MDEV-27088: Server crash on ARM (WMM architecture) due to missing barriers in lf-hash

MariaDB server crashes on ARM (weak memory model architecture) while
concurrently executing l_find to load node->key and add_to_purgatory
to store node->key = NULL. l_find then uses key (which is NULL), to
pass it to a comparison function.

The specific problem is the out-of-order execution that happens on a
weak memory model architecture. Two essential reorderings are possible,
which need to be prevented.

a) As l_find has no barriers in place between the optimistic read of
the key field lf_hash.cc#L117 and the verification of link lf_hash.cc#L124,
the processor can reorder the load to happen after the while-loop.

In that case, a concurrent thread executing add_to_purgatory on the same
node can be scheduled to store NULL at the key field lf_alloc-pin.c#L253
before key is loaded in l_find.

b) A node is marked as deleted by a CAS in l_delete lf_hash.cc#L247 and
taken off the list with an upfollowing CAS lf_hash.cc#L252. Only if both
CAS succeed, the key field is written to by add_to_purgatory. However,
due to a missing barrier, the relaxed store of key lf_alloc-pin.c#L253
can be moved ahead of the two CAS operations, which makes the value of
the local purgatory list stored by add_to_purgatory visible to all threads
operating on the list. As the node is not marked as deleted yet, the
same error occurs in l_find.

This change three accesses to be atomic.

* optimistic read of key in l_find lf_hash.cc#L117
* read of link for verification lf_hash.cc#L124
* write of key in add_to_purgatory lf_alloc-pin.c#L253

Reviewers: Sergei Vojtovich, Sergei Golubchik

Fixes: MDEV-23510 / d30c1331a18d875e553f3fcf544997e4f33fb943

mysys/lf_alloc-pin.c

@@ -256,8 +256,9 @@ static int ptr_cmp(void **a, void **b)
 #define add_to_purgatory(PINS, ADDR)                                    \
   do                                                                    \
   {                                                                     \
-    *(void **)((char *)(ADDR)+(PINS)->pinbox->free_ptr_offset)=         \
-      (PINS)->purgatory;                                                \
+    my_atomic_storeptr_explicit(                                        \
+      (void **)((char *)(ADDR)+(PINS)->pinbox->free_ptr_offset),        \
+      (PINS)->purgatory, MY_MEMORY_ORDER_RELEASE);                      \
     (PINS)->purgatory= (ADDR);                                          \
     (PINS)->purgatory_count++;                                          \
   } while (0)

mysys/lf_hash.c

@@ -111,13 +111,16 @@ static int l_find(LF_SLIST **head, CHARSET_INFO *cs, uint32 hashnr,
 
     cur_hashnr= cursor->curr->hashnr;
     cur_keylen= cursor->curr->keylen;
-    cur_key= cursor->curr->key;
+    cur_key= my_atomic_loadptr_explicit((void **) &cursor->curr->key,
+                                        MY_MEMORY_ORDER_ACQUIRE);
 
     do {
-      link= cursor->curr->link;
+      link= (intptr) my_atomic_loadptr_explicit((void **) &cursor->curr->link,
+                                                MY_MEMORY_ORDER_RELAXED);
       cursor->next= PTR(link);
       lf_pin(pins, 0, cursor->next);
-    } while (link != cursor->curr->link && LF_BACKOFF);
+    } while (link != (intptr) my_atomic_loadptr((void *) &cursor->curr->link)
+             && LF_BACKOFF);
 
     if (!DELETED(link))
     {