C++ Details of Asymmetric Fences

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• Claude, deal with this
  by CamperBob2
• I can't help myself digging into the referenced source code. The membarrier syscall can fail to allocate, returning ENOMEM. The way Folly calls it, the program would abort. Which I guess is a fair strategy but it's good to know when your synchronization primitive is actually SynchronizeOrCrash.
  by jeffbee
• In Linux everything is XOrCrash, since allocations never fail but the OOM killer can get you later.
  by ot
• > The membarrier syscall can fail to allocate, returning ENOMEM.

  This is mentioned in the source comment, but not documented in the membarrier(2) manual page.

  https://elixir.bootlin.com/linux/v7.1.2/source/kernel/sched/...

  In reality... what is it allocating?

  https://elixir.bootlin.com/linux/v7.1.2/source/kernel/sched/...

  Ok, so it needs to allocate a cpumask (tiny) under some usages.

  Similar requirement in other sites, e.g.: https://elixir.bootlin.com/linux/v7.1.2/source/kernel/sched/...

  Depending on kernel config (e.g., !CONFIG_CPUMAKS_OFFSTACK), this allocation literally cannot fail:

  https://elixir.bootlin.com/linux/v7.1.2/source/include/linux...

  On OFFSTACK configs, it's merely a tiny heap allocation:

  https://elixir.bootlin.com/linux/v7.1.2/source/lib/cpumask.c...

  If this fails, your system is already hosed; program abort vs kernel lock up a few seconds later is probably immaterial. I think folly's choice to abort is a reasonable one.

  by loeg
• I also just added these to Julia for 1.14 (https://github.com/JuliaLang/julia/pull/60311).
  by KenoFischer
• This is a great article but it goes into a lot of detail that can be intimidating at first.

  For me, the reading that made asymmetric fences "click" is this: https://pvk.ca/Blog/2019/01/09/preemption-is-gc-for-memory-r...

  It might be easier to read that first, as it also goes into practical applications, and then this one.

  by ot
• Thanks, this is very useful. Reading it I was wondering how applications typically exploit the asymmetric fence, I hope the article you linked help in that regard.
  by fschutze