netdev - [PATCH v1 net-next 15/16] af_unix: Remove lock dance in unix_peek

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Date: Fri, 2 Feb 2024 19:00:57 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>
CC: Kuniyuki Iwashima <kuniyu@...zon.com>, Kuniyuki Iwashima
	<kuni1840@...il.com>, <netdev@...r.kernel.org>
Subject: [PATCH v1 net-next 15/16] af_unix: Remove lock dance in unix_peek_fds().

In the previous GC implementation, the shape of the inflight graph
was not expected to change while GC was in progress.

MSG_PEEK was tricky because it could install inflight fd silently
and transform the graph.

Let's say we peeked a fd, which was a listening socket, and accept()ed
some embryo sockets from it.  The garbage collection algorithm would
have been confused as the set of sockets visited in scan_inflight()
were different within the same GC invocation.

That's why we placed spin_lock(&unix_gc_lock) and spin_unlock() in
unix_peek_fds() with a fat comment.

In a new implementation, we no longer garbage-collect the socket if
it exists in another queue, that is, if it has a bridge to another
SCC.  Also, accept() will require the lock if it has edges.

Thus, we need not do the complicated lock dance.

Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
---
 include/net/af_unix.h |  1 -
 net/unix/af_unix.c    | 42 ------------------------------------------
 net/unix/garbage.c    |  2 +-
 3 files changed, 1 insertion(+), 44 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 78ab1107ffb3..33ddfe27bf50 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -17,7 +17,6 @@ static inline struct unix_sock *unix_get_socket(struct file *filp)
 }
 #endif
 
-extern spinlock_t unix_gc_lock;
 extern unsigned int unix_tot_inflight;
 
 void unix_init_vertex(struct unix_sock *u);
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 9022a3a5dccc..d55fc3e9875b 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1827,48 +1827,6 @@ static void unix_detach_fds(struct scm_cookie *scm, struct sk_buff *skb)
 static void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb)
 {
 	scm->fp = scm_fp_dup(UNIXCB(skb).fp);
-
-	/*
-	 * Garbage collection of unix sockets starts by selecting a set of
-	 * candidate sockets which have reference only from being in flight
-	 * (total_refs == inflight_refs).  This condition is checked once during
-	 * the candidate collection phase, and candidates are marked as such, so
-	 * that non-candidates can later be ignored.  While inflight_refs is
-	 * protected by unix_gc_lock, total_refs (file count) is not, hence this
-	 * is an instantaneous decision.
-	 *
-	 * Once a candidate, however, the socket must not be reinstalled into a
-	 * file descriptor while the garbage collection is in progress.
-	 *
-	 * If the above conditions are met, then the directed graph of
-	 * candidates (*) does not change while unix_gc_lock is held.
-	 *
-	 * Any operations that changes the file count through file descriptors
-	 * (dup, close, sendmsg) does not change the graph since candidates are
-	 * not installed in fds.
-	 *
-	 * Dequeing a candidate via recvmsg would install it into an fd, but
-	 * that takes unix_gc_lock to decrement the inflight count, so it's
-	 * serialized with garbage collection.
-	 *
-	 * MSG_PEEK is special in that it does not change the inflight count,
-	 * yet does install the socket into an fd.  The following lock/unlock
-	 * pair is to ensure serialization with garbage collection.  It must be
-	 * done between incrementing the file count and installing the file into
-	 * an fd.
-	 *
-	 * If garbage collection starts after the barrier provided by the
-	 * lock/unlock, then it will see the elevated refcount and not mark this
-	 * as a candidate.  If a garbage collection is already in progress
-	 * before the file count was incremented, then the lock/unlock pair will
-	 * ensure that garbage collection is finished before progressing to
-	 * installing the fd.
-	 *
-	 * (*) A -> B where B is on the queue of A or B is on the queue of C
-	 * which is on the queue of listening socket A.
-	 */
-	spin_lock(&unix_gc_lock);
-	spin_unlock(&unix_gc_lock);
 }
 
 static void unix_destruct_scm(struct sk_buff *skb)
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 84c445c79f8c..d9dd97d3d4a6 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -127,7 +127,7 @@ static void unix_graph_update(struct unix_edge *edge)
 	unix_graph_maybe_cyclic = true;
 }
 
-DEFINE_SPINLOCK(unix_gc_lock);
+static DEFINE_SPINLOCK(unix_gc_lock);
 static LIST_HEAD(unix_unvisited_vertices);
 unsigned int unix_tot_inflight;
 
-- 
2.30.2