Re: [PATCH v8 6/7] rust: Add read_poll_timeout functions

[Alice Ryhl]

On Thu, Jan 16, 2025 at 5:43 AM FUJITA Tomonori
<fujita.tomonori@xxxxxxxxx> wrote:
>
> Add read_poll_timeout functions which poll periodically until a
> condition is met or a timeout is reached.
>
> C's read_poll_timeout (include/linux/iopoll.h) is a complicated macro
> and a simple wrapper for Rust doesn't work. So this implements the
> same functionality in Rust.
>
> The C version uses usleep_range() while the Rust version uses
> fsleep(), which uses the best sleep method so it works with spans that
> usleep_range() doesn't work nicely with.
>
> Unlike the C version, __might_sleep() is used instead of might_sleep()
> to show proper debug info; the file name and line
> number. might_resched() could be added to match what the C version
> does but this function works without it.
>
> The sleep_before_read argument isn't supported since there is no user
> for now. It's rarely used in the C version.
>
> core::panic::Location::file() doesn't provide a null-terminated string
> so add __might_sleep_precision() helper function, which takes a
> pointer to a string with its length.
>
> read_poll_timeout() can only be used in a nonatomic context. This
> requirement is not checked by these abstractions, but it is intended
> that klint [1] or a similar tool will be used to check it in the
> future.
>
> Link: https://rust-for-linux.com/klint [1]
> Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@xxxxxxxxx>

[...]

> +void __might_sleep(const char *file, int line)
> +{
> +       long len = strlen(file);
> +
> +       __might_sleep_precision(file, len, line);
>  }
>  EXPORT_SYMBOL(__might_sleep);

I think these strlen() calls could be pretty expensive. You run them
every time might_sleep() runs even if the check does not fail.

How about changing __might_resched_precision() to accept a length of
-1 for nul-terminated strings, and having it compute the length with
strlen only *if* we know that we actually need the length?

if (len < 0) len = strlen(file);
pr_err("BUG: sleeping function called from invalid context at %.*s:%d\n",
       len, file, line);

Another option might be to compile the lengths at compile-time by
having the macros use sizeof on __FILE__, but that sounds more tricky
to get right.

Alice