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Historically, much (I would probably argue most) C concurrency has been implemented with fork. Certainly not all, and there are many ways to handle it in C...bu

Historically, much (I would probably argue most) C concurrency has been implemented with fork. Certainly not all, and there are many ways to handle it in C...but fork is really common, and I don't think it's considered all that big of a deal to do so. It is idiomatic (at least historically and across maybe billions of lines of C code), and not much harder to reason about than many kinds of thread implementation in C; in fact, it's easier to reason about fork than something like POSIX threads, IMHO. But, are you saying that to switch to a namespace in Go one must fork, whereas one wouldn't need to fork in C unless you need to switch namespace and you need concurrency (because you can determine when things will happen with precision in C)? I don't know. Again, this is beyond my understanding of Go right now. I may just not be understanding the implications of this. The code to deal with it looks reasonable enough to me; it's a smallish function, easily isolated. I think the author did a great job explaining the problem, the troubleshooting, and the solution. I just didn't see the problem as being all that damning of Go...but, that may be a reflection of my shallow understanding of the problem, or of the implications of the cost spawning a new process (to me, I always think back to the old adage "fork is cheap on Linux").

cyphar 9y ago
> C concurrency has been implemented with fork. Arguably it has been implemented with clone(2) which has flags. > in fact, it's easier to reason about fork than something like POSIX threads, IMHO. Not if you call fork() in a multi-threaded program. That ends _exceptionally_ badly (let's just say there's a reason Go doesn't expose syscall.Fork and it has to do with horrific deadlocks). > I just didn't see the problem as being all that damning of Go. The problem is more subtle, and it comes down to maintainability and understandably. If you ever decide to read the runc codebase, I apologise. One of the reasons the codebase is so scattered is because of these sorts of hacks where you have to work around issues in the Go runtime (because it doesn't give you enough control). In the article, whenever you read the small function you have to keep in mind that it's actually spawning a subprocess (which then means you have to think about what namespaces had the process joined and so on). Go is an okay language, but it simply wasn't designed for stuff this low-level. We would be much better served with Rust in my opinion. > But, are you saying that to switch to a namespace in Go one must fork, whereas one wouldn't need to fork in C unless you need to switch namespace and you need concurrency In C you don't need to fork to switch namespaces, you just call setns(...). For the PID namespace you need to fork, but that's just a quirk of the interface. In Go, you theoretically don't need to fork either (syscall.Setns is available). However, there is no real way to safely use it. First of all, the namespace interfaces in Linux are quite fragile when it comes to multi-threaded processes, but combine that with a runtime that will switch you between OS threads at random. And while the documentation on runtime.GOMAXPROC and runtime.LockOSThread might trick you into believing it's possible to stop the Go runtime from doing a clone(CLONE_THREAD|CLONE_PARENT), you can't.
- jankedeen 9y ago
  No. You can call fork pretty cleanly in a pthread based program. This post is not informed criticism. There is some prep for ugly contingencies(pthread_atfork) but it usually just works. Namespace based code while doing the same is like setting your hair on fire while running through the gasoline forest..but caveat emptor is usually spelled out pretty critically in the API and docs. Oh, btw: Fuck RUST. For once and for all fuck this tyranny of coercion by potential IP holders. It is a shit language.
  - klodolph 9y ago
    > There is some prep for ugly contingencies(pthread_atfork) but it usually just works. Actually, that's not true. Perhaps it's theoretically possible, in some cases, to make your threaded code work with fork with the appropriate pthread_atfork() handlers, but in general, it's a total mess. Thread A has a lock, thread B calls fork(), thread B tries to acquire the lock. The lock is held, but thread A is not running in the child. That much should be obvious. The usual fix is to write a pthread_atfork() handler for every lock in your program. Before forking, it locks all of your mutexes. After forking, in both the child and parent, it unlocks them. You can see the holes in this. One simple hole is that fork() is async signal safe but pthread_mutex_lock() is not, but most people won't fork() in a signal anyway. The big problem is that you now need to make all of your locks recursive and figure out, globally, what order to lock them in. This means registering your atfork() handlers in exactly the right order, something which is more difficult than it sounds. Another problem is the performance implications of acquiring all the locks in your program. Namespaces + threads are fine, the only problem here is namespaces + M:N threads + no method for pinning a green thread to an OS thread, like forkOS in Haskell.
    - jankedeen 9y ago
      *...it's a total mess...theoretically it's possible....speculation on standard behavior based on platform behavior...namespace(x)<-threads->chosen implementation detail...functional language reference... -- Translation: Design is broken
dullgiulio 9y ago
I mostly agree. Of course, for as cheap as a fork can be (and in Linux it's very cheap), it's not as cheap as a systemcall. When changing namespace happens often in a hot path, forking might not be fast enough. I disagree with Walton that the blame is on the N:M threading. It's really on the Linux kernel that has never made a clean distinction between threads and processes, both in kernel and in the APIs.
- YZF 9y ago
  I agree. Bestowing certain permissions/properties on a thread doesn't make sense. The thread shouldn't "enter the namespace", an API should retrieve a handle to that namespace that should then be usable from any thread in the process via some appropriate calls.
  - the8472 9y ago
    On the other hand having individual threads entering namespaces means your broker process does not have to constantly switch namespaces, it can use shared memory between differently privileged threads to do the brokering.
    - YZF 9y ago
      I know nothing of the implementation here but presumably these namespaces can exist side by side in a way that doesn't require any "switching". If switching is expensive that would make context switching between a thread in one namespace and a thread in another namespace just as expensive? If you have one thread in one namespace and another in another you now have to worry about what you can do in the context of a callback. This asymmetry just makes any multi-threaded program more complicated than it needs to be (and already is).
      - the8472 9y ago
        Switching is a system call (setns), in principle shared-memory IPC does not involve context switches, just lock-free data structures. I'm not sure how common this is in practice since shared memory also has some downsides if you're doing this for security. But there also are non-security applications of namespaces. And it's not like namespaces are the only per-thread thing in linux. Capabilities, uid and signal handlers come to mind.
aidenn0 9y ago
Separate threads for IO was really common in C and C++ in the 90s. In addition fork() style concurrency was essentially nonexistent outside of Unix.
pyre 9y ago
> But, are you saying that to switch to a namespace in Go one must fork The article is stating that to control the namespace that the threads execute in, that a separate process must be spawned so that the entire process can be forced into the correct namespace. Otherwise, the runtime can spawn new threads as it sees fit and you don't have control over which namespace they are in. It might be possible to work around this from within the same process if it were possible to force the runtime to not spawn new threads in particular cases. If you could control when the runtime was allowed to spawn new threads, then you could organize the program / threads in a way that would keep the correct code operating in the correct namespace. Unfortunately, you can't. Disclaimer: I don't know Go, but this is my understanding from reading the article.

