[patch v3] malloc: avoid need for tcache == NULL checks
DJ Delorie
dj@redhat.com
Wed Sep 3 00:49:28 GMT 2025
Avoid needing to check for tcache == NULL by initializing it
to a dummy read-only tcache structure. This dummy is all zeros,
so logically it is both full (when you want to put) and empty (when
you want to get). Also, there are two dummies, one used for
"not yet initialized" and one for "tunables say we shouldn't have
a tcache".
The net result is twofold:
1. Checks for tcache == NULL may be removed from the fast path.
Whether this makes the fast path faster when tcache is
disabled is TBD, but the normal case is tcache enabled.
2. no memory for tcache is allocated if tunables disable caching.
Co-authored-by: Florian Weimer <fweimer@redhat.com>
----- >8 -----
v2: removed inits from malloc/calloc hot paths. Added to malloc slow
path. Redid tcache_init to avoid recursion.
v3: replaced macros with functions. Moved tcache_init() calls deeper
into malloc logic. Had to leave init in free() else tests failed.
diff --git a/malloc/malloc.c b/malloc/malloc.c
index 6c02af5a27..584b3eb700 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -3118,8 +3118,57 @@ typedef struct tcache_perthread_struct
tcache_entry *entries[TCACHE_MAX_BINS];
} tcache_perthread_struct;
-static __thread bool tcache_shutting_down = false;
-static __thread tcache_perthread_struct *tcache = NULL;
+static const union
+{
+ struct tcache_perthread_struct inactive;
+ struct
+ {
+ char pad;
+ struct tcache_perthread_struct disabled;
+ };
+} __tcache_dummy;
+
+/* TCACHE is never NULL; it's either "live" or points to one of the
+ above dummy entries. The dummy entries are all zero so act like an
+ empty/unusable tcache. */
+static __thread tcache_perthread_struct *tcache = (tcache_perthread_struct *) &__tcache_dummy.inactive;
+
+/* This is the default, and means "check to see if a real tcache
+ should be allocated." */
+static __always_inline bool
+tcache_inactive (void)
+{
+ return (tcache == &__tcache_dummy.inactive);
+}
+
+/* This means "the user has disabled the tcache but we have to point
+ to something." */
+static __always_inline bool
+tcache_disabled (void)
+{
+ return (tcache == &__tcache_dummy.disabled);
+}
+
+/* This means the tcache is active. */
+static __always_inline bool
+tcache_enabled (void)
+{
+ return (! tcache_inactive () && ! tcache_disabled ());
+}
+
+/* Sets the tcache to INACTIVE state. */
+static __always_inline void
+tcache_set_inactive (void)
+{
+ tcache = (tcache_perthread_struct *) &__tcache_dummy.inactive;
+}
+
+/* Sets the tcache to DISABLED state. */
+static __always_inline void
+tcache_set_disabled (void)
+{
+ tcache = (tcache_perthread_struct *) &__tcache_dummy.disabled;
+}
/* Process-wide key to try and catch a double-free in the same thread. */
static uintptr_t tcache_key;
@@ -3272,19 +3321,13 @@ tcache_get_large (size_t tc_idx, size_t nb)
return tcache_get_n (tc_idx, entry, mangled);
}
-static void tcache_init (void);
+static void tcache_init (mstate av);
static __always_inline void *
tcache_get_align (size_t nb, size_t alignment)
{
if (nb < mp_.tcache_max_bytes)
{
- if (__glibc_unlikely (tcache == NULL))
- {
- tcache_init ();
- return NULL;
- }
-
size_t tc_idx = csize2tidx (nb);
if (__glibc_unlikely (tc_idx >= TCACHE_SMALL_BINS))
tc_idx = large_csize2tidx (nb);
@@ -3347,14 +3390,12 @@ tcache_thread_shutdown (void)
{
int i;
tcache_perthread_struct *tcache_tmp = tcache;
-
- tcache_shutting_down = true;
-
- if (!tcache)
- return;
+ int need_free = tcache_enabled ();
/* Disable the tcache and prevent it from being reinitialized. */
- tcache = NULL;
+ tcache_set_disabled ();
+ if (! need_free)
+ return;
/* Free all of the entries and the tcache itself back to the arena
heap for coalescing. */
@@ -3377,15 +3418,30 @@ tcache_thread_shutdown (void)
/* Initialize tcache. In the rare case there isn't any memory available,
