linux-kernel - Re: [PATCH] powerpc: mm: radix_tlb: rearrange the if-else block

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Message-ID: <6b1e51a8-2f4d-2024-df90-a35c926d7a30@csgroup.eu>
Date:   Fri, 26 Nov 2021 14:43:22 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Anders Roxell <anders.roxell@...aro.org>, benh@...nel.crashing.org,
        paulus@...ba.org
Cc:     Arnd Bergmann <arnd@...db.de>, llvm@...ts.linux.dev,
        ndesaulniers@...gle.com, linux-kernel@...r.kernel.org,
        nathan@...nel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc: mm: radix_tlb: rearrange the if-else block



Le 25/11/2021 à 16:44, Anders Roxell a écrit :
> Clang warns:
> 
> arch/powerpc/mm/book3s64/radix_tlb.c:1191:23: error: variable 'hstart' is uninitialized when used here [-Werror,-Wuninitialized]
>                                  __tlbiel_va_range(hstart, hend, pid,
>                                                    ^~~~~~
> arch/powerpc/mm/book3s64/radix_tlb.c:1175:23: note: initialize the variable 'hstart' to silence this warning
>                  unsigned long hstart, hend;
>                                      ^
>                                       = 0
> arch/powerpc/mm/book3s64/radix_tlb.c:1191:31: error: variable 'hend' is uninitialized when used here [-Werror,-Wuninitialized]
>                                  __tlbiel_va_range(hstart, hend, pid,
>                                                            ^~~~
> arch/powerpc/mm/book3s64/radix_tlb.c:1175:29: note: initialize the variable 'hend' to silence this warning
>                  unsigned long hstart, hend;
>                                            ^
>                                             = 0
> 2 errors generated.
> 
> Rework the if-else to pull the 'IS_ENABLE(CONFIG_TRANSPARENT_HUGEPAGE)'
> check one level up, this will silent the warnings. That will also
> simplify the 'else' path. Clang is getting confused with these warnings,
> but the warnings is a false-positive.

But you are duplicating a significant part of the code by doing that, 
and duplicated code generaly leads to bugs.

And we already have redundant stuff between FLUSH_TYPE_LOCAL leg and 
cputlb_use_tlbie() leg.

Can't you fix CLANG instead :) ?

Or just add an else to the IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) that 
sets hstart and hend to 0 ?

Or just put hstart and hend calculation outside the IS_ENABLED() ? After 
all GCC should drop the calculation when not used.


> 
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>
> ---
>   arch/powerpc/mm/book3s64/radix_tlb.c | 31 +++++++++++++++++++++-------
>   1 file changed, 24 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
> index 7724af19ed7e..e494a45ce1b4 100644
> --- a/arch/powerpc/mm/book3s64/radix_tlb.c
> +++ b/arch/powerpc/mm/book3s64/radix_tlb.c
> @@ -1170,16 +1170,14 @@ static inline void __radix__flush_tlb_range(struct mm_struct *mm,
>   				_tlbiel_pid_multicast(mm, pid, RIC_FLUSH_ALL);
>   			}
>   		}
> -	} else {
> +	} else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
>   		bool hflush = false;
>   		unsigned long hstart, hend;
>   
> -		if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> -			hstart = (start + PMD_SIZE - 1) & PMD_MASK;
> -			hend = end & PMD_MASK;
> -			if (hstart < hend)
> -				hflush = true;
> -		}
> +		hstart = (start + PMD_SIZE - 1) & PMD_MASK;
> +		hend = end & PMD_MASK;
> +		if (hstart < hend)
> +			hflush = true;
>   
>   		if (type == FLUSH_TYPE_LOCAL) {
>   			asm volatile("ptesync": : :"memory");
> @@ -1207,6 +1205,25 @@ static inline void __radix__flush_tlb_range(struct mm_struct *mm,
>   				_tlbiel_va_range_multicast(mm,
>   					hstart, hend, pid, PMD_SIZE, MMU_PAGE_2M, flush_pwc);
>   		}
> +	} else {
> +
> +		if (type == FLUSH_TYPE_LOCAL) {
> +			asm volatile("ptesync" : : : "memory");
> +			if (flush_pwc)
> +				/* For PWC, only one flush is needed */
> +				__tlbiel_pid(pid, 0, RIC_FLUSH_PWC);
> +			__tlbiel_va_range(start, end, pid, page_size, mmu_virtual_psize);
> +			ppc_after_tlbiel_barrier();
> +		} else if (cputlb_use_tlbie()) {
> +			asm volatile("ptesync" : : : "memory");
> +			if (flush_pwc)
> +				__tlbie_pid(pid, RIC_FLUSH_PWC);
> +			__tlbie_va_range(start, end, pid, page_size, mmu_virtual_psize);
> +			asm volatile("eieio; tlbsync; ptesync" : : : "memory");
> +		} else {
> +			_tlbiel_va_range_multicast(mm,
> +					start, end, pid, page_size, mmu_virtual_psize, flush_pwc);
> +		}
>   	}
>   out:
>   	preempt_enable();
>