Hi,
On the 6.6-rc2 vanilla torvalds tree kernel, KCSAN had reported a number of data-races:
[ 6413.367326] ==================================================================
[ 6413.367349] BUG: KCSAN: data-race in mas_topiary_replace / mtree_range_walk
[ 6413.367375] write to 0xffff8883a0c5db00 of 8 bytes by task 5475 on cpu 24:
[ 6413.367386] mas_topiary_replace (/home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:491 /home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:1701 /home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:2590)
[ 6413.367399] mas_spanning_rebalance.isra.0 (/home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:2664 /home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:2961)
[ 6413.367413] mas_wr_spanning_store.isra.0 (/home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:3894)
[ 6413.367428] mas_wr_store_entry.isra.0 (/home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:4242)
[ 6413.367442] mas_store_prealloc (/home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:256 /home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:5408)
[ 6413.367455] vma_merge (/home/marvin/linux/kernel/torvalds2/mm/internal.h:1127 /home/marvin/linux/kernel/torvalds2/mm/mmap.c:1005)
[ 6413.367466] mprotect_fixup (/home/marvin/linux/kernel/torvalds2/mm/mprotect.c:632)
[ 6413.367480] do_mprotect_pkey (/home/marvin/linux/kernel/torvalds2/mm/mprotect.c:819)
[ 6413.367494] __x64_sys_mprotect (/home/marvin/linux/kernel/torvalds2/mm/mprotect.c:837)
[ 6413.367503] do_syscall_64 (/home/marvin/linux/kernel/torvalds2/arch/x86/entry/common.c:50 /home/marvin/linux/kernel/torvalds2/arch/x86/entry/common.c:80)
[ 6413.367517] entry_SYSCALL_64_after_hwframe (/home/marvin/linux/kernel/torvalds2/arch/x86/entry/entry_64.S:120)
[ 6413.367534] read to 0xffff8883a0c5db00 of 8 bytes by task 5558 on cpu 11:
[ 6413.367545] mtree_range_walk (/home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:539 /home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:2789)
[ 6413.367556] mas_walk (/home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:251 /home/marvin/linux/kernel/torvalds2/lib/maple_tree.c:4844)
[ 6413.367567] lock_vma_under_rcu (/home/marvin/linux/kernel/torvalds2/mm/memory.c:5436)
[ 6413.367579] do_user_addr_fault (/home/marvin/linux/kernel/torvalds2/arch/x86/mm/fault.c:1357)
[ 6413.367593] exc_page_fault (/home/marvin/linux/kernel/torvalds2/./arch/x86/include/asm/paravirt.h:695 /home/marvin/linux/kernel/torvalds2/arch/x86/mm/fault.c:1513 /home/marvin/linux/kernel/torvalds2/arch/x86/mm/fault.c:1561)
[ 6413.367602] asm_exc_page_fault (/home/marvin/linux/kernel/torvalds2/./arch/x86/include/asm/idtentry.h:570)
[ 6413.367617] value changed: 0xffff888341d43116 -> 0xffff888340f92116
[ 6413.367632] Reported by Kernel Concurrency Sanitizer on:
[ 6413.367640] CPU: 11 PID: 5558 Comm: ThreadPoolForeg Tainted: G L 6.6.0-rc2-kcsan-00143-gb5cbe7c00aa0 #41
[ 6413.367653] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
[ 6413.367660] ==================================================================
For your convenience, took the trouble of finding those suspicious lines of code:
The write side:
lib/maple_tree.c:491
--------------------
456 /*
457 * mas_set_parent() - Set the parent node and encode the slot
458 * @enode: The encoded maple node.
459 * @parent: The encoded maple node that is the parent of @enode.
460 * @slot: The slot that @enode resides in @parent.
461 *
462 * Slot number is encoded in the enode->parent bit 3-6 or 2-6, depending on the
463 * parent type.
464 */
465 static inline
466 void mas_set_parent(struct ma_state *mas, struct maple_enode *enode,
467 const struct maple_enode *parent, unsigned char slot)
468 {
469 unsigned long val = (unsigned long)parent;
470 unsigned long shift;
471 unsigned long type;
472 enum maple_type p_type = mte_node_type(parent);
473
474 MAS_BUG_ON(mas, p_type == maple_dense);
475 MAS_BUG_ON(mas, p_type == maple_leaf_64);
476
477 switch (p_type) {
478 case maple_range_64:
479 case maple_arange_64:
480 shift = MAPLE_PARENT_SLOT_SHIFT;
481 type = MAPLE_PARENT_RANGE64;
482 break;
483 default:
484 case maple_dense:
485 case maple_leaf_64:
486 shift = type = 0;
487 break;
488 }
489
490 val &= ~MAPLE_NODE_MASK; /* Clear all node metadata in parent */
→ 491 val |= (slot << shift) | type;
492 mte_to_node(enode)->parent = ma_parent_ptr(val);
493 }
lib/maple_tree.c:1701
---------------------
1682 /*
1683 * mas_adopt_children() - Set the parent pointer of all nodes in @parent to
1684 * @parent with the slot encoded.
1685 * @mas - the maple state (for the tree)
1686 * @parent - the maple encoded node containing the children.
