sr[MAX_FFT_SIZE/(4*16)] = {
@@ -314,15 +316,29 @@ const int32_t ff_w_tab_sr[MAX_FFT_SIZE/(4*16)] = {
 
 uint16_t ff_fft_offsets_lut[21845];
 
-void ff_fft_lut_init(uint16_t *table, int off, int size, int *index)
+static void fft_lut_init(uint16_t *table, int off, int size, int *index)
 {
     if (size < 16) {
         table[*index] = off >> 2;
         (*index)++;
     }
     else {
-        ff_fft_lut_init(table, off, size>>1, index);
-        ff_fft_lut_init(table, off+(size>>1), size>>2, index);
-        ff_fft_lut_init(table, off+3*(size>>2), size>>2, index);
+        fft_lut_init(table, off,                   size >> 1, index);
+        fft_lut_init(table, off +     (size >> 1), size >> 2, index);
+        fft_lut_init(table, off + 3 * (size >> 2), size >> 2, index);
     }
 }
+
+static void fft_lut_init_start(void)
+{
+    int n = 0;
+
+    fft_lut_init(ff_fft_offsets_lut, 0, 1 << 17, &n);
+}
+
+void ff_fft_lut_init(void)
+{
+    static AVOnce init_once = AV_ONCE_INIT;
+
+    ff_thread_once(&init_once, fft_lut_init_start);
+}

libavcodec/fft_table.h

@@ -61,6 +61,6 @@
 
 extern const int32_t ff_w_tab_sr[];
 extern uint16_t ff_fft_offsets_lut[];
-void ff_fft_lut_init(uint16_t *table, int off, int size, int *index);
+void ff_fft_lut_init(void);
 
 #endif /* AVCODEC_FFT_TABLE_H */

libavcodec/fft_template.c

@@ -35,13 +35,6 @@
 
 #if FFT_FIXED_32
 #include "fft_table.h"
-
-static void av_cold fft_lut_init(void)
-{
-    int n = 0;
-    ff_fft_lut_init(ff_fft_offsets_lut, 0, 1 << 17, &n);
-}
-
 #else /* FFT_FIXED_32 */
 
 /* cos(2*pi*x/n) for 0<=x<=n/4, followed by its reverse */
@@ -236,10 +229,7 @@ av_cold int ff_fft_init(FFTContext *s, int nbits, int inverse)
 #endif
 
 #if FFT_FIXED_32
-    {
-        static AVOnce control = AV_ONCE_INIT;
-        ff_thread_once(&control, fft_lut_init);
-    }
+    ff_fft_lut_init();
 #else /* FFT_FIXED_32 */
 #if FFT_FLOAT
     if (ARCH_AARCH64) ff_fft_init_aarch64(s);

libavcodec/mips/fft_mips.c

@@ -500,9 +500,7 @@ static void ff_imdct_calc_mips(FFTContext *s, FFTSample *output, const FFTSample
 
 av_cold void ff_fft_init_mips(FFTContext *s)
 {
-    int n=0;
-
-    ff_fft_lut_init(ff_fft_offsets_lut, 0, 1 << 17, &n);
+    ff_fft_lut_init();
     ff_init_ff_cos_tabs(17);
 
 #if HAVE_INLINE_ASM