https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=265382
Bug ID: 265382
Summary: VLAN hash-list table thrashing when adding and
removing entries
Product: Base System
Version: Unspecified
Hardware: Any
OS: Any
Status: New
Severity: Affects Some People
Priority: ---
Component: kern
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
vlan_remhash() uses incorrect value for b.
When using the default value for VLAN_DEF_HWIDTH (4), the VLAN hash-list table
expands from 16 chains to 32 chains as the 129th entry is added. trunk->hwidth
becomes 5. Say a few more entries are added and there are now 135 entries.
trunk-hwidth will still be 5. If an entry is removed, vlan_remhash() will
calculate a value of 32 for b. refcnt will be decremented to 134. The if
comparison at line 473 will return true and vlan_growhash() will be called. The
VLAN hash-list table will be compressed from 32 chains wide to 16 chains wide.
hwidth will become 4. This is an error, and it can be seen when a new VLAN is
added. The table will again be expanded. If an entry is then removed, again
the table is contracted.
If the number of VLANS stays in the range of 128-512, each time an insert
follows a remove, the table will expand. Each time a remove follows an
insert, the table will be contracted.
The fix is simple. The line 473 should test that the number of entries has
decreased such that the table should be contracted using what would be the new
value of hwidth. line 467 should be:
b = 1 << (trunk->hwidth - 1);
For reference:
458 static int
459 vlan_remhash(struct ifvlantrunk *trunk, struct ifvlan *ifv)
460 {
461 int i, b;
462 struct ifvlan *ifv2;
463
464 VLAN_XLOCK_ASSERT();
465 KASSERT(trunk->hwidth > 0, ("%s: hwidth not positive", __func__));
466
467 b = 1 << trunk->hwidth;
468 i = HASH(ifv->ifv_vid, trunk->hmask);
469 CK_SLIST_FOREACH(ifv2, &trunk->hash[i], ifv_list)
470 if (ifv2 == ifv) {
471 trunk->refcnt--;
472 CK_SLIST_REMOVE(&trunk->hash[i], ifv2, ifvlan,
ifv_list);
473 if (trunk->refcnt < (b * b) / 2)
474 vlan_growhash(trunk, -1);
475 return (0);
476 }
477
478 panic("%s: vlan not found\n", __func__);
479 return (ENOENT); /*NOTREACHED*/
480 }
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