linux-kernel - Re: [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes

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Date:   Fri, 28 Feb 2020 20:07:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     André Almeida <andrealmeid@...labora.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        kernel@...labora.com, krisman@...labora.com, shuah@...nel.org,
        linux-kselftest@...r.kernel.org, rostedt@...dmis.org,
        ryao@...too.org, dvhart@...radead.org, mingo@...hat.com,
        z.figura12@...il.com, steven@...vesoftware.com,
        pgriffais@...vesoftware.com, steven@...uorix.net
Subject: Re: [PATCH v3 1/4] futex: Implement mechanism to wait on any of
 several futexes

On Thu, Feb 13, 2020 at 06:45:22PM -0300, André Almeida wrote:
> @@ -150,4 +153,21 @@ struct robust_list_head {
>    (((op & 0xf) << 28) | ((cmp & 0xf) << 24)		\
>     | ((oparg & 0xfff) << 12) | (cmparg & 0xfff))
>  
> +/*
> + * Maximum number of multiple futexes to wait for
> + */
> +#define FUTEX_MULTIPLE_MAX_COUNT	128
> +
> +/**
> + * struct futex_wait_block - Block of futexes to be waited for
> + * @uaddr:	User address of the futex
> + * @val:	Futex value expected by userspace
> + * @bitset:	Bitset for the optional bitmasked wakeup
> + */
> +struct futex_wait_block {
> +	__u32 __user *uaddr;
> +	__u32 val;
> +	__u32 bitset;
> +};

So I have a problem with this vector layout, it doesn't allow for
per-futex flags, and esp. with that multi-size futex support that
becomes important, but also with the already extand private/shared and
wait_bitset flags this means you cannot have a vector with mixed wait
types.

>  #endif /* _UAPI_LINUX_FUTEX_H */
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 0cf84c8664f2..58cf9eb2b851 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -215,6 +215,8 @@ struct futex_pi_state {
>   * @rt_waiter:		rt_waiter storage for use with requeue_pi
>   * @requeue_pi_key:	the requeue_pi target futex key
>   * @bitset:		bitset for the optional bitmasked wakeup
> + * @uaddr:             userspace address of futex
> + * @uval:              expected futex's value
>   *
>   * We use this hashed waitqueue, instead of a normal wait_queue_entry_t, so
>   * we can wake only the relevant ones (hashed queues may be shared).
> @@ -237,6 +239,8 @@ struct futex_q {
>  	struct rt_mutex_waiter *rt_waiter;
>  	union futex_key *requeue_pi_key;
>  	u32 bitset;
> +	u32 __user *uaddr;
> +	u32 uval;
>  } __randomize_layout;

That creates a hole for no reason.