Re: [PATCH v3 1/7] asm-generic: ticket-lock: New generic ticket-based spinlock

From: Waiman Long <longman@redhat.com>
To: Palmer Dabbelt <palmer@rivosinc.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	heiko@sntech.de, guoren@kernel.org, shorne@gmail.com,
	peterz@infradead.org, mingo@redhat.com,
	Will Deacon <will@kernel.org>,
	boqun.feng@gmail.com, jonas@southpole.se,
	stefan.kristiansson@saunalahti.fi,
	Paul Walmsley <paul.walmsley@sifive.com>,
	aou@eecs.berkeley.edu, macro@orcam.me.uk,
	Greg KH <gregkh@linuxfoundation.org>,
	sudipm.mukherjee@gmail.com, wangkefeng.wang@huawei.com,
	jszhang@kernel.org, linux-csky@vger.kernel.org,
	linux-kernel@vger.kernel.org, openrisc@lists.librecores.org,
	linux-riscv@lists.infradead.org, linux-arch@vger.kernel.org
Subject: Re: [PATCH v3 1/7] asm-generic: ticket-lock: New generic ticket-based spinlock
Date: Fri, 15 Apr 2022 13:02:42 -0400	[thread overview]
Message-ID: <8f2a3903-ab49-23cc-a362-a9857dc38410@redhat.com> (raw)
In-Reply-To: <mhng-f4d144dd-b6ed-4f3f-bfc3-6dc29fab14e4@palmer-ri-x1c9>

On 4/15/22 12:46, Palmer Dabbelt wrote:
>
>>> diff --git a/include/asm-generic/spinlock_types.h 
>>> b/include/asm-generic/spinlock_types.h
>>> new file mode 100644
>>> index 000000000000..e56ddb84d030
>>> --- /dev/null
>>> +++ b/include/asm-generic/spinlock_types.h
>>> @@ -0,0 +1,17 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +
>>> +#ifndef __ASM_GENERIC_TICKET_LOCK_TYPES_H
>>> +#define __ASM_GENERIC_TICKET_LOCK_TYPES_H
>>> +
>>> +#include <linux/types.h>
>>> +typedef atomic_t arch_spinlock_t;
>>> +
>>> +/*
>>> + * qrwlock_types depends on arch_spinlock_t, so we must typedef 
>>> that before the
>>> + * include.
>>> + */
>>> +#include <asm/qrwlock_types.h>
>>
>> I believe that if you guard the include line by
>>
>> #ifdef CONFIG_QUEUED_RWLOCK
>> #include <asm/qrwlock_types.h>
>> #endif
>>
>> You may not need to do the hack in patch 5.
>
> Yes, and we actually had it that way the first time around 
> (specifically the ARCH_USES_QUEUED_RWLOCKS, but IIUC that's the same 
> here).  The goal was to avoid adding the ifdef to the asm-generic code 
> and instead keep the oddness in arch/riscv, it's only there for that 
> one commit (and just so we can split out the spinlock conversion from 
> the rwlock conversion, in case there's a bug and these need to be 
> bisected later).
>
> I'd also considered renaming qrwlock* to rwlock*, which would avoid 
> the ifdef and make it a touch easier to override the rwlock 
> implementation, but that didn't seem useful enough to warrant the 
> diff.  These all seem a bit more coupled than I expected them to be 
> (both {spin,qrw}lock{,_types}.h and the bits in linux/), I looked into 
> cleaning that up a bit but it seemed like too much for just the one 
> patch set.

Then you are forcing arches that use asm_generic/spinlock.h to use 
qrwlock as well. Even though most of them probably will, but forcing it 
this way remove the flexibility an arch may want to have.

The difference between CONFIG_QUEUED_RWLOCK and ARCH_USES_QUEUED_RWLOCKS 
is that qrwlock will not be compiled in when PREEMPT_RT || !SMP. So 
CONFIG_QUEUED_RWLOCK is a more accurate guard as to whether qrwlock 
should really be used.

Cheers,
Longman