Re: sched: memory corruption on completing completions

[Sasha Levin <sasha.levin@oracle.com>]

On 02/05/2015 03:59 PM, Davidlohr Bueso wrote:
> On Wed, 2015-02-04 at 16:16 -0800, Linus Torvalds wrote:
>> And looking at the arch version, I think the paravirtualized code is crap.
>>
>> It does:
>>
>>                 prev = *lock;
>>                 add_smp(&lock->tickets.head, TICKET_LOCK_INC);
>>
>>                 /* add_smp() is a full mb() */
>>
>>                 if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
>>                         __ticket_unlock_slowpath(lock, prev);
>>
>>
>> which is *exactly* the kind of things you cannot do with spinlocks,
>> because after you've done the "add_smp()" and released the spinlock
>> for the fast-path, you can't access the spinlock any more.  Exactly
>> because a fast-path lock migth come in, and release the whole data
>> structure.
>>
>> As usual, the paravirt code is a horribly buggy heap of crud. Film at 11.
> 
> Per http://lwn.net/Articles/495597/ which clearly describes the intent
> of the slowpath unlocking. Cc'ing Raghavendra.

Interestingly enough, according to that article this behaviour seems to be
"by design":

"""
This version of the patch uses a locked add to do this, followed by a test
to see if the slowflag is set.  The lock prefix acts as a full memory barrier,
so we can be sure that other CPUs will have seen the unlock before we read
the flag
"""

Thanks,
Sasha