Re: [PATCH v6 13/13] riscv: Add qspinlock support

[Will Deacon <will@kernel.org>]

On Sun, Nov 03, 2024 at 03:51:53PM +0100, Alexandre Ghiti wrote:
> In order to produce a generic kernel, a user can select
> CONFIG_COMBO_SPINLOCKS which will fallback at runtime to the ticket
> spinlock implementation if Zabha or Ziccrse are not present.
> 
> Note that we can't use alternatives here because the discovery of
> extensions is done too late and we need to start with the qspinlock
> implementation because the ticket spinlock implementation would pollute
> the spinlock value, so let's use static keys.

I think the static key toggling takes a mutex (jump_label_lock()) which
can take a spinlock (lock->wait_lock) internally, so I don't grok how
this works:

> +static void __init riscv_spinlock_init(void)
> +{
> +	char *using_ext = NULL;
> +
> +	if (IS_ENABLED(CONFIG_RISCV_TICKET_SPINLOCKS)) {
> +		pr_info("Ticket spinlock: enabled\n");
> +		return;
> +	}
> +
> +	if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA) &&
> +	    IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) &&
> +	    riscv_isa_extension_available(NULL, ZABHA) &&
> +	    riscv_isa_extension_available(NULL, ZACAS)) {
> +		using_ext = "using Zabha";
> +	} else if (riscv_isa_extension_available(NULL, ZICCRSE)) {
> +		using_ext = "using Ziccrse";
> +	}
> +#if defined(CONFIG_RISCV_COMBO_SPINLOCKS)
> +	else {
> +		static_branch_disable(&qspinlock_key);
> +		pr_info("Ticket spinlock: enabled\n");
> +		return;
> +	}
> +#endif

i.e. we've potentially already used the qspinlock at this point.

Will