cyphar 9y ago

> C concurrency has been implemented with fork. Arguably it has been implemented with clone(2) which has flags. > in fact, it's easier to reason about fork than something like POSIX threads, IMHO. Not if you call fork() in a multi-threaded program. That ends _exceptionally_ badly (let's just say there's a reason Go doesn't expose syscall.Fork and it has to do with horrific deadlocks). > I just didn't see the problem as being all that damning of Go. The problem is more subtle, and it comes down to maintainability and understandably. If you ever decide to read the runc codebase, I apologise. One of the reasons the codebase is so scattered is because of these sorts of hacks where you have to work around issues in the Go runtime (because it doesn't give you enough control). In the article, whenever you read the small function you have to keep in mind that it's actually spawning a subprocess (which then means you have to think about what namespaces had the process joined and so on). Go is an okay language, but it simply wasn't designed for stuff this low-level. We would be much better served with Rust in my opinion. > But, are you saying that to switch to a namespace in Go one must fork, whereas one wouldn't need to fork in C unless you need to switch namespace and you need concurrency In C you don't need to fork to switch namespaces, you just call setns(...). For the PID namespace you need to fork, but that's just a quirk of the interface. In Go, you theoretically don't need to fork either (syscall.Setns is available). However, there is no real way to safely use it. First of all, the namespace interfaces in Linux are quite fragile when it comes to multi-threaded processes, but combine that with a runtime that will switch you between OS threads at random. And while the documentation on runtime.GOMAXPROC and runtime.LockOSThread might trick you into believing it's possible to stop the Go runtime from doing a clone(CLONE_THREAD|CLONE_PARENT), you can't.

