Hi Slawa, On 10/10/16 7:35 PM, Slawa Olhovchenkov wrote: > On Mon, Oct 10, 2016 at 05:44:21PM +0200, Julien Charbon wrote: >>>> can check the current other usages of goto findpcb in tcp_input(). The >>>> rational here being: >>>> >>>> - Behavior before the patch: If the inp we found was deleted then goto >>>> findpcb. >>>> - Behavior after the patch: If the inp we found was deleted or dropped >>>> then goto findpcb. >>>> >>>> I just prefer having the same behavior applied everywhere: If >>>> tcp_input() loses the inp lock race and the inp was deleted or dropped >>>> then retry to find a new inpcb to deliver to. >>>> >>>> But you are right dropping the packet here will also fix the issue. >>>> >>>> Then the review process becomes quite helpful because people can argue: >>>> Dropping here is better because "blah", or goto findpcb is better >>>> because "bluh", etc. And at the review end you have a nice final patch. >>>> >>>> https://reviews.freebsd.org/D8211 >>> >>> I am not sure, I am see to >>> >>> sys/netinet/in_pcb.h:#define INP_DROPPED 0x04000000 /* >> protocol drop flag */ >>> >>> and think this is a flag 'all packets must be droped' >> >> Hm, I believe this flag means "this inp has been dropped by the TCP >> stack, so don't use it anymore". Actually this flag is better described >> in the function that sets it: >> >> "(INP_DROPPED) is used by TCP to mark an inpcb as unused and avoid >> future packet delivery or event notification when a socket remains open >> but TCP has closed." >> >> https://github.com/freebsd/freebsd/blob/release/11.0.0/sys/netinet/in_pcb.c#L1320 >> >> /* >> * in_pcbdrop() removes an inpcb from hashed lists, releasing its >> address and >> * port reservation, and preventing it from being returned by inpcb lookups. >> * >> * It is used by TCP to mark an inpcb as unused and avoid future packet >> * delivery or event notification when a socket remains open but TCP has >> * closed. This might occur as a result of a shutdown()-initiated TCP close >> * or a RST on the wire, and allows the port binding to be reused while >> still >> * maintaining the invariant that so_pcb always points to a valid inpcb >> until >> * in_pcbdetach(). >> * >> */ >> void >> in_pcbdrop(struct inpcb *inp) >> { >> inp->inp_flags |= INP_DROPPED; >> ... >> >> The classical example where "goto findpcb" is useful: You receive a >> new connection request with a TCP SYN packet and this packet is unlucky >> and reached a inp being dropped: >> >> - with "goto findpcb" approach, the next lookup will most likely find >> the LISTEN inp and start the TCP hand-shake as usual >> - with "drop the packet" approach, the TCP client will need to >> re-transmit a TCP SYN packet >> >> It is not because a packet was unlucky once that it deserves to be >> dropped. :) > > Thanks for explaining, very helpfull. > In this situation (TCP SYN with same 4-tuple as existing socket) > allocate new PCB is best. But for this we must destroy current PCB. I > am think INP_WUNLOCK(inp) don't destroy it and in_pcblookup_mbuf find > it again (I am think in_pcblookup_mbuf find this PCB on first turn). > I am assume for classical example in_pcbrele_wlocked(inp) free and > destroy current PCB for possibility in_pcblookup_mbuf allocate new > one.
Astute question: Here, the same inp cannot be find again by in_pcblookup_mbuf(), the explanation is a bit long though: in_pcbdrop() does two things under INP_WLOCK lock: https://github.com/freebsd/freebsd/blob/release/11.0.0/sys/netinet/in_pcb.c#L1334 1. Add INP_DROPPED flag 2. Remove the inp from the TCP hash table And once removed from the TCP hash table, in_pcblookup_mbuf() will return NULL when doing the same TCP hash table lookup again. It means that under a INP_WLOCK lock, these two changes are atomic: - Either an inp does not have INP_DROPPED flag and can be in TCP hash table - Either an inp has INP_DROPPED flag and is not in TCP hash table But when you don't have the INP_WLOCK lock, then you can witness the intermediate state where a inp is still in TCP hash table while a thread is adding the INP_DROPPED flag. Nothing unusual here. Then threads are competing for the INP_WLOCK lock. For the example, let's say the thread A wants to run tcp_input()/in_pcblookup_mbuf() and racing for this INP_WLOCK: https://github.com/freebsd/freebsd/blob/release/11.0.0/sys/netinet/in_pcb.c#L1964 And thread B wants to run tcp_timer_2msl()/tcp_close()/in_pcbdrop() and racing for this INP_WLOCK: https://github.com/freebsd/freebsd/blob/release/11.0.0/sys/netinet/tcp_timer.c#L323 That leads to two cases: o Thread A wins the race: Thread A will continue tcp_input() as usal and INP_DROPPED flags is not set and inp is still in TCP hash table. Thread B is waiting on thread A to release INP_WLOCK after finishing tcp_input() processing, and thread B will continue tcp_timer_2msl()/tcp_close()/in_pcbdrop() processing. o Thread B wins the race: Thread B runs tcp_timer_2msl()/tcp_close()/in_pcbdrop() and inp INP_DROPPED is set and inp being removed from TCP hash table. In parallel, thread A has found the inp in TCP hash before is was removed, and waiting on the found inp INP_WLOCK lock. Once thread B has released the INP_WLOCK lock, thread A gets this lock and sees the INP_DROPPED flag and do "goto findpcb" but here because the inp is not more in TCP hash table and it will not be find again by in_pcblookup_mbuf(). Hopefully I am clear enough here. -- Julien
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