Re: sched: memory corruption on completing completions

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 02/06/2015 04:07 AM, Davidlohr Bueso wrote:
> On Thu, 2015-02-05 at 13:34 -0800, Linus Torvalds wrote:
>> On Thu, Feb 5, 2015 at 1:02 PM, Sasha Levin <sasha.levin@oracle.com> wrote:
>>>
>>> Interestingly enough, according to that article this behaviour seems to be
>>> "by design":
>>
>> Oh, it's definitely by design, it's just that the design looked at
>> spinlocks without the admittedly very subtle issue of lifetime vs
>> unlocking.
>>
>> Spinlocks (and completions) are special - for other locks we have
>> basically allowed lifetimes to be separate from the lock state, and if
>> you have a data structure with a mutex in it, you'll have to have some
>> separate lifetime rule outside of the lock itself. But spinlocks and
>> completions have their locking state tied into their lifetime.
>
> For spinlocks I find this very much a virtue. Tight lifetimes allow the
> overall locking logic to be *simple* - keeping people from being "smart"
> and bloating up spinlocks. Similarly, I hate how the paravirt
> alternative blends in with regular (sane) bare metal code. What was
> preventing this instead??
>
> #ifdef CONFIG_PARAVIRT_SPINLOCKS
> static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
> {
> 	if (!static_key_false(&paravirt_ticketlocks_enabled))
> 		return;
>
> 	add_smp(&lock->tickets.head, TICKET_LOCK_INC);
> 	/* Do slowpath tail stuff... */
> }
> #else
> static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
> {
> 	__add(&lock->tickets.head, TICKET_LOCK_INC, UNLOCK_LOCK_PREFIX);
> }
> #endif
>
> I just don't see the point to all this TICKET_SLOWPATH_FLAG:
>
> #ifdef CONFIG_PARAVIRT_SPINLOCKS
> #define __TICKET_LOCK_INC	2
> #define TICKET_SLOWPATH_FLAG	((__ticket_t)1)
> #else
> #define __TICKET_LOCK_INC	1
> #define TICKET_SLOWPATH_FLAG	((__ticket_t)0)
> #endif
>
> when it is only for paravirt -- and the word slowpath implies the
> general steps as part of the generic algorithm. Lets keep code for
> simple locks simple.
>

Good point, I will send this as a separate cleanup once I test the
patch I have to correct the current problem.