• David Howells's avatar
    rxrpc: Rewrite the data and ack handling code · 248f219c
    David Howells authored
    Rewrite the data and ack handling code such that:
    
     (1) Parsing of received ACK and ABORT packets and the distribution and the
         filing of DATA packets happens entirely within the data_ready context
         called from the UDP socket.  This allows us to process and discard ACK
         and ABORT packets much more quickly (they're no longer stashed on a
         queue for a background thread to process).
    
     (2) We avoid calling skb_clone(), pskb_pull() and pskb_trim().  We instead
         keep track of the offset and length of the content of each packet in
         the sk_buff metadata.  This means we don't do any allocation in the
         receive path.
    
     (3) Jumbo DATA packet parsing is now done in data_ready context.  Rather
         than cloning the packet once for each subpacket and pulling/trimming
         it, we file the packet multiple times with an annotation for each
         indicating which subpacket is there.  From that we can directly
         calculate the offset and length.
    
     (4) A call's receive queue can be accessed without taking locks (memory
         barriers do have to be used, though).
    
     (5) Incoming calls are set up from preallocated resources and immediately
         made live.  They can than have packets queued upon them and ACKs
         generated.  If insufficient resources exist, DATA packet #1 is given a
         BUSY reply and other DATA packets are discarded).
    
     (6) sk_buffs no longer take a ref on their parent call.
    
    To make this work, the following changes are made:
    
     (1) Each call's receive buffer is now a circular buffer of sk_buff
         pointers (rxtx_buffer) rather than a number of sk_buff_heads spread
         between the call and the socket.  This permits each sk_buff to be in
         the buffer multiple times.  The receive buffer is reused for the
         transmit buffer.
    
     (2) A circular buffer of annotations (rxtx_annotations) is kept parallel
         to the data buffer.  Transmission phase annotations indicate whether a
         buffered packet has been ACK'd or not and whether it needs
         retransmission.
    
         Receive phase annotations indicate whether a slot holds a whole packet
         or a jumbo subpacket and, if the latter, which subpacket.  They also
         note whether the packet has been decrypted in place.
    
     (3) DATA packet window tracking is much simplified.  Each phase has just
         two numbers representing the window (rx_hard_ack/rx_top and
         tx_hard_ack/tx_top).
    
         The hard_ack number is the sequence number before base of the window,
         representing the last packet the other side says it has consumed.
         hard_ack starts from 0 and the first packet is sequence number 1.
    
         The top number is the sequence number of the highest-numbered packet
         residing in the buffer.  Packets between hard_ack+1 and top are
         soft-ACK'd to indicate they've been received, but not yet consumed.
    
         Four macros, before(), before_eq(), after() and after_eq() are added
         to compare sequence numbers within the window.  This allows for the
         top of the window to wrap when the hard-ack sequence number gets close
         to the limit.
    
         Two flags, RXRPC_CALL_RX_LAST and RXRPC_CALL_TX_LAST, are added also
         to indicate when rx_top and tx_top point at the packets with the
         LAST_PACKET bit set, indicating the end of the phase.
    
     (4) Calls are queued on the socket 'receive queue' rather than packets.
         This means that we don't need have to invent dummy packets to queue to
         indicate abnormal/terminal states and we don't have to keep metadata
         packets (such as ABORTs) around
    
     (5) The offset and length of a (sub)packet's content are now passed to
         the verify_packet security op.  This is currently expected to decrypt
         the packet in place and validate it.
    
         However, there's now nowhere to store the revised offset and length of
         the actual data within the decrypted blob (there may be a header and
         padding to skip) because an sk_buff may represent multiple packets, so
         a locate_data security op is added to retrieve these details from the
         sk_buff content when needed.
    
     (6) recvmsg() now has to handle jumbo subpackets, where each subpacket is
         individually secured and needs to be individually decrypted.  The code
         to do this is broken out into rxrpc_recvmsg_data() and shared with the
         kernel API.  It now iterates over the call's receive buffer rather
         than walking the socket receive queue.
    
    Additional changes:
    
     (1) The timers are condensed to a single timer that is set for the soonest
         of three timeouts (delayed ACK generation, DATA retransmission and
         call lifespan).
    
     (2) Transmission of ACK and ABORT packets is effected immediately from
         process-context socket ops/kernel API calls that cause them instead of
         them being punted off to a background work item.  The data_ready
         handler still has to defer to the background, though.
    
     (3) A shutdown op is added to the AF_RXRPC socket so that the AFS
         filesystem can shut down the socket and flush its own work items
         before closing the socket to deal with any in-progress service calls.
    
    Future additional changes that will need to be considered:
    
     (1) Make sure that a call doesn't hog the front of the queue by receiving
         data from the network as fast as userspace is consuming it to the
         exclusion of other calls.
    
     (2) Transmit delayed ACKs from within recvmsg() when we've consumed
         sufficiently more packets to avoid the background work item needing to
         run.
    Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
    248f219c
af_rxrpc.h 1.77 KB