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 <al...@nicira.com>
Signed-off-by: Ben Pfaff <b...@nicira.com>
---
lib/netlink-socket.c | 10 ++++++++++
lib/netlink-socket.h | 2 ++
2 files changed, 12 insertions(+)
diff --git a/lib/netlink-socket.c b/lib/netlink-socket.c
index 42ba7e1..8f8d603 100644
--- a/lib/netlink-socket.c
+++ b/lib/netlink-socket.c
@@ -714,6 +714,7 @@ nl_dump_start(struct nl_dump *dump, int protocol, const
struct ofpbuf *request)
atomic_init(&dump->status, status << 1);
dump->nl_seq = nl_msg_nlmsghdr(request)->nlmsg_seq;
dump->status_seq = seq_create();
+ ovs_mutex_init(&dump->mutex);
}
/* Attempts to retrieve another reply from 'dump' into 'buffer'. 'dump' must
@@ -756,7 +757,15 @@ nl_dump_next(struct nl_dump *dump, struct ofpbuf *reply,
struct ofpbuf *buffer)
return false;
}
+ /* Take the mutex here to avoid a in-kernel race. If two threads try
+ * to read from a Netlink dump socket at once, then the socket error
+ * can be set to EINVAL, which will be encountered on the next recv on
+ * that socket, which could be anywhere due to the way that we pool
+ * Netlink sockets. Serializing the recv calls avoids the issue. */
+ ovs_mutex_lock(&dump->mutex);
retval = nl_sock_recv__(dump->sock, buffer, false);
+ ovs_mutex_unlock(&dump->mutex);
+
if (retval) {
ofpbuf_clear(buffer);
if (retval == EAGAIN) {
@@ -835,6 +844,7 @@ nl_dump_done(struct nl_dump *dump)
}
nl_pool_release(dump->sock);
seq_destroy(dump->status_seq);
+ ovs_mutex_destroy(&dump->mutex);
return status >> 1;
}
diff --git a/lib/netlink-socket.h b/lib/netlink-socket.h
index dd32409..8ac201a 100644
--- a/lib/netlink-socket.h
+++ b/lib/netlink-socket.h
@@ -52,6 +52,7 @@
#include <stdint.h>
#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,
--
1.7.10.4
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