Re: [PATCH RFC 06/26] ipc: Replace spin_unlock_wait() with lock/unlock pair

[Manfred Spraul <manfred@colorfullife.com>]

On 06/30/2017 02:01 AM, Paul E. McKenney wrote:
> There is no agreed-upon definition of spin_unlock_wait()'s semantics,
> and it appears that all callers could do just as well with a lock/unlock
> pair.  This commit therefore replaces the spin_unlock_wait() call in
> exit_sem() with spin_lock() followed immediately by spin_unlock().
> This should be safe from a performance perspective because exit_sem()
> is rarely invoked in production.
>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Davidlohr Bueso <dave@stgolabs.net>
> Cc: Manfred Spraul <manfred@colorfullife.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: Andrea Parri <parri.andrea@gmail.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
> ---
>   ipc/sem.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/ipc/sem.c b/ipc/sem.c
> index 947dc2348271..e88d0749a929 100644
> --- a/ipc/sem.c
> +++ b/ipc/sem.c
> @@ -2096,7 +2096,8 @@ void exit_sem(struct task_struct *tsk)
>   			 * possibility where we exit while freeary() didn't
>   			 * finish unlocking sem_undo_list.
>   			 */
> -			spin_unlock_wait(&ulp->lock);
> +			spin_lock(&ulp->lock);
> +			spin_unlock(&ulp->lock);
>   			rcu_read_unlock();
>   			break;
>   		}