later calls will retry initialization. */
static void
-tcache_init (void)
+tcache_init (mstate av)
{
- if (tcache_shutting_down)
+ /* Check minimum mmap chunk is larger than max tcache size. This means
+ mmap chunks with their different layout are never added to tcache. */
+ if (MAX_TCACHE_SMALL_SIZE >= GLRO (dl_pagesize) / 2)
+ malloc_printerr ("max tcache size too large");
+
+ /* Set this unconditionally to avoid infinite loops in
+ __libc_malloc2 below. */
+ tcache_set_disabled ();
+ if (mp_.tcache_count == 0)
return;
size_t bytes = sizeof (tcache_perthread_struct);
- tcache = (tcache_perthread_struct *) __libc_malloc2 (bytes);
+ if (av)
+ tcache = (tcache_perthread_struct *) _int_malloc (av, request2size (bytes));
+ else
+ tcache = (tcache_perthread_struct *) __libc_malloc2 (bytes);
- if (tcache != NULL)
+ if (tcache == NULL)
+ {
+ tcache_set_inactive ();
+ }
+ else
{
memset (tcache, 0, bytes);
for (int i = 0; i < TCACHE_MAX_BINS; i++)
@@ -3393,20 +3449,6 @@ tcache_init (void)
}
}
-static void * __attribute_noinline__
-tcache_calloc_init (size_t bytes)
-{
- tcache_init ();
- return __libc_calloc2 (bytes);
-}
-
-static void * __attribute_noinline__
-tcache_malloc_init (size_t bytes)
-{
- tcache_init ();
- return __libc_malloc2 (bytes);
-}
-
#else /* !USE_TCACHE */
static void
@@ -3464,8 +3506,6 @@ __libc_malloc (size_t bytes)
if (nb < mp_.tcache_max_bytes)
{
size_t tc_idx = csize2tidx (nb);
- if(__glibc_unlikely (tcache == NULL))
- return tcache_malloc_init (bytes);
if (__glibc_likely (tc_idx < TCACHE_SMALL_BINS))
{
@@ -3510,8 +3550,11 @@ __libc_free (void *mem)
return malloc_printerr_tail ("free(): invalid pointer");
#if USE_TCACHE
- if (__glibc_likely (size < mp_.tcache_max_bytes && tcache != NULL))
+ if (__glibc_likely (size < mp_.tcache_max_bytes))
{
+ if (__glibc_unlikely (tcache_inactive ()))
+ tcache_init (NULL);
+
/* Check to see if it's already in the tcache. */
tcache_entry *e = (tcache_entry *) chunk2mem (p);
@@ -3896,9 +3939,6 @@ __libc_calloc (size_t n, size_t elem_size)
if (nb < mp_.tcache_max_bytes)
{
- if (__glibc_unlikely (tcache == NULL))
- return tcache_calloc_init (bytes);
-
size_t tc_idx = csize2tidx (nb);
if (__glibc_unlikely (tc_idx < TCACHE_SMALL_BINS))
@@ -4030,10 +4070,14 @@ _int_malloc (mstate av, size_t bytes)
/* While we're here, if we see other chunks of the same size,
stash them in the tcache. */
size_t tc_idx = csize2tidx (nb);
- if (tcache != NULL && tc_idx < mp_.tcache_small_bins)
+ if (tc_idx < mp_.tcache_small_bins)
{
mchunkptr tc_victim;
+#if USE_TCACHE
+ if (__glibc_unlikely (tcache_inactive ()))
+ tcache_init (av);
+#endif
/* While bin not empty and tcache not full, copy chunks. */
while (tcache->num_slots[tc_idx] != 0 && (tc_victim = *fb) != NULL)
{
@@ -4090,10 +4134,14 @@ _int_malloc (mstate av, size_t bytes)
/* While we're here, if we see other chunks of the same size,
stash them in the tcache. */
size_t tc_idx = csize2tidx (nb);
- if (tcache != NULL && tc_idx < mp_.tcache_small_bins)
+ if (tc_idx < mp_.tcache_small_bins)
{
mchunkptr tc_victim;
+#if USE_TCACHE
+ if (__glibc_unlikely (tcache_inactive ()))
+ tcache_init (av);
+#endif
/* While bin not empty and tcache not full, copy chunks over. */
while (tcache->num_slots[tc_idx] != 0
&& (tc_victim = last (bin)) != bin)
@@ -4152,7 +4200,7 @@ _int_malloc (mstate av, size_t bytes)
#if USE_TCACHE
INTERNAL_SIZE_T tcache_nb = 0;
size_t tc_idx = csize2tidx (nb);
- if (tcache != NULL && tc_idx < mp_.tcache_small_bins)
+ if (tc_idx < mp_.tcache_small_bins)
tcache_nb = nb;
int return_cached = 0;
@@ -4230,6 +4278,8 @@ _int_malloc (mstate av, size_t bytes)
if (av != &main_arena)
set_non_main_arena (victim);
#if USE_TCACHE
+ if (__glibc_unlikely (tcache_inactive ()))
+ tcache_init (av);
/* Fill cache first, return to user only if cache fills.
We may return one of these chunks later. */
if (tcache_nb > 0
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