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MDEV-23510: arm64 lf_hash alignment of pointers

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volatile != atomic.

volatile has no memory barrier schemantics, its for mmaped IO
so lets allow some optimizer gains and stop pretending it helps
with memory atomicity.

The MDEV lists a SEGV an assumption is made that an address was
partially read. As C packs structs strictly in order and on arm64 the
cache line size is 128 bits. A pointer (link - 64 bits), followed
by a hashnr (uint32 - 32 bits), leaves the following key (uchar *
64 bits), neither naturally aligned to any pointer and worse, split
across a cache line which is the processors view of an atomic
reservation of memory.

lf_dynarray_lvalue is assumed to return a 64 bit aligned address.

As a solution move the 32bit hashnr to the end so we don't get the
*key pointer split across two cache lines.

Tested by: Krunal Bauskar
Reviewer: Marko Mäkelä
This commit is contained in:
Daniel Black 2021-02-15 12:31:31 +11:00
parent 9e259d58c2
commit 1635686b50
1 changed files with 14 additions and 14 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

28
mysys/lf_hash.c
View file

@ -31,10 +31,10 @@
/* An element of the list */
typedef struct {
intptr volatile link; /* a pointer to the next element in a list and a flag */
uint32 hashnr; /* reversed hash number, for sorting */
intptr link; /* a pointer to the next element in a list and a flag */
const uchar *key;
size_t keylen;
uint32 hashnr; /* reversed hash number, for sorting */
/*
data is stored here, directly after the keylen.
thus the pointer to data is (void*)(slist_element_ptr+1)
@ -48,7 +48,7 @@ const int LF_HASH_OVERHEAD= sizeof(LF_SLIST);
in a list) from l_find to l_insert/l_delete
*/
typedef struct {
intptr volatile *prev;
intptr *prev;
LF_SLIST *curr, *next;
} CURSOR;
@ -85,7 +85,7 @@ typedef struct {
0 - ok
1 - error (callbck returned 1)
*/
static int l_find(LF_SLIST * volatile *head, CHARSET_INFO *cs, uint32 hashnr,
static int l_find(LF_SLIST **head, CHARSET_INFO *cs, uint32 hashnr,
const uchar *key, uint keylen, CURSOR *cursor, LF_PINS *pins,
my_hash_walk_action callback)
{
@ -168,7 +168,7 @@ retry:
it uses pins[0..2], on return all pins are removed.
if there're nodes with the same key value, a new node is added before them.
*/
static LF_SLIST *l_insert(LF_SLIST * volatile *head, CHARSET_INFO *cs,
static LF_SLIST *l_insert(LF_SLIST **head, CHARSET_INFO *cs,
LF_SLIST *node, LF_PINS *pins, uint flags)
{
CURSOR cursor;
@ -220,7 +220,7 @@ static LF_SLIST *l_insert(LF_SLIST * volatile *head, CHARSET_INFO *cs,
NOTE
it uses pins[0..2], on return all pins are removed.
*/
static int l_delete(LF_SLIST * volatile *head, CHARSET_INFO *cs, uint32 hashnr,
static int l_delete(LF_SLIST **head, CHARSET_INFO *cs, uint32 hashnr,
const uchar *key, uint keylen, LF_PINS *pins)
{
CURSOR cursor;
@ -278,7 +278,7 @@ static int l_delete(LF_SLIST * volatile *head, CHARSET_INFO *cs, uint32 hashnr,
it uses pins[0..2], on return the pin[2] keeps the node found
all other pins are removed.
*/
static LF_SLIST *l_search(LF_SLIST * volatile *head, CHARSET_INFO *cs,
static LF_SLIST *l_search(LF_SLIST **head, CHARSET_INFO *cs,
uint32 hashnr, const uchar *key, uint keylen,
LF_PINS *pins)
{
@ -319,7 +319,7 @@ static inline my_hash_value_type calc_hash(CHARSET_INFO *cs,
#define MAX_LOAD 1.0 /* average number of elements in a bucket */
static int initialize_bucket(LF_HASH *, LF_SLIST * volatile*, uint, LF_PINS *);
static int initialize_bucket(LF_HASH *, LF_SLIST **, uint, LF_PINS *);
static void default_initializer(LF_HASH *hash, void *dst, const void *src)
{
@ -398,7 +398,7 @@ void lf_hash_destroy(LF_HASH *hash)
int lf_hash_insert(LF_HASH *hash, LF_PINS *pins, const void *data)
{
int csize, bucket, hashnr;
LF_SLIST *node, * volatile *el;
LF_SLIST *node, **el;
node= (LF_SLIST *)lf_alloc_new(pins);
if (unlikely(!node))
@ -437,7 +437,7 @@ int lf_hash_insert(LF_HASH *hash, LF_PINS *pins, const void *data)
*/
int lf_hash_delete(LF_HASH *hash, LF_PINS *pins, const void *key, uint keylen)
{
LF_SLIST * volatile *el;
LF_SLIST **el;
uint bucket, hashnr;
hashnr= hash->hash_function(hash->charset, (uchar *)key, keylen) & INT_MAX32;
@ -473,7 +473,7 @@ void *lf_hash_search_using_hash_value(LF_HASH *hash, LF_PINS *pins,
my_hash_value_type hashnr,
const void *key, uint keylen)
{
LF_SLIST * volatile *el, *found;
LF_SLIST **el, *found;
uint bucket;
/* hide OOM errors - if we cannot initialize a bucket, try the previous one */
@ -507,7 +507,7 @@ int lf_hash_iterate(LF_HASH *hash, LF_PINS *pins,
CURSOR cursor;
uint bucket= 0;
int res;
LF_SLIST * volatile *el;
LF_SLIST **el;
el= lf_dynarray_lvalue(&hash->array, bucket);
if (unlikely(!el))
@ -539,13 +539,13 @@ static const uchar *dummy_key= (uchar*)"";
0 - ok
-1 - out of memory
*/
static int initialize_bucket(LF_HASH *hash, LF_SLIST * volatile *node,
static int initialize_bucket(LF_HASH *hash, LF_SLIST **node,
uint bucket, LF_PINS *pins)
{
uint parent= my_clear_highest_bit(bucket);
LF_SLIST *dummy= (LF_SLIST *)my_malloc(sizeof(LF_SLIST), MYF(MY_WME));
LF_SLIST **tmp= 0, *cur;
LF_SLIST * volatile *el= lf_dynarray_lvalue(&hash->array, parent);
LF_SLIST **el= lf_dynarray_lvalue(&hash->array, parent);
if (unlikely(!el || !dummy))
return -1;
if (*el == NULL && bucket &&