linux-kernel - Re: [PATCH v3 2/6] futex2: Implement vectorized wait

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Date:   Tue, 14 Sep 2021 14:18:58 -0300
From:   André Almeida <andrealmeid@...labora.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        kernel@...labora.com, linux-api@...r.kernel.org,
        libc-alpha@...rceware.org, mtk.manpages@...il.com,
        Davidlohr Bueso <dave@...olabs.net>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 2/6] futex2: Implement vectorized wait

Hi Gabriel, thanks for the feedback! A few clarifications:

Às 22:03 de 13/09/21, Gabriel Krisman Bertazi escreveu:
> André Almeida <andrealmeid@...labora.com> writes:
> 
>> Add support to wait on multiple futexes. This is the interface
>> implemented by this syscall:
>>

[...]

>>  
>> +/*
>> + * Flags to specify the bit length of the futex word for futex2 syscalls.
>> + * Currently, only 32 is supported.
>> + */
>> +#define FUTEX_32		2
> 
> Why start at 2?

I was planning to do:

FUTEX_8		0
FUTEX_16	1
FUTEX_32	2
FUTEX_64	3

> 
>> +
>> +/*
>> + * Max numbers of elements in a futex_waitv array
>> + */
>> +#define FUTEX_WAITV_MAX		128
>> +
>> +/**
>> + * struct futex_waitv - A waiter for vectorized wait
>> + * @val:	Expected value at uaddr
>> + * @uaddr:	User address to wait on
>> + * @flags:	Flags for this waiter
>> + * @__reserved:	Reserved member to preserve data alignment. Should be 0.
>> + */
>> +struct futex_waitv {
>> +	__u64 val;
>> +	__u64 uaddr;
>> +	__u32 flags;
>> +	__u32 __reserved;
>> +};
> 
> why force uaddr  to be __u64, even for 32-bit?  uaddr could be a (void*) for
> all we care, no?  Also, by adding a reserved field, you are wasting 32
> bits even on 32-bit architectures.
> 

We do that to make the structure layout compatible with both entry
points, remove the need for special cast and duplicated code, as
suggested by Thomas and Arnd:

https://lore.kernel.org/lkml/87v94310gm.ffs@tglx/

https://lore.kernel.org/lkml/CAK8P3a0MO1qJLRkCH8KrZ3+=L66KOsMRmcbrUvYdMoKykdKoyQ@mail.gmail.com/