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The calling of siw_cm_upcall and detaching new_cep with its listen_cep
should be atomistic semantics. Otherwise siw_reject may be called in a
temporary state, e,g, siw_cm_upcall is called but the new_cep->listen_cep
has not being cleared.

This fixes a WARN:

  WARNING: CPU: 7 PID: 201 at drivers/infiniband/sw/siw/siw_cm.c:255 siw_cep_put+0x125/0x130 [siw]
  CPU: 2 PID: 201 Comm: kworker/u16:22 Kdump: loaded Tainted: G            E     5.17.0-rc7 #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
  Workqueue: iw_cm_wq cm_work_handler [iw_cm]
  RIP: 0010:siw_cep_put+0x125/0x130 [siw]
  Call Trace:
   <TASK>
   siw_reject+0xac/0x180 [siw]
   iw_cm_reject+0x68/0xc0 [iw_cm]
   cm_work_handler+0x59d/0xe20 [iw_cm]
   process_one_work+0x1e2/0x3b0
   worker_thread+0x50/0x3a0
   ? rescuer_thread+0x390/0x390
   kthread+0xe5/0x110
   ? kthread_complete_and_exit+0x20/0x20
   ret_from_fork+0x1f/0x30
   </TASK>

Fixes: 6c52fdc2 ("rdma/siw: connection management")
Link: https://lore.kernel.org/r/d528d83466c44687f3872eadcb8c184528b2e2d4.1650526554.git.chengyou@linux.alibaba.com

Reported-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Cheng Xu <chengyou@linux.alibaba.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>