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

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