Re: [PATCH 3/6] btrfs: introduce try-lock semantics for exclusive op start

[Anand Jain <anand.jain@oracle.com>]

On 28/05/2021 20:30, David Sterba wrote:
> On Thu, May 27, 2021 at 03:43:46PM +0800, Anand Jain wrote:
>> On 21/05/2021 20:06, David Sterba wrote:
>>    Nit:
>>    This function implements a conditional lock. But the function name
>>    misleads to some operation similar to spin_trylock() or
>>    mutex_trylock(). How about btrfs_exclop_start_cond_lock() instead?
> 
> The semantics is same as spin_trylock so it's named like it. Try lock is
> a conditional lock so I would not want to cause confusion by using
> another naming scheme.
> 
> https://elixir.bootlin.com/linux/latest/source/include/linux/spinlock_api_smp.h#L86
> 
> static inline int __raw_spin_trylock(raw_spinlock_t *lock)
> {
> 	preempt_disable();
> 	if (do_raw_spin_trylock(lock)) {
> 		spin_acquire(&lock->dep_map, 0, 1, _RET_IP_);
> 		return 1;
> 	}
> 	preempt_enable();
> 	return 0;
> }
> 
> bool btrfs_exclop_start_try_lock(struct btrfs_fs_info *fs_info,
>                                  enum btrfs_exclusive_operation type)
> {
>         spin_lock(&fs_info->super_lock);
>         if (fs_info->exclusive_operation == type)
>                 return true;
> 
>         spin_unlock(&fs_info->super_lock);
>         return false;
> }
> 
> The code flow is the same.
> 

Oh. Ok, now I understand your POV.

I looked up the document to check what it says, and it matched with
my understanding too, as below.

https://www.kernel.org/doc/htmldocs/kernel-locking/trylock-functions.html
-----
::
They can be used if you need no access to the data protected with the 
lock when some other thread is holding the lock.
::
----

Mainly ...trylocks are non-blocking/non-waiting locks.

However, btrfs_exclop_start_try_lock() can be blocking.

But as this is trivial should be ok. Thanks for clarifying.

- Anand