linux-kernel - Re: [PATCH-tip 04/22] locking/rwsem: Remove arch specific rwsem files

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Date:   Thu, 7 Feb 2019 14:43:10 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH-tip 04/22] locking/rwsem: Remove arch specific rwsem files

On 02/07/2019 02:36 PM, Peter Zijlstra wrote:
> On Thu, Feb 07, 2019 at 02:07:08PM -0500, Waiman Long wrote:
>
>> +static inline int __down_read_trylock(struct rw_semaphore *sem)
>> +{
>> +	long tmp;
>> +
>> +	while ((tmp = atomic_long_read(&sem->count)) >= 0) {
>> +		if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
>> +				   tmp + RWSEM_ACTIVE_READ_BIAS)) {
>> +			return 1;
>> +		}
>> +	}
> Nah, you're supposed to write that like:
>
> 	for (;;) {
> 		val = atomic_long_cond_read_relaxed(&sem->count, VAL < 0);
> 		if (atomic_long_try_cmpxchg_acquire(&sem->count, &val,
> 						    val + RWSEM_ACTIVE_READ_BIAS))
> 			break;
> 	}
>

Ah, you are right. I just took it directly from the current generic
asm.h file and didn't pay much attention to it. I will fix that in the
next iteration.

>> +	return 0;
>> +}
>
> Anyway, yuck, you're keeping all that BIAS nonsense :/ I was so hoping
> for a rwsem implementation without that impenetrable crap.

Well, I am going to take out all the BIAS handling in a later patch.
This one is just for removing the architecture specific files.

I can't change the rwsem count encoding before I take out all the arch
specific files.

Cheers,
Longman