semaphores between processes

Daniel Eischen deischen at
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/...


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