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af_unix: Remove lock dance in unix_peek_fds().

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In the previous GC implementation, the shape of the inflight socket
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 because the set of sockets visited in scan_inflight()
would change 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 the new GC 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@amazon.com>
Link: https://lore.kernel.org/r/20240401173125.92184-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Kuniyuki Iwashima 2024-04-01 10:31:25 -07:00 committed by Jakub Kicinski
parent 7c349ed090
commit 118f457da9
3 changed files with 1 additions and 44 deletions
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1
include/net/af_unix.h
View File

@ -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_add_edges(struct scm_fp_list *fpl, struct unix_sock *receiver);
void unix_del_edges(struct scm_fp_list *fpl);

42
net/unix/af_unix.c
View File

@ -1814,48 +1814,6 @@ 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)

2
net/unix/garbage.c
View File

@ -183,7 +183,7 @@ static void unix_free_vertices(struct scm_fp_list *fpl)
}
}
DEFINE_SPINLOCK(unix_gc_lock);
static DEFINE_SPINLOCK(unix_gc_lock);
unsigned int unix_tot_inflight;
void unix_add_edges(struct scm_fp_list *fpl, struct unix_sock *receiver)