4 ms·

The only part needed to build a lock is to be able to 'sleep' the current thread and the latter to be able to be awaken, plus compare-and-swap or load linked/st

by xxs 5y ago

The only part needed to build a lock is to be able to 'sleep' the current thread and the latter to be able to be awaken, plus compare-and-swap or load linked/store conditional. There is nothing really special about the locks as long as the current thread can be made dormant. Technically a single OS lock (e.g. a futex), associated with the current thread would suffice to implement as many locks as one desires. The lock would be released when the thread dies in the clean up code. The locks that result in thread awaiting are still in-memory, of course. They are just OS provided primitives. Yet again, a well behaved/designed lock should be non-contested for the most calls (easily 99.9%+). In that case there is no mode-switch to get to the kernel. A contested lock that goes to the kernel is a context switch. In the end there is no need to do anything special about locks as they can implemented in the user-space unlike the file descriptors.

tialaramex 5y ago
> In the end there is no need to do anything special about locks as they can implemented in the user-space unlike the file descriptors. If you get semaphores from create_sem() you will need to give them back, you can't just say "This shouldn't be an OS resource" and throw it away, exactly like a file descriptor it's your responsibility to dispose of it properly. Speaking of which, depending on the OS, your file descriptors might also just be implemented in user space. They're just integers, it's no big deal, of course you must still properly open() and close() files, the fact it's "implemented in user space" does not magically mean it isn't your problem to clean up properly. Likewise, if you opened a sqlite database, you must close it properly, you can't just say "Eh, it's only userspace, it will be fine" and throw away the memory without closing properly.
- xxs 5y ago
  >If you get semaphores from create_sem() you will need to give them back... I don't quite get the continued argument - the created locks in the user space have 'zero' extra dependency on the kernel. In a GC setup they would be an object no different than a mere string (likely less memory as well). They can be GC'd at any time as long as there are no references to them. If you wish to see an example: java.util.concurrent package. ReentrantLock[0] is a classic lock, and there is even a Semaphore in the same package (technically not in the very same). Originated in 2004 with JMM and the availability of CAS. You can create as many as you choose - no kernel mode switch would occur. For the record I do no object that proper resource managed is needed - incl. sqldatabase close. The latter does use a file descriptor any time you establish a connection. However, locks need no special support from the kernel. Also in GC'd setup file descriptors would be GC'd in pre (or post) morterm via some finalization mechanism, however relying on the GC to perform resource management is a terrible practice. [0]: https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/concurrent/locks/ReentrantLock.java https://github.com/openjdk/jdk/blob/master/src/java.base/sha...
  - tialaramex 5y ago
    > I don't quite get the continued argument I keep telling you that, in fact, these primitives can literally be OS resources such as the semaphores from the BeOS system call create_sem() or the POSIX sem_open or Windows CreateSemaphore. Your insistence that these don't have a dependency on the kernel, that they're just memory to be garbage collected is wrong, they aren't, this won't work. You propose the Java ReentrantLock class as an example, but, that's just a thin wrapper for the underlying Java runtime parking mechanism, so, sure it's just a Java object no big deal, but the actual implementation (which is not in the file you just linked) requires an OS primitive that you've not examined at all, that OS primitive is being managed by the runtime (via the "Unsafe" interface) and is excluded from your garbage collection entirely. In essence you get to "just" garbage collect these locks because they're only a superficial Java language shadow of the real mechanism, which you aren't allowed to touch and can't garbage collect.
    - xxs 5y ago
      >requires an OS primitive that you've not examined at all, that OS primitive is being managed by the runtime (via the "Unsafe" interface) and is excluded from your garbage collection entirely. This is most definitely false. The methods needed are the compare-and-set + LockSupport/park (parkNanos) and LockSupport.unpark. The former is a standard CPU instruction w/o any special magic about and the latter is setting the current thread to sleep and being awaken. As for examine: I know how j.u.c works down the OS calls and generated assembly. Unsafe is a class (not an interface), albeit mostly resulting in compiler magic. The use of unsafe is solely for: LockSupport::park/unpark and AbstractQueuedSynchronizer::compareAndSet. ReentrantLock is NOT any thing wrapper.
      - tialaramex 5y ago
        > As for examine: I know how j.u.c works down the OS calls and generated assembly. And here's the big reveal. It's actually calling the OS for "setting the current thread to sleep and being awaken" ie it's just using the OS locks and those are hidden from you so that you can't garbage collect them. > Unsafe is a class (not an interface), albeit mostly resulting in compiler magic. Unfortunate choice of language, I didn't mean a Java interface, but an interface in the broader sense. I assure you that "setting the current thread to sleep" is not "compiler magic", it's just using the OS locks, on a modern system it will presumably use a lightweight lock but on older systems it'll be something more heavyweight like the semaphores we saw earlier. The claim that LockSupport, which is calling the OS is not a wrapper is bogus. But all this is a distraction, even on systems where old semaphore APIs are now rarely needed they still exist and if you hold one you are still responsible for cleaning it up properly even if you think you know better.

