[Mesa-dev] [PATCH V2 3/3] util/disk_cache: use rand_xorshift128plus() to get our random int

[Timothy Arceri]

Otherwise for apps that don't seed the regular rand() we will always
remove old cache entries from the same dirs.

V2: assume bits returned by rand are independent uniformly distributed
    bits and grab our hex value without taking the modulus of the whole
    value, this also fixes a bug where 'f' was always missing.
---
 src/util/disk_cache.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/src/util/disk_cache.c b/src/util/disk_cache.c
index f2d67c9..f8677e3 100644
--- a/src/util/disk_cache.c
+++ b/src/util/disk_cache.c
@@ -34,20 +34,21 @@
 #include <sys/statvfs.h>
 #include <sys/mman.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <pwd.h>
 #include <errno.h>
 #include <dirent.h>
 #include "zlib.h"
 
 #include "util/crc32.h"
+#include "util/rand_xor.h"
 #include "util/u_atomic.h"
 #include "util/u_queue.h"
 #include "util/mesa-sha1.h"
 #include "util/ralloc.h"
 #include "main/errors.h"
 #include "util/macros.h"
 
 #include "disk_cache.h"
 
 /* Number of bits to mask off from a cache key to get an index. */
@@ -59,20 +60,23 @@
 /* The number of keys that can be stored in the index. */
 #define CACHE_INDEX_MAX_KEYS (1 << CACHE_INDEX_KEY_BITS)
 
 struct disk_cache {
    /* The path to the cache directory. */
    char *path;
 
    /* Thread queue for compressing and writing cache entries to disk */
    struct util_queue cache_queue;
 
+   /* Seed for rand, which is used to pick a random directory */
+   uint64_t seed_xorshift128plus[2];
+
    /* A pointer to the mmapped index file within the cache directory. */
    uint8_t *index_mmap;
    size_t index_mmap_size;
 
    /* Pointer to total size of all objects in cache (within index_mmap) */
    uint64_t *size;
 
    /* Pointer to stored keys, (within index_mmap). */
    uint8_t *stored_keys;
 
@@ -354,20 +358,23 @@ disk_cache_create(const char *gpu_name, const char *timestamp)
    cache->max_size = max_size;
 
    /* A limit of 32 jobs was choosen as observations of Deus Ex start-up times
     * showed that we reached at most 11 jobs on an Intel i5-6400 CPU at 2.70GHz
     * (a fairly modest desktop CPU). 1 thread was chosen because we don't
     * really care about getting things to disk quickly just that it's not
     * blocking other tasks.
     */
    util_queue_init(&cache->cache_queue, "disk_cache", 32, 1);
 
+   /* Seed our rand function */
+   s_rand_xorshift128plus(cache->seed_xorshift128plus, true);
+
    ralloc_free(local);
 
    return cache;
 
  fail:
    if (fd != -1)
       close(fd);
    if (cache)
       ralloc_free(cache);
    ralloc_free(local);
@@ -559,37 +566,32 @@ is_two_character_sub_directory(const char *path, const struct stat *sb,
    /* If dir only contains '.' and '..' it must be empty */
    if (subdir_entries <= 2)
       return false;
 
    return true;
 }
 
 static void
 evict_lru_item(struct disk_cache *cache)
 {
-   const char hex[] = "0123456789abcde";
    char *dir_path;
-   int a, b;
-   size_t size;
 
    /* With a reasonably-sized, full cache, (and with keys generated
     * from a cryptographic hash), we can choose two random hex digits
     * and reasonably expect the directory to exist with a file in it.
     * Provides pseudo-LRU eviction to reduce checking all cache files.
     */
-   a = rand() % 16;
-   b = rand() % 16;
-
-   if (asprintf(&dir_path, "%s/%c%c", cache->path, hex[a], hex[b]) < 0)
+   uint64_t rand64 = rand_xorshift128plus(cache->seed_xorshift128plus);
+   if (asprintf(&dir_path, "%s/%02" PRIx64 , cache->path, rand64 & 0xff) < 0)
       return;
 
-   size = unlink_lru_file_from_directory(dir_path);
+   size_t size = unlink_lru_file_from_directory(dir_path);
 
    free(dir_path);
 
    if (size) {
       p_atomic_add(cache->size, - (uint64_t)size);
       return;
    }
 
    /* In the case where the random choice of directory didn't find
     * something, we choose the least recently accessed from the
-- 
2.9.3