Reading Comprehension - Subtle memory ordering rules

[shaharhada]

In the source:
https://en.wikipedia.org/wiki/Spinlock
I have question about the sentence:
On later implementations of the x86 architecture, spin_unlock can safely use an unlocked MOV instead of the slower locker XCHG. This is due to subtle memory ordering rules which support this, even though MOV is not a full memory barrier.
What is sutble memory ordering rules?
Why MOV is not a full memory barrier and what is instruction(s) that has a full memory barrier?
And what the meaning of the sentence? And how it can express in other simple words?
Thanks for answering...

 

[Ralston18]

You already appear to understand "memory ordering tools", etc..

Focus on the word "subtle".

Reference:

https://dictionary.cambridge.org/us/dictionary/english/subtle

Small little changes to the memory ordering rules that most people (including myself) would not notice and/or know how to apply such small little changes.

Changes that allow more flexibility and application of MOV but are likely not obvious in appearance nor application.

 

[shaharhada]

You already appear to understand "memory ordering tools", etc..

Focus on the word "subtle".

Reference:

https://dictionary.cambridge.org/us/dictionary/english/subtle

Small little changes to the memory ordering rules that most people (including myself) would not notice and/or know how to apply such small little changes.

Changes that allow more flexibility and application of MOV but are likely not obvious in appearance nor application.

What is to "acquire a lock" meaning?

 

[Eximo]

When a spinlock is active, it has acquired (gained) a lock. Meaning other code can't utilize that thread.

 

[shaharhada]

When a spinlock is active, it has acquired (gained) a lock. Meaning other code can't utilize that thread.

I think that I understand you, but I am not exactly.
What the meaning of the sentence "when a spinlock is active, it has acquired a lock."
It is a kind of procedure?
The meaning is that Everything I need a lock, I use spinlock and create a one ? i.e. acquired a lock???

 

[kanewolf]

I think that I understand you, but I am not exactly.
What the meaning of the sentence "when a spinlock is active, it has acquired a lock."
It is a kind of procedure?
The meaning is that Everything I need a lock, I use spinlock and create a one ? i.e. acquired a lock???

First, spinlocks are for multi-threaded software. They are used to synchronize threads when they are accessing a shared resource. That could be memory or I/O.
So if you are using only memory that is exclusively mapped into your thread then you don't need a lock. If you are trying to exchange information into shared memory, then you need a spinlock when you are reading or writing that memory.
The examples shown in Wikipedia, in assembly language would never be encountered in real life programming, unless you were debugging a compiler or OS. You would use higher level language constructs. Maybe this would help -- https://www.justsoftwaresolutions.co.uk/threading/locks-mutexes-semaphores.html