linux-kernel - Re: [PATCH v2] x86/alternatives: Let __text_poke() acquire the pte lock with enabled interrupts

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Message-ID: <20200813111351.GV2674@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Aug 2020 13:13:51 +0200
From:   peterz@...radead.org
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86/alternatives: Let __text_poke() acquire the pte
 lock with enabled interrupts

On Thu, Aug 13, 2020 at 12:50:26PM +0200, Sebastian Andrzej Siewior wrote:
> The pte lock is never acquired in-IRQ context so it does not require the
> interrupts to be disabled.
> 
> RT complains here because the spinlock_t must not be acquired with
> disabled interrupts.
> 
> use_temporary_mm() expects interrupts to be off because it invokes
> switch_mm_irqs_off() and uses per-CPU (current active mm) data.
> 
> Move the PTE lock handling outside the interrupt disabled region.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Agreed, this should be fine.

Acked-by; Peter Zijlstra (Intel) <peterz@...radead.org>

> ---
> v1…v2: Reword the patch description.
> 
>  arch/x86/kernel/alternative.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -875,8 +875,6 @@ static void *__text_poke(void *addr, con
>  	 */
>  	BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));
>  
> -	local_irq_save(flags);
> -
>  	/*
>  	 * Map the page without the global bit, as TLB flushing is done with
>  	 * flush_tlb_mm_range(), which is intended for non-global PTEs.
> @@ -893,6 +891,8 @@ static void *__text_poke(void *addr, con
>  	 */
>  	VM_BUG_ON(!ptep);
>  
> +	local_irq_save(flags);
> +
>  	pte = mk_pte(pages[0], pgprot);
>  	set_pte_at(poking_mm, poking_addr, ptep, pte);
>  
> @@ -942,8 +942,8 @@ static void *__text_poke(void *addr, con
>  	 */
>  	BUG_ON(memcmp(addr, opcode, len));
>  
> -	pte_unmap_unlock(ptep, ptl);
>  	local_irq_restore(flags);
> +	pte_unmap_unlock(ptep, ptl);
>  	return addr;
>  }
>