Attilio Rao wrote:
> 2006/12/20, Duane Whitty :
>
>> Hello again,
>>
>> It seems to me that understanding locking holds the key to
>> understanding fbsd internals.
>>
>> Could someone review my understanding of fbsd locking fundamentals.
>> (No assertions here, just questions)
>>
>> lock_mgr
>> --------------------
>> mutexes|sx_lock
>> ------------------- ^
>> atomic | mem barriers |

>
>
> Our current locking hierarchy is basically different:
>
> III level: lockmgr - sema - sx
> II level: mutex (sleep/spin/pool) - rwlock - refcount - cv - msleep
> I level: atomic instructions - memory barriers - sleepqueues/turnstiles
>
> (a lower lever means that the upper layer primitives use it as a base.
> ie: sx locks are build using 1 pool
> mutex and 2 condition variables).
>
> This scheme is far from being perfect due to the presence of 'level 3
> primitives' which should never exist.
> Currently, there is an ongoing efforts to take all the top layer
> primitives to the level II.
>
> On the other side, level I primitives should never be used directly by
> kernel code, but should only be used as a bottom layer for
> syncronizing primitives. All you need to care is in the layer 2 and 3
> (and possibly should switch to layer 2).


I disagree. There are many uses of atomic operations in the kernel that are not for locks or refcounts. It's a bad idea to use locks if you can achieve the same thing locklessly, with atomic operations.

I would personally also add "critical sections" (critical_enter()/critical_exit()) at level I. They can be used instead of locks when you know your data will only be accessed on one CPU, and you only need to protect it from (non-FAST) interrupt handlers.

-- Suleiman
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