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Date:   Wed, 15 Jun 2022 13:30:15 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Stephen Rothwell <sfr@canb.auug.org.au>,
        Frederic Weisbecker <frederic@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org, linux-next@vger.kernel.org
Subject: Re: Contextual conflict between kspp and rcu trees
Message-ID: <20220615203015.GF1790663@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <Yqo5SequJuC2qX6S@dev-arch.thelio-3990X>
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On Wed, Jun 15, 2022 at 12:55:53PM -0700, Nathan Chancellor wrote:
> Hi Stephen et al.,
> 
> There is a contextual conflict between commit e1d337335207 ("cfi: Fix
> __cfi_slowpath_diag RCU usage with cpuidle") in the kspp tree and commit
> dcc0c11aa87b ("rcu/context-tracking: Remove rcu_irq_enter/exit()") in
> the rcu tree, which is visible when building ARCH=arm64 defconfig +
> CONFIG_LTO_CLANG_THIN=y + CONFIG_CFI_CLANG=y with clang:
> 
>   kernel/cfi.c:298:3: error: call to undeclared function 'rcu_irq_enter'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
>                   rcu_irq_enter();
>                   ^
>   kernel/cfi.c:298:3: note: did you mean 'ct_irq_enter'?
>   ./include/linux/context_tracking_irq.h:6:6: note: 'ct_irq_enter' declared here
>   void ct_irq_enter(void);
>        ^
>   kernel/cfi.c:307:3: error: call to undeclared function 'rcu_irq_exit'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
>                   rcu_irq_exit();
>                   ^
>   kernel/cfi.c:307:3: note: did you mean 'ct_irq_exit'?
>   ./include/linux/context_tracking_irq.h:7:6: note: 'ct_irq_exit' declared here
>   void ct_irq_exit(void);
>        ^
>   2 errors generated.
> 
> 
> Per the above RCU commit and commit 6c5218715286 ("context_tracking:
> Take IRQ eqs entrypoints over RCU"), it appears that the following diff
> is the proper fix up. Would you mind applying it to the merge of
> whichever tree comes second if possible? I did build and boot test it
> but it would not be a bad idea for Sami and Frederic to verify that it
> is correct so that Kees/Paul can mention it to Linus :)

Agreed on Frederic verifying it, but in the meantime, it looks good to
me as well.

							Thanx, Paul

> Cheers,
> Nathan
> 
> diff --git a/kernel/cfi.c b/kernel/cfi.c
> index 08102d19ec15..2046276ee234 100644
> --- a/kernel/cfi.c
> +++ b/kernel/cfi.c
> @@ -295,7 +295,7 @@ static inline cfi_check_fn find_check_fn(unsigned long ptr)
>  	rcu_idle = !rcu_is_watching();
>  	if (rcu_idle) {
>  		local_irq_save(flags);
> -		rcu_irq_enter();
> +		ct_irq_enter();
>  	}
>  
>  	if (IS_ENABLED(CONFIG_CFI_CLANG_SHADOW))
> @@ -304,7 +304,7 @@ static inline cfi_check_fn find_check_fn(unsigned long ptr)
>  		fn = find_module_check_fn(ptr);
>  
>  	if (rcu_idle) {
> -		rcu_irq_exit();
> +		ct_irq_exit();
>  		local_irq_restore(flags);
>  	}
>