Bug #2337

Wishlist: We need named POSIX semaphores

Added by vsrinivas over 4 years ago. Updated over 2 years ago.

Status:ResolvedStart date:03/26/2012
Priority:NormalDue date:
Assignee:nthery% Done:


Target version:-


DragonFly is missing named POSIX semaphores (sem_open / sem_close / sem_unlink). python-multiprocessing, for example, needs this to work.


#1 Updated by nthery almost 3 years ago

  • Status changed from New to In Progress
  • Assignee set to nthery

I've got an skeleton prototype roughly similar to the fbsd implementation.

Semaphore names are regular files. Each semaphore has a system-wide state, basically its value, stored in a shared memory region. There is also a per-process state for dealing with multiple sem_open() calls.

The semaphore value is manipulated via atomic instructions. sem_wait() calls into the kernel (umtx_sleep()) if and only if the calling thread must sleep (semaphore value < 0). Similarly sem_post() calls into the kernel (umtx_wakeup()) if and only if there are waiters (value 0 ->1). umtx_sleep() could be replaced in a subsequent step with a semaphore-optimized syscall for avoiding user<->kernel transitions when several threads concurrently call sem_wait() with value <= 0.

I'll post the code when fleshed out and cleaned up.

#2 Updated by jalcazar over 2 years ago

I haven't had enough time but, I did some research and was going to do the same thing (implement it similarly to FreeBSD).

nthery: Keep up the work

#3 Updated by jorisgio over 2 years ago

  • Status changed from In Progress to Resolved

Implemented by commit 1a09a5c7d3dc38e61fc28441d066b071ad659dc0

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