4 ms·

I wonder if any software rely on these bugs EDIT: windows famously had bugs and had to add special code to preserve the buggy behaviour to keep certain applica

• chrisseaton 7y ago
  Seems very tricky, almost by definition, to rely on a race condition?
  • nitwit005 7y ago
    My previous company preserved bad server caching behavior for one or two customers, as they had custom code depending on it. Given that it was a cache, it was a race condition, just not one likely to trigger (would only get evicted if there was a storm of requests).
• asveikau 7y ago
  You assume every race is a bug. Part of the job of a kernel is to resolve races. Requests come to it from multiple threads and processes in a parallel fashion. Sometimes your timing will be one way and get you one result, sometimes it will go another way. That's OK. It's the nature of the beast.
  • sanxiyn 7y ago
    This tool finds data races. All data races are bugs.
    • asveikau 7y ago
      You cannot say all data races are bugs independent of what the data represents or how it is handled. Perhaps a read not seeing someone else's write in a timely fashion is OK in your situation. Perhaps you will resolve conflicts later using proper synchronization.
      • sanxiyn 7y ago
        Data races are undefined behaviors hence bugs under the C standard. You do have a point in that Linux kernel is not written against the C standard and could declare to be written in the dialect of C where data races are defined...
        • asveikau 7y ago
          > You do have a point in that Linux kernel is not written against the C standard and could declare to be written in the dialect of C where data races are defined... You had my retort already written. Thank you. And in fact the linux kernel does rely on very specific compilers, and breaks the standard's machine abstraction quite a bit.
          • gpderetta 7y ago
            It is not a given that those compilers will support the unspecified memory model forever. And in fact the linux kernel has been slowly moving to the C++11 memory model for a while.
            • deleted 7y ago
              [deleted]
        • wahern 7y ago
          Undefined behavior is not necessarily a "bug", even in the context of the C standard. Undefined behavior is undefined behavior--the whole point of term is that the standard has nothing to say about the actual behavior. Undefined behavior includes behaviors about which the standard literally says nothing, like the effect of a supernova. But the term is normally used to distinguish certain behaviors that for various reasons the standard chooses (and chooses loudly) not to impose any requirements even though in context it could easily choose to do so; behaviors people might otherwise mistakenly assume are defined (or "natural") in reliance on their own outside experience and knowledge. Considering that C didn't support threading or even an atomics API until C11, and that most C code was written well before C11, by your definition the entire Linux kernel has been one giant "bug" ever since it added SMP support. Are data race conditions bugs? Usually--the term itself is conclusory of that fact. They're bugs not because they constitute undefined behavior as defined by the C standard, but because neither the hardware, compiler, application logic, or anything else guarantees consistent behavior; and if you have no guarantee of behavior then your software is ipso facto incorrect. If something were to guarantee the behavior, such as the notably generous cache coherency semantics of the x86 ISA, and nothing else conflicts with that guarantee, like an aggressive compiler, then it wouldn't be a bug, at least to the extent you knowingly relied on that guarantee. Undefined behavior is not an epithet. Just because a language specification doesn't use the term doesn't mean it doesn't have undefined behavior. Gobs of code in languages like Python and PHP rely on undefined behavior because those languages have very loose specifications. Even Java has undefined behavior despite its otherwise extremely rigorous specifications. (What Java does have is a very generous memory model which usually cabins the consequences of undefined behavior. Even then you can never rule out undefined behavior resulting in nasal daemons through some cascade of unexpected code paths in the application.) One of the reasons the term is so important in C and C++ is because they're two of the few languages with numerous, diverse implementations. The odds of two implementations diverging while programmers unwittingly rely on one of several potential behaviors is much greater. If there are only a few relevant implementations then there's less pressure to explicitly distinguish undefined behavior. Behaviors will tend to converge through informal communication. There's almost no pressure beyond the good diligence of the engineers if there's only a single implementation. Any accidental semantics relied upon by users can be made retrospectively well defined, and if there's no reliance then behaviors silently change (sometimes breaking code). For reference, here's what undefined behavior means, per C11 3.4.3 (N1570): 1 undefined behavior behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements 2 NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message). 3 EXAMPLE An example of undefined behavior is the behavior on integer overflow.
  • pjc50 7y ago
    This is a misunderstanding. A situation where "either process A or process B gets a resource first" is not a race condition. A situation where both of them get it at the same time is.
    • asveikau 7y ago
      Ok, I was being imprecise. Even if there is parallelism and multi-core, it is still a question of ordering, and from there we can use language like who gets there "first" [indeed "race" suggests this]. Or, did this read of a machine word see somebody else's write, etc.
  • KirinDave 7y ago
    Is this actually what the system was attempting to observe? I was under the impression that most of these race conditions were unknown and indeed were supposed to be implicit.