Subject: Re: [PATCH 8/8] membarrier: Rewrite sync_core_before_usermode() and improve documentation

From: Mathieu Desnoyers <[email-masked]>
Date: Thu, 17 Jun 2021 10:47:57 -0400 (EDT)

----- On Jun 15, 2021, at 11:21 PM, Andy Lutomirski [email-masked] wrote:

> The old sync_core_before_usermode() comments suggested that a non-icache-syncing
> return-to-usermode instruction is x86-specific and that all other
> architectures automatically notice cross-modified code on return to
> userspace.
> 
> This is misleading.  The incantation needed to modify code from one
> CPU and execute it on another CPU is highly architecture dependent.
> On x86, according to the SDM, one must modify the code, issue SFENCE
> if the modification was WC or nontemporal, and then issue a "serializing
> instruction" on the CPU that will execute the code.  membarrier() can do
> the latter.
> 
> On arm64 and powerpc, one must flush the icache and then flush the pipeline
> on the target CPU, although the CPU manuals don't necessarily use this
> language.
> 
> So let's drop any pretense that we can have a generic way to define or
> implement membarrier's SYNC_CORE operation and instead require all
> architectures to define the helper and supply their own documentation as to
> how to use it.

Agreed. Documentation of the sequence of operations that need to be performed
when cross-modifying code on SMP should be per-architecture. The documentation
of the architectural effects of membarrier sync-core should be per-arch as well.

> This means x86, arm64, and powerpc for now.

And also arm32, as discussed in the other leg of the patchset's email thread.

> Let's also
> rename the function from sync_core_before_usermode() to
> membarrier_sync_core_before_usermode() because the precise flushing details
> may very well be specific to membarrier, and even the concept of
> "sync_core" in the kernel is mostly an x86-ism.

OK

[...]
> 
> static void ipi_rseq(void *info)
> {
> @@ -368,12 +373,14 @@ static int membarrier_private_expedited(int flags, int
> cpu_id)
> 	smp_call_func_t ipi_func = ipi_mb;
> 
> 	if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
> -		if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
> +#ifndef CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE
> 			return -EINVAL;
> +#else
> 		if (!(atomic_read(&mm->membarrier_state) &
> 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
> 			return -EPERM;
> 		ipi_func = ipi_sync_core;
> +#endif

Please change back this #ifndef / #else / #endif within function for

if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE)) {
  ...
} else {
  ...
}

I don't think mixing up preprocessor and code logic makes it more readable.

Thanks,

Mathieu

> 	} else if (flags == MEMBARRIER_FLAG_RSEQ) {
> 		if (!IS_ENABLED(CONFIG_RSEQ))
> 			return -EINVAL;
> --
> 2.31.1

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com