Re: [PATCH v4 03/13] task_isolation: userspace hard isolation from kernel

[Frederic Weisbecker]

On Wed, Jul 22, 2020 at 02:49:49PM +0000, Alex Belits wrote:
> +/**
> + * task_isolation_kernel_enter() - clear low-level task isolation flag
> + *
> + * This should be called immediately after entering kernel.
> + */
> +static inline void task_isolation_kernel_enter(void)
> +{
> +	unsigned long flags;
> +
> +	/*
> +	 * This function runs on a CPU that ran isolated task.
> +	 *
> +	 * We don't want this CPU running code from the rest of kernel
> +	 * until other CPUs know that it is no longer isolated.
> +	 * When CPU is running isolated task until this point anything
> +	 * that causes an interrupt on this CPU must end up calling this
> +	 * before touching the rest of kernel. That is, this function or
> +	 * fast_task_isolation_cpu_cleanup() or stop_isolation() calling
> +	 * it. If any interrupt, including scheduling timer, arrives, it
> +	 * will still end up here early after entering kernel.
> +	 * From this point interrupts are disabled until all CPUs will see
> +	 * that this CPU is no longer running isolated task.
> +	 *
> +	 * See also fast_task_isolation_cpu_cleanup().
> +	 */
> +	smp_rmb();

I'm a bit confused what this read memory barrier is ordering. Also against
what it pairs.

> +	if((this_cpu_read(ll_isol_flags) & FLAG_LL_TASK_ISOLATION) == 0)
> +		return;
> +
> +	local_irq_save(flags);
> +
> +	/* Clear low-level flags */
> +	this_cpu_write(ll_isol_flags, 0);
> +
> +	/*
> +	 * If something happened that requires a barrier that would
> +	 * otherwise be called from remote CPUs by CPU kick procedure,
> +	 * this barrier runs instead of it. After this barrier, CPU
> +	 * kick procedure would see the updated ll_isol_flags, so it
> +	 * will run its own IPI to trigger a barrier.
> +	 */
> +	smp_mb();
> +	/*
> +	 * Synchronize instructions -- this CPU was not kicked while
> +	 * in isolated mode, so it might require synchronization.
> +	 * There might be an IPI if kick procedure happened and
> +	 * ll_isol_flags was already updated while it assembled a CPU
> +	 * mask. However if this did not happen, synchronize everything
> +	 * here.
> +	 */
> +	instr_sync();

It's the first time I meet an instruction barrier. I should get information
about that but what is it ordering here?

> +	local_irq_restore(flags);
> +}

Thanks.