Most operating systems have implemented realtime signals incorrectly

http://zinascii.com/2014/crossed-signals.html

by rzezeski • 12 years ago

105 23 12 years ago

Since they all do such a spectacular job of delivering different (buggy) results, I have to believe support was 'roll-your-own' on each platform. There's no reference implementation of POSIX features? No test cases? I guess there are now.

I helped deliver the 1st POSIX implementation, the year it was specified, on an old real-time OS called CTOS (BTOS if you bought it from Burroughs/Unisys). I wonder if we got it right...

noselasd• 12 years ago

There's test suites: http://www.opengroup.org/testing/downloads.html , though I don't know what they cover - posix is highly componentized.

acqq• 12 years ago

The kernel delivers the the signals in the correct order only to have userland invert them on the stack.

So does it mean that it's not a kernel bug at all? Just the libc? Then it's not surprising that different kernels, all using the same libc, have the same bug?

chawco• 12 years ago

These different kernels do not use the same libc. In particular, BSD derivatives are using a BSD libc, while Linux and friends are using glibc (or some cousin). I'm not certain which libc the others are using, but they are likely separate forks as well.

rzezeski2• 12 years ago

All kernels do not use the same libc. In fact, Linux is different in the fact that it doesn't ship a libc with the kernel. Illumos, the OS I investigated, goes by the philosophy that you _have_ to ship libc with your kernel, and thus they have their own libc.

My diagnosis for Illumos may not hold up on other OSs, they may invert priority for other reasons. In any case the spec is fairly clear that a signal is considered delivered when the "appropriate action" is taken. In this case the appropriate action is to invoke the user's handler, which is above libc on the stack.

Finally, in case you missed it, I witnessed 4 unique orderings across the systems I tested. They were not the same across the board.

rdtsc• 12 years ago

I have seen realtime signals before but never quite know what the intended use of them would be. They are relegated in my head to the same category as POSIX message queues -- remember for trivia question purposes.

So anyone used real-time signals and why did you pick them instead of something else?

pjc502• 12 years ago

The intended use case is people trying to write realtime systems - that is, systems with a latency guarantee, like antilock brakes or flight control systems - in UNIX.

However, no sensible person would write such a thing in a general-purpose multi-user operating system. Realtime is hard, see the Toyota mess: http://www.viva64.com/en/a/0083/

"Move fast and break things" is not a great motto for automotive control software.

amelius• 12 years ago

How about games, where, e.g., sound effects have a maximum acceptable delay w.r.t. the physics that generates them?

gaius• 12 years ago

Well, any sort of process control too (as in, controlling an assembly line or a chemical plant).

rzezeski2• 12 years ago

According to POSIX.1b RT signals are used for other features defined in .1b. An excerpt from "POSIX.4 Programmers Guide":

So far, we've talked about real-time signals sent by the sigqueue function. However, most POSIX.4 signals are sent as a result of three other facilities in POSIX.4. We'll cover these facilities in detail in their own chapters, but for now, you need to know that realtime signals can be generated as a result of expiration of a POSIX.4 timer, completion of asynchronous I/O, and by arrival of a message on an empty message queue.

Linux reserves 2-3 RT signals for use in its threading library:

http://man7.org/linux/man-pages/man7/signal.7.html

riffraff• 12 years ago

I remember studying POSIX message queues in the first OS class I took at the university. Nowadays, I have no I idea if anybody is actually using them.

pjc502• 12 years ago

Conclusion: hardly anyone uses realtime signals, and if they do they have no way of testing that they actually get the desired preemption behavior.

(I've used them once to workaround per-thread signal masking not behaving well for non-realtime signals)

apaprocki2• 12 years ago

I would not have that take-away.. I would instead say that probably most of the users of the signal features described in the article are not concerned with making their code cross-platform.

javert• 12 years ago

(I've used them once to workaround per-thread signal masking not behaving well for non-realtime signals)

Do you remember what the misbehavior was? I ask because I actually work with these kinds of things.

apaprocki2• 12 years ago

Happy (if you could call it that!) to report AIX 7.1 works correctly :)

    RT sig 50 received with sival 0
    RT sig 50 received with sival 1
    RT sig 50 received with sival 2
    RT sig 51 received with sival 0
    RT sig 51 received with sival 1
    RT sig 51 received with sival 2
    RT sig 52 received with sival 0
    RT sig 52 received with sival 1
    RT sig 52 received with sival 2

Solaris 11 still appears backwards:

    RT sig 43 received with sival 0
    RT sig 43 received with sival 1
    RT sig 43 received with sival 2
    RT sig 42 received with sival 0
    RT sig 42 received with sival 1
    RT sig 42 received with sival 2
    RT sig 41 received with sival 0
    RT sig 41 received with sival 1
    RT sig 41 received with sival 2

aidenn03• 12 years ago

Here's another POSIXism that everybody gets wrong: command lookup for sh; follow this algorithm for executing a command with no slashes, consider what would happen if "echo" is not in your path. Try it out on any shell and see that it just runs the "echo" builtin despite it being required to return 127.

Text below taken from: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3...

If the command name matches the name of a special built-in utility, that special built-in utility shall be invoked.

If the command name matches the name of a function known to this shell, the function shall be invoked as described in Function Definition Command. If the implementation has provided a standard utility in the form of a function, it shall not be recognized at this point. It shall be invoked in conjunction with the path search in step 1d.

If the command name matches the name of a utility listed in the following table, that utility shall be invoked.

alias bg cd command

false fc fg getopts

jobs kill newgrp pwd

read true umask unalias

wait

Otherwise, the command shall be searched for using the PATH environment variable as described in XBD Environment Variables :

If the search is successful:

<snip>

If the search is unsuccessful, the command shall fail with an exit status of 127 and the shell shall write an error message.

EdiX• 12 years ago

Step 1.d.i.a:

If the system has implemented the utility as a regular built-in or as a shell function, it shall be invoked at this point in the path search.

aidenn03• 12 years ago

1.d.i only happens if the search in PATH is successful.

I think the logic behind this was to ensure that as shells add more built-ins scripts that depend on an executable failing when not in the path won't break.

Borealid2• 12 years ago

Quoting from your post:

"If the command name matches the name of a special built-in utility, that special built-in utility shall be invoked."

Let echo be a "special built-in utility", and sh is compliant.

aidenn03• 12 years ago

"special built-in utilities" are specifically listed elsewhere. echo is not among them:

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3...

fotoetienne• 12 years ago

Minor nitpick: The HP-UX version that you are likely referring to is HP-UX 11i which is also known as HP-UX 11.11. 11.11i is not an HP-UX version number.

As a point of curiosity, this version is still employed in many nuclear plants.

Most operating systems have implemented realtime signals incorrectly

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