Hi Slawa,

On 10/11/16 2:11 PM, Slawa Olhovchenkov wrote:
> On Tue, Oct 11, 2016 at 09:20:17AM +0200, Julien Charbon wrote:
>>  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.
> Thanks, very clear.
> Small qeustion: when both thread run on same CPU core, INP_WLOCK will
> be re-schedule?

 Hmm, a thread can re-scheduled but not a lock. Thus no sure I
understand your question here.  :)

> As I remeber race created by call tcp_twstart() at time of end
> tcp_close(), at path sofree()-tcp_usr_detach() and unexpected
> INP_TIMEWAIT state in the tcp_usr_detach(). INP_TIMEWAIT set in tcp_twstart()

 Exactly, thus the current fix is:  If you already have the INP_DROPPED
flag set you are not allowed to call tcp_twstart(), actually it is a
good candidate for a new INVARIANT.  Let me add that.

> After check source code I am found invocation of tcp_twstart() in
> sys/netinet/tcp_stacks/fastpath.c, sys/netinet/tcp_input.c,
> sys/dev/cxgb/ulp/tom/cxgb_cpl_io.c, sys/dev/cxgbe/tom/t4_cpl_io.c.
> Invocation from sys/netinet/tcp_stacks/fastpath.c and
> sys/netinet/tcp_input.c guarded by INP_WLOCK in tcp_input(), and now
> will be OK.
> Invocation from sys/dev/cxgb/ulp/tom/cxgb_cpl_io.c and
> sys/dev/cxgbe/tom/t4_cpl_io.c is not clear to me, I am see independed
> INP_WLOCK. Is this OK?
> Can be thread A wants do_peer_close() directed from chelsio IRQ
> handler, bypass tcp_input()?

 If you look carefully INP_WLOCK is used in cxgb_cpl_io.c and
t4_cpl_io.c before calling tcp_twstart().


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