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Re: [Xen-devel] [PATCH] x86 spinlock: Fix memory corruption on completing completions

On 02/10, Raghavendra K T wrote:
> On 02/10/2015 06:23 AM, Linus Torvalds wrote:
>>          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 << 
>>          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.


--- 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) + 
        /* cmpxchg is a full barrier, so nothing can move before it */
        return cmpxchg(&lock->head_tail, old.head_tail, new.head_tail) == 

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