On Fri, 2006-05-19 at 10:47 +0900, minchan Kim wrote:
quoted text
> Hello. I am a kernel newbie. 
> I knew the kernel-mentor site today. 
> Please, I hope that mentors help me to have a question in the future.
>  
> I had a one question. 
>  
> I know new feature "Completions" in Linux kernel 2.6 , but I don't
> know difference semaphore and completion. 
> I wonder that When i use completion instead of semaphore ?

they're different usage patterns:

the basic completion pattern is when you set something up that at some
point in the future gets finished, for example, you could schedule a
disk IO, and when the IO actually finishes, the completion would be
called. Eg there generally is a "task" that at some point gets
'completed'.

A semaphore.. isn't used much anymore, a mutex is used instead.
A mutex is basically a short term exclusion tool, eg the pattern is

<take mutex>
<do some actions>
<release mutex>

unlike completions, there is a strong sense of "who takes the mutex
releases it", and the there is a well defined task "in the middle",
while for completions it is "something else finishes it".

Does this help?


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