> > From: Miklos Szeredi <[EMAIL PROTECTED]> > > Date: Mon, 04 Jun 2007 11:45:32 +0200 > > > > > > A recv() on an AF_UNIX, SOCK_STREAM socket can race with a > > > > send()+close() on the peer, causing recv() to return zero, even though > > > > the sent data should be received. > > > > > > > > This happens if the send() and the close() is performed between > > > > skb_dequeue() and checking sk->sk_shutdown in unix_stream_recvmsg(): > > > > > > > > process A skb_dequeue() returns NULL, there's no data in the socket > > > > queue > > > > process B new data is inserted onto the queue by unix_stream_sendmsg() > > > > process B sk->sk_shutdown is set to SHUTDOWN_MASK by > > > > unix_release_sock() > > > > process A sk->sk_shutdown is checked, unix_release_sock() returns zero > > > > > > This is only part of the story. It turns out, there are other races > > > involving the garbage collector, that can throw away perfectly good > > > packets with AF_UNIX sockets in them. > > > > > > The problems arise when a socket goes from installed to in-flight or > > > vica versa during garbage collection. Since gc is done with a > > > spinlock held, this only shows up on SMP. > > > > > > The following patch fixes it for me, but it's possibly the wrong > > > approach. > > > > > > Signed-off-by: Miklos Szeredi <[EMAIL PROTECTED]> > > Concerning this specific patch I think we need to rethink it > a bit. > > Holding a global mutex over recvmsg() calls under AF_UNIX is pretty > much a non-starter, this will kill performance for multi-threaded > apps.
That's an rwsem held for read. It's held for write in unix_gc() only for a short duration, and unix_gc() should only rarely be called. So I don't think there's any performance problem here. > > One possible solution is for the garbage collection code to hold the > u->readlock while processing a socket, but be careful about deadlocks. That would have exactly the same effect. Only the code would be more complicated. Miklos - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
