linux-kernel - Re: [PATCH 03/20] mmiowb: Hook up mmiowb helpers to spinlocks and generic I/O accessors

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Date:   Sun, 03 Mar 2019 11:47:22 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     linux-arch@...r.kernel.org, Will Deacon <will.deacon@....com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Rich Felker <dalias@...c.org>,
        David Howells <dhowells@...hat.com>,
        Daniel Lustig <dlustig@...dia.com>,
        linux-kernel@...r.kernel.org,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Burton <paul.burton@...s.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Tony Luck <tony.luck@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 03/20] mmiowb: Hook up mmiowb helpers to spinlocks and
 generic I/O accessors

Will Deacon's on March 2, 2019 12:03 am:
> @@ -177,6 +178,7 @@ do {								\
>  static inline void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock)
>  {
>  	__acquire(lock);
> +	mmiowb_spin_lock();
>  	arch_spin_lock(&lock->raw_lock);
>  }
>  
> @@ -188,16 +190,23 @@ static inline void
>  do_raw_spin_lock_flags(raw_spinlock_t *lock, unsigned long *flags) __acquires(lock)
>  {
>  	__acquire(lock);
> +	mmiowb_spin_lock();
>  	arch_spin_lock_flags(&lock->raw_lock, *flags);
>  }

You'd be better to put these inside the spin lock, to match your 
trylock.

Also it means the mmiowb state can be used inside a lock/unlock pair
without a compiler barrer forcing it to be reloaded, should be better
code generation for very small critical sections on archs which inline
lock and unlock.

>  
>  static inline int do_raw_spin_trylock(raw_spinlock_t *lock)
>  {
> -	return arch_spin_trylock(&(lock)->raw_lock);
> +	int ret = arch_spin_trylock(&(lock)->raw_lock);
> +
> +	if (ret)
> +		mmiowb_spin_lock();
> +
> +	return ret;
>  }
>  
>  static inline void do_raw_spin_unlock(raw_spinlock_t *lock) __releases(lock)
>  {
> +	mmiowb_spin_unlock();
>  	arch_spin_unlock(&lock->raw_lock);
>  	__release(lock);
>  }

Thanks,
Nick