From 540b1c48c37ac0ad66212004db21e1ff7e2d78be Mon Sep 17 00:00:00 2001 From: David Howells Date: Mon, 27 Feb 2017 15:43:06 +0000 Subject: rxrpc: Fix deadlock between call creation and sendmsg/recvmsg All the routines by which rxrpc is accessed from the outside are serialised by means of the socket lock (sendmsg, recvmsg, bind, rxrpc_kernel_begin_call(), ...) and this presents a problem: (1) If a number of calls on the same socket are in the process of connection to the same peer, a maximum of four concurrent live calls are permitted before further calls need to wait for a slot. (2) If a call is waiting for a slot, it is deep inside sendmsg() or rxrpc_kernel_begin_call() and the entry function is holding the socket lock. (3) sendmsg() and recvmsg() or the in-kernel equivalents are prevented from servicing the other calls as they need to take the socket lock to do so. (4) The socket is stuck until a call is aborted and makes its slot available to the waiter. Fix this by: (1) Provide each call with a mutex ('user_mutex') that arbitrates access by the users of rxrpc separately for each specific call. (2) Make rxrpc_sendmsg() and rxrpc_recvmsg() unlock the socket as soon as they've got a call and taken its mutex. Note that I'm returning EWOULDBLOCK from recvmsg() if MSG_DONTWAIT is set but someone else has the lock. Should I instead only return EWOULDBLOCK if there's nothing currently to be done on a socket, and sleep in this particular instance because there is something to be done, but we appear to be blocked by the interrupt handler doing its ping? (3) Make rxrpc_new_client_call() unlock the socket after allocating a new call, locking its user mutex and adding it to the socket's call tree. The call is returned locked so that sendmsg() can add data to it immediately. From the moment the call is in the socket tree, it is subject to access by sendmsg() and recvmsg() - even if it isn't connected yet. (4) Lock new service calls in the UDP data_ready handler (in rxrpc_new_incoming_call()) because they may already be in the socket's tree and the data_ready handler makes them live immediately if a user ID has already been preassigned. Note that the new call is locked before any notifications are sent that it is live, so doing mutex_trylock() *ought* to always succeed. Userspace is prevented from doing sendmsg() on calls that are in a too-early state in rxrpc_do_sendmsg(). (5) Make rxrpc_new_incoming_call() return the call with the user mutex held so that a ping can be scheduled immediately under it. Note that it might be worth moving the ping call into rxrpc_new_incoming_call() and then we can drop the mutex there. (6) Make rxrpc_accept_call() take the lock on the call it is accepting and release the socket after adding the call to the socket's tree. This is slightly tricky as we've dequeued the call by that point and have to requeue it. Note that requeuing emits a trace event. (7) Make rxrpc_kernel_send_data() and rxrpc_kernel_recv_data() take the new mutex immediately and don't bother with the socket mutex at all. This patch has the nice bonus that calls on the same socket are now to some extent parallelisable. Note that we might want to move rxrpc_service_prealloc() calls out from the socket lock and give it its own lock, so that we don't hang progress in other calls because we're waiting for the allocator. We probably also want to avoid calling rxrpc_notify_socket() from within the socket lock (rxrpc_accept_call()). Signed-off-by: David Howells Tested-by: Marc Dionne Signed-off-by: David S. Miller --- include/trace/events/rxrpc.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'include') diff --git a/include/trace/events/rxrpc.h b/include/trace/events/rxrpc.h index 593f586545eb..39123c06a566 100644 --- a/include/trace/events/rxrpc.h +++ b/include/trace/events/rxrpc.h @@ -119,6 +119,7 @@ enum rxrpc_recvmsg_trace { rxrpc_recvmsg_full, rxrpc_recvmsg_hole, rxrpc_recvmsg_next, + rxrpc_recvmsg_requeue, rxrpc_recvmsg_return, rxrpc_recvmsg_terminal, rxrpc_recvmsg_to_be_accepted, @@ -277,6 +278,7 @@ enum rxrpc_congest_change { EM(rxrpc_recvmsg_full, "FULL") \ EM(rxrpc_recvmsg_hole, "HOLE") \ EM(rxrpc_recvmsg_next, "NEXT") \ + EM(rxrpc_recvmsg_requeue, "REQU") \ EM(rxrpc_recvmsg_return, "RETN") \ EM(rxrpc_recvmsg_terminal, "TERM") \ EM(rxrpc_recvmsg_to_be_accepted, "TBAC") \ -- cgit v1.2.3