On 02/10, Raghavendra K T wrote:

On 02/10/2015 06:23 AM, Linus Torvalds wrote:

quoted

         add_smp(&lock->tickets.head, TICKET_LOCK_INC);
         if (READ_ONCE(lock->tickets.tail) & TICKET_SLOWPATH_FLAG) ..

into something like

         val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC << TICKET_SHIFT);
         if (unlikely(val & TICKET_SLOWPATH_FLAG)) ...

would be the right thing to do. Somebody should just check that I got
that shift right, and that the tail is in the high bytes (head really
needs to be high to work, if it's in the low byte(s) the xadd would
overflow from head into tail which would be wrong).

Unfortunately xadd could result in head overflow as tail is high.

The other option was repeated cmpxchg which is bad I believe.
Any suggestions?

Stupid question... what if we simply move SLOWPATH from .tail to .head?
In this case arch_spin_unlock() could do xadd(tickets.head) and check
the result

In this case __ticket_check_and_clear_slowpath() really needs to cmpxchg
the whole .head_tail. Plus obviously more boring changes. This needs a
separate patch even _if_ this can work.



BTW. If we move "clear slowpath" into "lock" path, then probably trylock
should be changed too? Something like below, we just need to clear SLOWPATH
before cmpxchg.

Oleg.

--- x/arch/x86/include/asm/spinlock.h
+++ x/arch/x86/include/asm/spinlock.h

@@ -109,7 +109,8 @@ static __always_inline int arch_spin_try
 	if (old.tickets.head != (old.tickets.tail & ~TICKET_SLOWPATH_FLAG))
 		return 0;
 
-	new.head_tail = old.head_tail + (TICKET_LOCK_INC << TICKET_SHIFT);
+	new.tickets.head = old.tickets.head;
+	new.tickets.tail = (old.tickets.tail & ~TICKET_SLOWPATH_FLAG) + TICKET_LOCK_INC;
 
 	/* cmpxchg is a full barrier, so nothing can move before it */
 	return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == old.head_tail;