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UPSTREAM: af_unix: Save O(n) setup of Tarjan's algo.

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commit ba31b4a4e1018f5844c6eb31734976e2184f2f9a upstream.

Before starting Tarjan's algorithm, we need to mark all vertices
as unvisited.  We can save this O(n) setup by reserving two special
indices (0, 1) and using two variables.

The first time we link a vertex to unix_unvisited_vertices, we set
unix_vertex_unvisited_index to index.

During DFS, we can see that the index of unvisited vertices is the
same as unix_vertex_unvisited_index.

When we finalise SCC later, we set unix_vertex_grouped_index to each
vertex's index.

Then, we can know (i) that the vertex is on the stack if the index
of a visited vertex is >= 2 and (ii) that it is not on the stack and
belongs to a different SCC if the index is unix_vertex_grouped_index.

After the whole algorithm, all indices of vertices are set as
unix_vertex_grouped_index.

Next time we start DFS, we know that all unvisited vertices have
unix_vertex_grouped_index, and we can use unix_vertex_unvisited_index
as the not-on-stack marker.

To use the same variable in __unix_walk_scc(), we can swap
unix_vertex_(grouped|unvisited)_index at the end of Tarjan's
algorithm.

Bug: 404256079
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-10-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit e0e23fc499)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I275a421f7ddbb80f327a7d5203293debd7a136ad
This commit is contained in:
Kuniyuki Iwashima
2025-05-21 14:45:23 +00:00
committed by Lee Jones
parent 0c40a05117
commit 250c362acd
4 changed files with 20 additions and 20 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
android/abi_gki_aarch64.stg
View File

@@ -148465,12 +148465,6 @@ member {
type_id: 0xc93e017b
offset: 288
}
member {
id: 0x19406067
name: "on_stack"
type_id: 0x6d7f5ff6
offset: 576
}
member {
id: 0x9de8e020
name: "on_time"
@@ -277537,14 +277531,13 @@ struct_union {
kind: STRUCT
name: "unix_vertex"
definition {
bytesize: 80
bytesize: 72
member_id: 0x76197573
member_id: 0x4d8789fe
member_id: 0xd741c1b9
member_id: 0x0fc2a957
member_id: 0xad0a7aac
member_id: 0x03c71124
member_id: 0x19406067
}
}
struct_union {

4
android/abi_gki_aarch64.stg.allowed_breaks
View File

@@ -184,3 +184,7 @@ type 'struct unix_sock' changed
4 members ('struct list_head link' .. 'unsigned long gc_flags') changed
offset changed by 64
type 'struct unix_vertex' changed
byte size changed from 80 to 72
member 'bool on_stack' was removed

1
include/net/af_unix.h
View File

@@ -37,7 +37,6 @@ struct unix_vertex {
unsigned long out_degree;
unsigned long index;
unsigned long lowlink;
bool on_stack;
};
struct unix_edge {

26
net/unix/garbage.c
View File

@@ -115,16 +115,20 @@ static struct unix_vertex *unix_edge_successor(struct unix_edge *edge)
static LIST_HEAD(unix_unvisited_vertices);
enum unix_vertex_index {
UNIX_VERTEX_INDEX_UNVISITED,
UNIX_VERTEX_INDEX_MARK1,
UNIX_VERTEX_INDEX_MARK2,
UNIX_VERTEX_INDEX_START,
};
static unsigned long unix_vertex_unvisited_index = UNIX_VERTEX_INDEX_MARK1;
static void unix_add_edge(struct scm_fp_list *fpl, struct unix_edge *edge)
{
struct unix_vertex *vertex = edge->predecessor->vertex;
if (!vertex) {
vertex = list_first_entry(&fpl->vertices, typeof(*vertex), entry);
vertex->index = unix_vertex_unvisited_index;
vertex->out_degree = 0;
INIT_LIST_HEAD(&vertex->edges);
@@ -265,6 +269,7 @@ void unix_destroy_fpl(struct scm_fp_list *fpl)
}
static LIST_HEAD(unix_visited_vertices);
static unsigned long unix_vertex_grouped_index = UNIX_VERTEX_INDEX_MARK2;
static void __unix_walk_scc(struct unix_vertex *vertex)
{
@@ -274,10 +279,10 @@ static void __unix_walk_scc(struct unix_vertex *vertex)
LIST_HEAD(edge_stack);
next_vertex:
/* Push vertex to vertex_stack.
/* Push vertex to vertex_stack and mark it as on-stack
* (index >= UNIX_VERTEX_INDEX_START).
* The vertex will be popped when finalising SCC later.
*/
vertex->on_stack = true;
list_add(&vertex->scc_entry, &vertex_stack);
vertex->index = index;
@@ -291,7 +296,7 @@ next_vertex:
if (!next_vertex)
continue;
if (next_vertex->index == UNIX_VERTEX_INDEX_UNVISITED) {
if (next_vertex->index == unix_vertex_unvisited_index) {
/* Iterative deepening depth first search
*
* 1. Push a forward edge to edge_stack and set
@@ -317,7 +322,7 @@ prev_vertex:
* to skip SCC finalisation.
*/
vertex->lowlink = min(vertex->lowlink, next_vertex->lowlink);
} else if (next_vertex->on_stack) {
} else if (next_vertex->index != unix_vertex_grouped_index) {
/* Loop detected by a back/cross edge.
*
* The successor is on vertex_stack, so two vertices are
@@ -344,7 +349,8 @@ prev_vertex:
/* Don't restart DFS from this vertex in unix_walk_scc(). */
list_move_tail(&vertex->entry, &unix_visited_vertices);
vertex->on_stack = false;
/* Mark vertex as off-stack. */
vertex->index = unix_vertex_grouped_index;
}
list_del(&scc);
@@ -357,20 +363,18 @@ prev_vertex:
static void unix_walk_scc(void)
{
struct unix_vertex *vertex;
list_for_each_entry(vertex, &unix_unvisited_vertices, entry)
vertex->index = UNIX_VERTEX_INDEX_UNVISITED;
/* Visit every vertex exactly once.
* __unix_walk_scc() moves visited vertices to unix_visited_vertices.
*/
while (!list_empty(&unix_unvisited_vertices)) {
struct unix_vertex *vertex;
vertex = list_first_entry(&unix_unvisited_vertices, typeof(*vertex), entry);
__unix_walk_scc(vertex);
}
list_replace_init(&unix_visited_vertices, &unix_unvisited_vertices);
swap(unix_vertex_unvisited_index, unix_vertex_grouped_index);
}
static LIST_HEAD(gc_candidates);