From 0791315e4dbf82e293a759086ee17c84b12b1d6e Mon Sep 17 00:00:00 2001 From: Ben Pfaff Date: Thu, 10 Jul 2014 16:48:16 -0700 Subject: netlink-socket: Work around kernel Netlink dump thread races. The Linux kernel Netlink implementation has two races that cause problems for processes that attempt to dump a table in a multithreaded manner. The first race is in the structure of the kernel netlink_recv() function. This function pulls a message from the socket queue and, if there is none, reports EAGAIN: skb = skb_recv_datagram(sk, flags, noblock, &err); if (skb == NULL) goto out; Only if a message is successfully read from the socket queue does the function, toward the end, try to queue up a new message to be dumped: if (nlk->cb && atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2) { ret = netlink_dump(sk); if (ret) { sk->sk_err = ret; sk->sk_error_report(sk); } } This means that if thread A reads a message from a dump, then thread B attempts to read one before A queues up the next, B will get EAGAIN. This means that, following EAGAIN, B needs to wait until A returns to userspace before it tries to read the socket again. nl_dump_next() already does this, using 'dump->status_seq' (although the need for it has never been explained clearly, to my knowledge). The second race is more serious. Suppose thread X and thread Y both simultaneously attempt to queue up a new message to be dumped, using the call to netlink_dump() quoted above. netlink_dump() begins with: mutex_lock(nlk->cb_mutex); cb = nlk->cb; if (cb == NULL) { err = -EINVAL; goto errout_skb; } Suppose that X gets cb_mutex first and finds that the dump is complete. It will therefore, toward the end of netlink_dump(), clear nlk->cb to NULL to indicate that no dump is in progress and release the mutex: nlk->cb = NULL; mutex_unlock(nlk->cb_mutex); When Y grabs cb_mutex afterward, it will see that nlk->cb is NULL and return -EINVAL as quoted above. netlink_recv() stuffs -EINVAL in sk_err, but that error is not reported immediately; instead, it is saved for the next read from the socket. Since Open vSwitch maintains a pool of Netlink sockets, that next failure can crop up pretty much anywhere. One of the worst places for it to crop up is in the execution of a later transaction (e.g. in nl_sock_transact_multiple__()), because userspace treats Netlink transactions as idempotent and will re-execute them when socket errors occur. For a transaction that sends a packet, this causes packet duplication, which we actually observed in practice. (ENOBUFS should actually cause transactions to be re-executed in many cases, but EINVAL should not; this is a separate bug in the userspace netlink code.) VMware-BZ: #1283188 Reported-and-tested-by: Alex Wang Signed-off-by: Ben Pfaff Acked-by: Alex Wang --- lib/netlink-socket.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'lib/netlink-socket.h') diff --git a/lib/netlink-socket.h b/lib/netlink-socket.h index dd3240907..8ac201aac 100644 --- a/lib/netlink-socket.h +++ b/lib/netlink-socket.h @@ -52,6 +52,7 @@ #include #include "ofpbuf.h" #include "ovs-atomic.h" +#include "ovs-thread.h" struct nl_sock; @@ -114,6 +115,7 @@ struct nl_dump { atomic_uint status; /* Low bit set if we read final message. * Other bits hold an errno (0 for success). */ struct seq *status_seq; /* Tracks changes to the above 'status'. */ + struct ovs_mutex mutex; }; void nl_dump_start(struct nl_dump *, int protocol, -- cgit v1.2.1