The following BUG has just surfaced with our 5.16 testing:

  BUG: using smp_processor_id() in preemptible [00000000] code: mpicheck/1581081
  caller is sdma_select_user_engine+0x72/0x210 [hfi1]
  CPU: 0 PID: 1581081 Comm: mpicheck Tainted: G S                5.16.0-rc1+ #1
  Hardware name: Intel Corporation S2600WT2R/S2600WT2R, BIOS SE5C610.86B.01.01.0016.033120161139 03/31/2016
  Call Trace:
   <TASK>
   dump_stack_lvl+0x33/0x42
   check_preemption_disabled+0xbf/0xe0
   sdma_select_user_engine+0x72/0x210 [hfi1]
   ? _raw_spin_unlock_irqrestore+0x1f/0x31
   ? hfi1_mmu_rb_insert+0x6b/0x200 [hfi1]
   hfi1_user_sdma_process_request+0xa02/0x1120 [hfi1]
   ? hfi1_write_iter+0xb8/0x200 [hfi1]
   hfi1_write_iter+0xb8/0x200 [hfi1]
   do_iter_readv_writev+0x163/0x1c0
   do_iter_write+0x80/0x1c0
   vfs_writev+0x88/0x1a0
   ? recalibrate_cpu_khz+0x10/0x10
   ? ktime_get+0x3e/0xa0
   ? __fget_files+0x66/0xa0
   do_writev+0x65/0x100
   do_syscall_64+0x3a/0x80

Fix this long standing bug by moving the smp_processor_id() to after the
rcu_read_lock().

The rcu_read_lock() implicitly disables preemption.

Link: https://lore.kernel.org/r/20211129191958.101968.87329.stgit@awfm-01.cornelisnetworks.com
Cc: stable@vger.kernel.org
Fixes: 0cb2aa69 ("IB/hfi1: Add sysfs interface for affinity setup")
Signed-off-by: NMike Marciniszyn <mike.marciniszyn@cornelisnetworks.com>
Signed-off-by: NDennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>