Messages in this thread | | | Date | Tue, 14 Sep 2021 14:18:58 -0300 | Subject | Re: [PATCH v3 2/6] futex2: Implement vectorized wait | From | André Almeida <> |
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Hi Gabriel, thanks for the feedback! A few clarifications:
Às 22:03 de 13/09/21, Gabriel Krisman Bertazi escreveu: > André Almeida <andrealmeid@collabora.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/
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