jankedeen 9y ago
No. You can call fork pretty cleanly in a pthread based program. This post is not informed criticism. There is some prep for ugly contingencies(pthread_atfork) but it usually just works. Namespace based code while doing the same is like setting your hair on fire while running through the gasoline forest..but caveat emptor is usually spelled out pretty critically in the API and docs. Oh, btw: Fuck RUST. For once and for all fuck this tyranny of coercion by potential IP holders. It is a shit language.
- klodolph 9y ago
  > There is some prep for ugly contingencies(pthread_atfork) but it usually just works. Actually, that's not true. Perhaps it's theoretically possible, in some cases, to make your threaded code work with fork with the appropriate pthread_atfork() handlers, but in general, it's a total mess. Thread A has a lock, thread B calls fork(), thread B tries to acquire the lock. The lock is held, but thread A is not running in the child. That much should be obvious. The usual fix is to write a pthread_atfork() handler for every lock in your program. Before forking, it locks all of your mutexes. After forking, in both the child and parent, it unlocks them. You can see the holes in this. One simple hole is that fork() is async signal safe but pthread_mutex_lock() is not, but most people won't fork() in a signal anyway. The big problem is that you now need to make all of your locks recursive and figure out, globally, what order to lock them in. This means registering your atfork() handlers in exactly the right order, something which is more difficult than it sounds. Another problem is the performance implications of acquiring all the locks in your program. Namespaces + threads are fine, the only problem here is namespaces + M:N threads + no method for pinning a green thread to an OS thread, like forkOS in Haskell.
  - jankedeen 9y ago
    *...it's a total mess...theoretically it's possible....speculation on standard behavior based on platform behavior...namespace(x)<-threads->chosen implementation detail...functional language reference... -- Translation: Design is broken

dullgiulio 9y ago

I mostly agree. Of course, for as cheap as a fork can be (and in Linux it's very cheap), it's not as cheap as a systemcall. When changing namespace happens often in a hot path, forking might not be fast enough. I disagree with Walton that the blame is on the N:M threading. It's really on the Linux kernel that has never made a clean distinction between threads and processes, both in kernel and in the APIs.

YZF 9y ago
I agree. Bestowing certain permissions/properties on a thread doesn't make sense. The thread shouldn't "enter the namespace", an API should retrieve a handle to that namespace that should then be usable from any thread in the process via some appropriate calls.
- the8472 9y ago
  On the other hand having individual threads entering namespaces means your broker process does not have to constantly switch namespaces, it can use shared memory between differently privileged threads to do the brokering.
  - YZF 9y ago
    I know nothing of the implementation here but presumably these namespaces can exist side by side in a way that doesn't require any "switching". If switching is expensive that would make context switching between a thread in one namespace and a thread in another namespace just as expensive? If you have one thread in one namespace and another in another you now have to worry about what you can do in the context of a callback. This asymmetry just makes any multi-threaded program more complicated than it needs to be (and already is).
    - the8472 9y ago
      Switching is a system call (setns), in principle shared-memory IPC does not involve context switches, just lock-free data structures. I'm not sure how common this is in practice since shared memory also has some downsides if you're doing this for security. But there also are non-security applications of namespaces. And it's not like namespaces are the only per-thread thing in linux. Capabilities, uid and signal handlers come to mind.

aidenn0 9y ago

Separate threads for IO was really common in C and C++ in the 90s. In addition fork() style concurrency was essentially nonexistent outside of Unix.

pyre 9y ago

> But, are you saying that to switch to a namespace in Go one must fork The article is stating that to control the namespace that the threads execute in, that a separate process must be spawned so that the entire process can be forced into the correct namespace. Otherwise, the runtime can spawn new threads as it sees fit and you don't have control over which namespace they are in. It might be possible to work around this from within the same process if it were possible to force the runtime to not spawn new threads in particular cases. If you could control when the runtime was allowed to spawn new threads, then you could organize the program / threads in a way that would keep the correct code operating in the correct namespace. Unfortunately, you can't. Disclaimer: I don't know Go, but this is my understanding from reading the article.