RFC 2: MT safe fiber context switch on AArch64#15581
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We need a store-store barrier between pushing the registers to the current stack and setting the resumable flag of the current fiber, otherwise the CPU is allowed to reorder the instructions at runtime and to store the resumable flag before or while storing the registers. This can happen for example: thread 1: enqueues current fiber A therad 1: swapcontext -> store resumable thread 1: is preempted thread 2: steals fiber A thread 2: resumes fiber A thread 2: loads registers => reads garbage => segfaults thread 1: stores registers (too late)
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We need a store-store barrier between pushing the registers to the current stack and setting the resumable flag of the current fiber, otherwise the CPU is allowed to reorder the instructions at runtime and to store the resumable flag before or while storing the registers thread 1: enqueues current fiber A therad 1: swapcontext -> store resumable thread 1: is preempted thread 2: steals fiber A thread 2: resumes fiber A thread 2: loads registers => reads garbage => segfaults thread 1: stores registers (too late) The chosen assembly is compatible with armv6, but might not work on older architectures, and it doesn't take advantage of armv7 supporting the `dmb ish` instruction. We might want to consider integrating #14524 (comment) into the compiler, so we could add flags based on features, for example `armv7` when any feature matches `/\+v7(ve|r|m|em|s|k|)/`. This is the ARM32 equivalent of #15581.
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We need a store-store barrier between pushing the registers to the current stack and setting the resumable flag of the current fiber, otherwise the CPU is allowed to reorder the instructions at runtime and to store the resumable flag before or while storing the registers.
This can happen for example:
The current implementation didn't need the barrier because fibers are stuck to a single thread (it must have finished swapcontext to be able to start another swapcontext), but RFC 2 adds work stealing to the MT environment.
Note: the same issue can happen in ARM32.