[#LU-15190] ptlrpc_server_check_resend_in_progress() can miss duplicate RPC

ptlrpc_server_check_resend_in_progress() has the following check at the beginning:

        if (!(lustre_msg_get_flags(req->rq_reqmsg) & MSG_RESENT) ||
            (atomic_read(&req->rq_export->exp_rpc_count) == 0))
                return NULL;

I think this can cause duplicate RPCs if none is in progress at the moment (due to high load, deep incoming queue).

and there is a crash dump in support of this theory. in that dump I was able to find lots of duplicate (up to 14). for example,

crash> p *(struct ptlrpc_request *)(0xffff9887c6c75ee0-0x60)
  rq_reqmsg = 0xffff9887c7f34000, 
  rq_xid = 1709012909603712, 
  rq_export = 0xffff988805ee5400, 
  rq_peer = {
    nid = 1407418002966021, 

crash> p *(struct ptlrpc_request *)(0xffff987eb906a8e0-0x60)
  rq_reqmsg = 0xffff987e7145c148, 
  rq_xid = 1709012909603712, 
  rq_export = 0xffff988805ee5400, 
  rq_peer = {
    nid = 1407418002966021, 

crash> ptlrpc_request_dump (0xffff98745d3a5a60-0x60)
req: 0xffff9875002d6520, xid: 2531069376, opc: 103, flags: 2, buf2: 0xffff9875002d6600/104
crash> ptlrpc_request_dump (0xffff98771dd06360-0x60)
req: 0xffff9884e2218520, xid: 2531069376, opc: 103, flags: 2, buf2: 0xffff9884e2218600/104
crash> ptlrpc_request_dump (0xffff9878a80c3ae0-0x60)
req: 0xffff9878ae8ae148, xid: 2531069376, opc: 103, flags: 2, buf2: 0xffff9878ae8ae228/104
crash> ptlrpc_request_dump (0xffff98789c049b60-0x60)
req: 0xffff98789c7403d8, xid: 2531069376, opc: 103, flags: 2, buf2: 0xffff98789c7404b8/104

crash> p ((struct ldlm_request *)0xffff9875002d6600)->lock_handle
    cookie = 13969718594132579448
crash> p ((struct ldlm_request *)0xffff9884e2218600)->lock_handle
    cookie = 13969718594132579448
crash> p ((struct ldlm_request *)0xffff9878ae8ae228)->lock_handle
    cookie = 13969718594132579448
crash> p ((struct ldlm_request *)0xffff98789c7404b8)->lock_handle
    cookie = 13969718594132579448

$ cat xid-sorted-list.txt | wc -l
877858
$ cat xid-sorted-list.txt | uniq |wc -l
213480

given ptlrpc_server_check_resend_in_progress() uses a linear scan to check for duplicates and a single spinlock, the check takes a lot and many CPUs were spinning for seconds.

[LU-15190] ptlrpc_server_check_resend_in_progress() can miss duplicate RPC Created: 03/Nov/21 Updated: 05/May/22 Resolved: 05/May/22
Status:	Resolved
Project:	Lustre
Component/s:	None
Affects Version/s:	Upstream
Fix Version/s:	Lustre 2.15.0

[LU-15190] ptlrpc_server_check_resend_in_progress() can miss duplicate RPC Created: 03/Nov/21 Updated: 05/May/22 Resolved: 05/May/22