linux-kernel - Re: [patch 3/3] x86/mm: Remove preempt_disable/enable() from __native_flush

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Message-ID: <20171230213145.rh26gyvfopujzumb@gmail.com>
Date:   Sat, 30 Dec 2017 22:31:45 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [patch 3/3] x86/mm: Remove preempt_disable/enable() from
 __native_flush_tlb()


The cleanup looks good to me, just a few speling nits:

* Thomas Gleixner <tglx@...utronix.de> wrote:

> The preempt_disable/enable() pair in __native_flush_tlb() was added in
> commit 5cf0791da5c1 ("x86/mm: Disable preemption during CR3 read+write") to
> protect the UP variant of flush_tlb_mm_range().
> 
> That preempt_disable/enable() pair should have been added to the UP variant
> of flush_tlb_mm_range() instead.
> 
> The UP variant was removed with commit ce4a4e565f52 ("x86/mm: Remove the UP
> asm/tlbflush.h code, always use the (formerly) SMP code"), but the
> preempt_disable/enable() pair stayed around.
> 
> The latest change to __native_flush_tlb() in commit 6fd166aae78c ("x86/mm:
> Use/Fix PCID to optimize user/kernel switches") added an access to a per
> cpu variable outside the preempt disabled regions which makes no sense at
> all. __native_flush_tlb() must always be called with at least preemption
> disabled.

s/cpu/CPU

> 
> Remove the preempt_disable/enable() pair and add a WARN_ON_ONCE() to catch
> bad callers independent of the smp_processor_id() debugging.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>  arch/x86/include/asm/tlbflush.h |   14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> --- a/arch/x86/include/asm/tlbflush.h
> +++ b/arch/x86/include/asm/tlbflush.h
> @@ -345,15 +345,17 @@ static inline void invalidate_user_asid(
>   */
>  static inline void __native_flush_tlb(void)
>  {
> -	invalidate_user_asid(this_cpu_read(cpu_tlbstate.loaded_mm_asid));
>  	/*
> -	 * If current->mm == NULL then we borrow a mm which may change
> -	 * during a task switch and therefore we must not be preempted
> -	 * while we write CR3 back:
> +	 * Preemption or interrupts must be disabled to protect the access
> +	 * to the per cpu variable and to prevent being preempted between
> +	 * read_cr3() and write_cr3().
>  	 */
> -	preempt_disable();
> +	WARN_ON_ONCE(preemptible());
> +
> +	invalidate_user_asid(this_cpu_read(cpu_tlbstate.loaded_mm_asid));
> +
> +	/* If current->mm == NULL then the read_cr3() "borrows" a mm */
>  	native_write_cr3(__native_read_cr3());
> -	preempt_enable();

s/a mm/an mm

Reviewed-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo