linux-kernel - Re: [PATCH] qspinlock: use signed temporaries for cmpxchg

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Date:   Mon, 26 Oct 2020 14:03:06 -0400
From:   Waiman Long <longman@...hat.com>
To:     Arnd Bergmann <arnd@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Matthew Wilcox <willy@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Will Deacon <will@...nel.org>
Cc:     Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nicholas Piggin <npiggin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] qspinlock: use signed temporaries for cmpxchg

On 10/26/20 12:57 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
>
> When building with W=2, the build log is flooded with
>
> include/asm-generic/qrwlock.h:65:56: warning: pointer targets in passing argument 2 of 'atomic_try_cmpxchg_acquire' differ in signedness [-Wpointer-sign]
> include/asm-generic/qrwlock.h:92:53: warning: pointer targets in passing argument 2 of 'atomic_try_cmpxchg_acquire' differ in signedness [-Wpointer-sign]
> include/asm-generic/qspinlock.h:68:55: warning: pointer targets in passing argument 2 of 'atomic_try_cmpxchg_acquire' differ in signedness [-Wpointer-sign]
> include/asm-generic/qspinlock.h:82:52: warning: pointer targets in passing argument 2 of 'atomic_try_cmpxchg_acquire' differ in signedness [-Wpointer-sign]
>
> The atomics are built on top of signed integers, but the caller
> doesn't actually care. Just use signed types as well.
>
> Fixes: 27df89689e25 ("locking/spinlocks: Remove an instruction from spin and write locks")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>   include/asm-generic/qrwlock.h   | 8 ++++----
>   include/asm-generic/qspinlock.h | 4 ++--
>   2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> index 3aefde23dcea..84ce841ce735 100644
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -37,7 +37,7 @@ extern void queued_write_lock_slowpath(struct qrwlock *lock);
>    */
>   static inline int queued_read_trylock(struct qrwlock *lock)
>   {
> -	u32 cnts;
> +	int cnts;
>   
>   	cnts = atomic_read(&lock->cnts);
>   	if (likely(!(cnts & _QW_WMASK))) {
> @@ -56,7 +56,7 @@ static inline int queued_read_trylock(struct qrwlock *lock)
>    */
>   static inline int queued_write_trylock(struct qrwlock *lock)
>   {
> -	u32 cnts;
> +	int cnts;
>   
>   	cnts = atomic_read(&lock->cnts);
>   	if (unlikely(cnts))
> @@ -71,7 +71,7 @@ static inline int queued_write_trylock(struct qrwlock *lock)
>    */
>   static inline void queued_read_lock(struct qrwlock *lock)
>   {
> -	u32 cnts;
> +	int cnts;
>   
>   	cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
>   	if (likely(!(cnts & _QW_WMASK)))
> @@ -87,7 +87,7 @@ static inline void queued_read_lock(struct qrwlock *lock)
>    */
>   static inline void queued_write_lock(struct qrwlock *lock)
>   {
> -	u32 cnts = 0;
> +	int cnts = 0;
>   	/* Optimize for the unfair lock case where the fair flag is 0. */
>   	if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
>   		return;
> diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
> index 4fe7fd0fe834..d74b13825501 100644
> --- a/include/asm-generic/qspinlock.h
> +++ b/include/asm-generic/qspinlock.h
> @@ -60,7 +60,7 @@ static __always_inline int queued_spin_is_contended(struct qspinlock *lock)
>    */
>   static __always_inline int queued_spin_trylock(struct qspinlock *lock)
>   {
> -	u32 val = atomic_read(&lock->val);
> +	int val = atomic_read(&lock->val);
>   
>   	if (unlikely(val))
>   		return 0;
> @@ -77,7 +77,7 @@ extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
>    */
>   static __always_inline void queued_spin_lock(struct qspinlock *lock)
>   {
> -	u32 val = 0;
> +	int val = 0;
>   
>   	if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
>   		return;

Yes, it shouldn't really matter if the value is defined as int or u32. 
However, the only caveat that I see is queued_spin_lock_slowpath() is 
expecting a u32 argument. Maybe you should cast it back to (u32) when 
calling it.

Cheers,
Longman