by xxs 5y ago

tialaramex 5y ago

> In the end there is no need to do anything special about locks as they can implemented in the user-space unlike the file descriptors. If you get semaphores from create_sem() you will need to give them back, you can't just say "This shouldn't be an OS resource" and throw it away, exactly like a file descriptor it's your responsibility to dispose of it properly. Speaking of which, depending on the OS, your file descriptors might also just be implemented in user space. They're just integers, it's no big deal, of course you must still properly open() and close() files, the fact it's "implemented in user space" does not magically mean it isn't your problem to clean up properly. Likewise, if you opened a sqlite database, you must close it properly, you can't just say "Eh, it's only userspace, it will be fine" and throw away the memory without closing properly.

xxs 5y ago
>If you get semaphores from create_sem() you will need to give them back... I don't quite get the continued argument - the created locks in the user space have 'zero' extra dependency on the kernel. In a GC setup they would be an object no different than a mere string (likely less memory as well). They can be GC'd at any time as long as there are no references to them. If you wish to see an example: java.util.concurrent package. ReentrantLock[0] is a classic lock, and there is even a Semaphore in the same package (technically not in the very same). Originated in 2004 with JMM and the availability of CAS. You can create as many as you choose - no kernel mode switch would occur. For the record I do no object that proper resource managed is needed - incl. sqldatabase close. The latter does use a file descriptor any time you establish a connection. However, locks need no special support from the kernel. Also in GC'd setup file descriptors would be GC'd in pre (or post) morterm via some finalization mechanism, however relying on the GC to perform resource management is a terrible practice. [0]: https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/concurrent/locks/ReentrantLock.java https://github.com/openjdk/jdk/blob/master/src/java.base/sha...
- tialaramex 5y ago
  > I don't quite get the continued argument I keep telling you that, in fact, these primitives can literally be OS resources such as the semaphores from the BeOS system call create_sem() or the POSIX sem_open or Windows CreateSemaphore. Your insistence that these don't have a dependency on the kernel, that they're just memory to be garbage collected is wrong, they aren't, this won't work. You propose the Java ReentrantLock class as an example, but, that's just a thin wrapper for the underlying Java runtime parking mechanism, so, sure it's just a Java object no big deal, but the actual implementation (which is not in the file you just linked) requires an OS primitive that you've not examined at all, that OS primitive is being managed by the runtime (via the "Unsafe" interface) and is excluded from your garbage collection entirely. In essence you get to "just" garbage collect these locks because they're only a superficial Java language shadow of the real mechanism, which you aren't allowed to touch and can't garbage collect.
  - xxs 5y ago
    >requires an OS primitive that you've not examined at all, that OS primitive is being managed by the runtime (via the "Unsafe" interface) and is excluded from your garbage collection entirely. This is most definitely false. The methods needed are the compare-and-set + LockSupport/park (parkNanos) and LockSupport.unpark. The former is a standard CPU instruction w/o any special magic about and the latter is setting the current thread to sleep and being awaken. As for examine: I know how j.u.c works down the OS calls and generated assembly. Unsafe is a class (not an interface), albeit mostly resulting in compiler magic. The use of unsafe is solely for: LockSupport::park/unpark and AbstractQueuedSynchronizer::compareAndSet. ReentrantLock is NOT any thing wrapper.
    - tialaramex 5y ago
      > As for examine: I know how j.u.c works down the OS calls and generated assembly. And here's the big reveal. It's actually calling the OS for "setting the current thread to sleep and being awaken" ie it's just using the OS locks and those are hidden from you so that you can't garbage collect them. > Unsafe is a class (not an interface), albeit mostly resulting in compiler magic. Unfortunate choice of language, I didn't mean a Java interface, but an interface in the broader sense. I assure you that "setting the current thread to sleep" is not "compiler magic", it's just using the OS locks, on a modern system it will presumably use a lightweight lock but on older systems it'll be something more heavyweight like the semaphores we saw earlier. The claim that LockSupport, which is calling the OS is not a wrapper is bogus. But all this is a distraction, even on systems where old semaphore APIs are now rarely needed they still exist and if you hold one you are still responsible for cleaning it up properly even if you think you know better.