On Fri, Jul 07, 2017 at 10:31:28AM +0200, Ingo Molnar wrote:

Here's a quick list of all the use cases:

 net/netfilter/nf_conntrack_core.c:

   - This is I believe the 'original', historic spin_unlock_wait() usecase that
     still exists in the kernel. spin_unlock_wait() is only used in a rare case, 
     when the netfilter hash is resized via nf_conntrack_hash_resize() - which is 
     a very heavy operation to begin with. It will no doubt get slower with the 
     proposed changes, but it probably does not matter. A networking person 
     Acked-by would be nice though.

 drivers/ata/libata-eh.c:

   - Locking of the ATA port in ata_scsi_cmd_error_handler(), presumably this can
     race with IRQs and ioctls() on other CPUs. Very likely not performance 
     sensitive in any fashion, on IO errors things stop for many seconds anyway.

 ipc/sem.c:

   - A rare race condition branch in the SysV IPC semaphore freeing code in 
     exit_sem() - where even the main code flow is not performance sensitive, 
     because typical database workloads get their semaphore arrays during startup 
     and don't ever do heavy runtime allocation/freeing of them.

 kernel/sched/completion.c:

   - completion_done(). This is actually a (comparatively) rarely used completion 
     API call - almost all the upstream usecases are in drivers, plus two in 
     filesystems - neither usecase seems in a performance critical hot path. 
     Completions typically involve scheduling and context switching, so in the 
     worst case the proposed change adds overhead to a scheduling slow path.

You missed the one in do_exit(), which I thought was the original one.