[PATCH 1/3] drm/amdgpu: fix a typo

[axie]

Hi Marek,

Spin lock and spin unlock is fast. But it is not so fast compared with 
atomic, which is a single CPU instruction in x86.


1. spinlock does NOT allow preemption at local CPU. Let us have a look 
at how spin lock was implemented.

static inline void __raw_spin_lock(raw_spinlock_t *lock)
{
     preempt_disable(); <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--This is 
memory barrier operation too.
     spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
     LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
}

2.  A function  __lock_acquire called by spinlock. The function is so 
long that I would not attach all of it here.

There is atomic operation inside and 12 meta data updates and 14 if 
statements and it calls quite some other functions.

Note that it disable IRQ...

static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
               int trylock, int read, int check, int hardirqs_off,
               struct lockdep_map *nest_lock, unsigned long ip,
               int references, int pin_count)
{
     struct task_struct *curr = current;
     struct lock_class *class = NULL;
     struct held_lock *hlock;
     unsigned int depth;
     int chain_head = 0;
     int class_idx;
     u64 chain_key;

     if (unlikely(!debug_locks))
         return 0;

     /*
      * Lockdep should run with IRQs disabled, otherwise we could
      * get an interrupt which would want to take locks, which would
      * end up in lockdep and have you got a head-ache already?
      */
     if (DEBUG_LOCKS_WARN_ON(!irqs_disabled())) <<<<<<<<<<<<<<<Disable IRQ
         return 0;

....

3. Another function called by spinlock in a higher level:

void lock_acquire(struct lockdep_map *lock, unsigned int subclass,

               int trylock, int read, int check,
               struct lockdep_map *nest_lock, unsigned long ip)
{
     unsigned long flags;

     if (unlikely(current->lockdep_recursion))
         return;

     raw_local_irq_save(flags);
     check_flags(flags);

     current->lockdep_recursion = 1;
     trace_lock_acquire(lock, subclass, trylock, read, check, nest_lock, 
ip);
     __lock_acquire(lock, subclass, trylock, read, check,
                irqs_disabled_flags(flags), nest_lock, ip, 0, 0);
     current->lockdep_recursion = 0;
     raw_local_irq_restore(flags);
}


Thanks,

Alex Bin


On 2017-06-22 12:27 PM, Marek Olšák wrote:
> On Thu, Jun 22, 2017 at 5:33 PM, Xie, AlexBin <AlexBin.Xie at amd.com> wrote:
>> Hi Christian,
>>
>>
>> In fact, the change from spinlock to atomic is quite painful. When I
>> started, I thought it was easy but later I found there might be race
>> condition here and there. Now I think the change looks more robust. In
>> kernel source, there are several other drivers used the same trick.
>>
>>
>> On the other hand, I think the logic itself might be optimized considering
>> the locking. But I had spent quite some effort to maintain original logic.
> It seems quite complicated and I don't know if there is any
> performance benefit. Spinlocks are nice because they allow preemption.
>
> It would be more interesting to merge the CS and BO_LIST ioctls into one.
>
> Marek