linux-kernel - Re: [PATCH -next] arm64/spinlock: fix a -Wunused-function warning

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Message-ID: <2fe3a534-feb6-490c-71c6-208607e6cdf6@redhat.com>
Date:   Thu, 23 Jan 2020 12:11:03 -0500
From:   Waiman Long <longman@...hat.com>
To:     Qian Cai <cai@....pw>, peterz@...radead.org
Cc:     will@...nel.org, mingo@...hat.com, catalin.marinas@....com,
        clang-built-linux@...glegroups.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] arm64/spinlock: fix a -Wunused-function warning

On 1/23/20 11:29 AM, Qian Cai wrote:
> The commit f5bfdc8e3947 ("locking/osq: Use optimized spinning loop for
> arm64") introduced a warning from Clang because vcpu_is_preempted() is
> compiled away,
>
> kernel/locking/osq_lock.c:25:19: warning: unused function 'node_cpu'
> [-Wunused-function]
> static inline int node_cpu(struct optimistic_spin_node *node)
>                   ^
> 1 warning generated.
>
> Since vcpu_is_preempted() had already been defined in
> include/linux/sched.h as false, just comment out the redundant macro, so
> it can still be served for the documentation purpose.
>
> Fixes: f5bfdc8e3947 ("locking/osq: Use optimized spinning loop for arm64")
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>  arch/arm64/include/asm/spinlock.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
> index 102404dc1e13..b05f82e8ba19 100644
> --- a/arch/arm64/include/asm/spinlock.h
> +++ b/arch/arm64/include/asm/spinlock.h
> @@ -17,7 +17,8 @@
>   *
>   * See:
>   * https://lore.kernel.org/lkml/20200110100612.GC2827@hirez.programming.kicks-ass.net
> + *
> + * #define vcpu_is_preempted(cpu)	false
>   */
> -#define vcpu_is_preempted(cpu)	false
>  
>  #endif /* __ASM_SPINLOCK_H */

Does adding a __maybe_unused tag help to prevent the warning? Like

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 6ef600aa0f47..0722655af34f 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -22,7 +22,7 @@ static inline int encode_cpu(int cpu_nr)
        return cpu_nr + 1;
 }
 
-static inline int node_cpu(struct optimistic_spin_node *node)
+static inline int __maybe_unused node_cpu(struct optimistic_spin_node
*node)
 {
        return node->cpu - 1;
 }

Cheers,
Longman