If you break the build for them by shipping with -Werror, the likely outcome is that they start making random changes to the code the compiler warns about until the warning goes away, possibly BREAKING THE CODE IN DANGEROUS WAYS.
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Summary: -Werror is only meaningful with a known compiler version and build target, and only to developers who can meaningfully act on the failures. Don't ship with -Werror. Ever.
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Since the Linux kernel doesn't follow the C memory model and disregards undefined behavior rules they don't agree with, it's fairly dangerous to use a newer compiler than what they're broadly using and testing themselves. Ideally, they'd actually list what's being tested / used.
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For Linux it's specific bare-metal targets so -Werror is less evil than in general, but still a bad idea. Linux uses -fno-strict-aliasing etc. so it's not subject to most C memory model issues a new compiler could break.
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I mean that they don't follow the C11 memory model for atomics and make extensive use of atomics. The compiler developers don't agree with their homegrown rules and don't respect them. There's a whole lot of complicated lock-free data structure stuff that's really quite fragile.
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Aren't they all accessed as volatiles? Assuming a C compiler with a reasonable sense of volatile, you can model your own atomics that way (other cores being async hardware modifying the volatile memory).
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No, they don't use volatile for that since it would hurt performance too much and the whole reason for them refusing to use C11 atomics is because they think even the acquire/release semantics are too expensive.
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Acquire/release are a lot more expensive than volatile. Volatile just means each load/store on the abstract machine has to translate to one on the real machine, with matching load/store size (no split/combining) where the machine admits it.
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Look at open-std.org/jtc1/sc22/wg21. They're got their own quite fleshed out way of doing things. They decided how C should work and fleshed that out but they aren't on the same page as the C standard or compiler authors right now. There are ongoing efforts to fix the problems.
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According to that they're all modeled as volatile accesses which makes them completely compatible with the C memory model assuming a compiler that implements volatile in any meaningful way (any way compatible with mmio registers, for example).
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Clang/GCC expect you to use volatile + atomics rather than volatile alone with the assumption that it gives atomic semantics.
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That is not the case. It is entirely consistent to model "other cores" or "threads running on other cores" as black-box hardware entities that modify volatile objects asynchronously. Of course you also need barriers if...
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...if you want them to impose any kind of consistency or ordering with other loads and stores. But there's no reason you can't do that.
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