1687 */
1688 static inline void mas_adopt_children(struct ma_state *mas,
1689 struct maple_enode *parent)
1690 {
1691 enum maple_type type = mte_node_type(parent);
1692 struct maple_node *node = mte_to_node(parent);
1693 void __rcu **slots = ma_slots(node, type);
1694 unsigned long *pivots = ma_pivots(node, type);
1695 struct maple_enode *child;
1696 unsigned char offset;
1697
1698 offset = ma_data_end(node, type, pivots, mas->max);
1699 do {
1700 child = mas_slot_locked(mas, slots, offset);
→ 1701 mas_set_parent(mas, child, parent, offset);
1702 } while (offset--);
1703 }
2562 static inline void mas_topiary_replace(struct ma_state *mas,
2563 struct maple_enode *old_enode)
2564 {
2565 struct ma_state tmp[3], tmp_next[3];
2566 MA_TOPIARY(subtrees, mas->tree);
2567 bool in_rcu;
2568 int i, n;
2569
2570 /* Place data in tree & then mark node as old */
2571 mas_put_in_tree(mas, old_enode);
2572
2573 /* Update the parent pointers in the tree */
2574 tmp[0] = *mas;
2575 tmp[0].offset = 0;
2576 tmp[1].node = MAS_NONE;
2577 tmp[2].node = MAS_NONE;
2578 while (!mte_is_leaf(tmp[0].node)) {
2579 n = 0;
2580 for (i = 0; i < 3; i++) {
2581 if (mas_is_none(&tmp[i]))
2582 continue;
2583
2584 while (n < 3) {
2585 if (!mas_find_child(&tmp[i], &tmp_next[n]))
2586 break;
2587 n++;
2588 }
2589
→ 2590 mas_adopt_children(&tmp[i], tmp[i].node);
2591 }
2592
2593 if (MAS_WARN_ON(mas, n == 0))
2594 break;
2595
2596 while (n < 3)
2597 tmp_next[n++].node = MAS_NONE;
2598
2599 for (i = 0; i < 3; i++)
2600 tmp[i] = tmp_next[i];
2601 }
2602
2603 /* Collect the old nodes that need to be discarded */
2604 if (mte_is_leaf(old_enode))
2605 return mas_free(mas, old_enode);
2606
2607 tmp[0] = *mas;
2608 tmp[0].offset = 0;
2609 tmp[0].node = old_enode;
2610 tmp[1].node = MAS_NONE;
2611 tmp[2].node = MAS_NONE;
2612 in_rcu = mt_in_rcu(mas->tree);
2613 do {
2614 n = 0;
2615 for (i = 0; i < 3; i++) {
2616 if (mas_is_none(&tmp[i]))
2617 continue;
The read side:
527 /*
528 * ma_dead_node() - check if the @enode is dead.
529 * @enode: The encoded maple node
530 *
531 * Return: true if dead, false otherwise.
532 */
533 static inline bool ma_dead_node(const struct maple_node *node)
534 {
535 struct maple_node *parent;
536
537 /* Do not reorder reads from the node prior to the parent check */
538 smp_rmb();
→ 539 parent = (void *)((unsigned long) node->parent & ~MAPLE_NODE_MASK);
540 return (parent == node);
541 }
2767 static inline void *mtree_range_walk(struct ma_state *mas)
2768 {
2769 unsigned long *pivots;
2770 unsigned char offset;
2771 struct maple_node *node;
2772 struct maple_enode *next, *last;
2773 enum maple_type type;
2774 void __rcu **slots;
2775 unsigned char end;
2776 unsigned long max, min;
2777 unsigned long prev_max, prev_min;
2778
2779 next = mas->node;
2780 min = mas->min;
2781 max = mas->max;
2782 do {
2783 offset = 0;
2784 last = next;
2785 node = mte_to_node(next);
2786 type = mte_node_type(next);
2787 pivots = ma_pivots(node, type);
2788 end = ma_data_end(node, type, pivots, max);
→ 2789 if (unlikely(ma_dead_node(node)))
2790 goto dead_node;
2791
2792 if (pivots[offset] >= mas->index) {
2793 prev_max = max;
2794 prev_min = min;
2795 max = pivots[offset];
2796 goto next;
2797 }
2798
2799 do {
2800 offset++;
2801 } while ((offset < end) && (pivots[offset] < mas->index));
As it is evident, ma_dead_node() expands to:
527 /*
528 * ma_dead_node() - check if the @enode is dead.
529 * @enode: The encoded maple node
530 *
531 * Return: true if dead, false otherwise.
532 */
533 static inline bool ma_dead_node(const struct maple_node *node)
534 {
535 struct maple_node *parent;
536
537 /* Do not reorder reads from the node prior to the parent check */
538 smp_rmb();
→ 539 parent = (void *)((unsigned long) node->parent & ~MAPLE_NODE_MASK);
540 return (parent == node);
541 }
as above, but the smb_rmb() protection is here, so the KCSAN warning should be double-checked for
validity.
But I do not really understand maple trees to their depth, I am only reporting the symptomatic
outlook of the reported data-race.
This is all-in-all a very interested subject, and I wish there was a way to just slurp all those
interesting kernel intrinsics into the brain, but it just ain't that easy. Forgive me for being
open ...
Best regards,
Mirsad Todorovac
