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

Commit 1af615683e4a1a858407afbaa2fd686842da7e49 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>

1 files changed, 20 insertions, 4 deletions

diff --git a/libavcodec/fft_init_table.c b/libavcodec/fft_init_table.c
index c488018f62..83e35ffb7c 100644
--- a/libavcodec/fft_init_table.c
+++ b/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);
+}