semaphores between processes
Daniel Eischen
deischen at freebsd.org
Fri Oct 23 17:18:05 UTC 2009
On Fri, 23 Oct 2009, Andrew Gallatin wrote:
> Daniel Eischen wrote:
>
>>
>> We already use umtx. This really is a hack and I wouldn't
>> advocate it. I'm not sure how you could make it work and
>> not break existing ability to return appropriate error
>> codes without slowing down the path in the non-shared
>> case. You'd have to check to see if the address space
>> was shared or not, which would require a system call.
>
> I'm probably missing something. What does it matter if the
> address space is shared, as long as the umtx struct is
> in shared memory?
>
> From my quick read, the umtx operations use a lock word
> in userspace. For uncontested locks, they use atomic
> ops to flip an id into the lock word. The kernel takes
> over for contested locks, and does sleeping, wakup, etc.
> Is this correct? Is there something here that matters
> if the address space (and not just the lock word) is
> shared?
>
>> All our public pthread_foo() symbols are weak. You
>> can easily override them in your application code in
>> the #ifdef freebsd case. What is wrong with providing
>> your own library that overrides them to do what you
>> require - this shouldn't change your application code?
>>
>
> For our code, I was thinking of something like:
>
> #ifdef FreeBSD
> #define lock(x) umtx_lock(x, getpid())
> #define unlock(x) umtx_unlock(x, getpid())
> #else
> #define lock(x) pthread_mutex_lock(x)
> #define unlock(x) pthread_mutex_lock(x)
> #endif
>
>
> I should probably just shut up and try it..
My apologies - I thought you were talking about changing
our pthread_mutex_* functions in src/lib/...
--
DE
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