Re: [PATCH 4/5] sched/core: Add __might_sleep_precision()

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, May 19, 2025 at 05:40:51AM -0700, Boqun Feng wrote:
> On Fri, May 09, 2025 at 12:19:03AM -0700, Boqun Feng wrote:
> > On Fri, May 09, 2025 at 08:00:32AM +0200, Ingo Molnar wrote:
> > > 
> > > * Boqun Feng <boqun.feng@gmail.com> wrote:
> > > 
> > > > From: FUJITA Tomonori <fujita.tomonori@gmail.com>
> > > > 
> > > > Add __might_sleep_precision(), Rust friendly version of
> > > > __might_sleep(), which takes a pointer to a string with the length
> > > > instead of a null-terminated string.
> > > > 
> > > > Rust's core::panic::Location::file(), which gives the file name of a
> > > > caller, doesn't provide a null-terminated
> > > > string. __might_sleep_precision() uses a precision specifier in the
> > > > printk format, which specifies the length of a string; a string
> > > > doesn't need to be a null-terminated.
> > > > 
> > > > Modify __might_sleep() to call __might_sleep_precision() but the
> > > > impact should be negligible. When printing the error (sleeping
> > > > function called from invalid context), the precision string format is
> > > > used instead of the simple string format; the precision specifies the
> > > > the maximum length of the displayed string.
> > > > 
> > > > Note that Location::file() providing a null-terminated string for
> > > > better C interoperability is under discussion [1].
> > > > 
> > > > [1]: https://github.com/rust-lang/libs-team/issues/466
> > > > 
> > > > Tested-by: Daniel Almeida <daniel.almeida@collabora.com>
> > > > Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> > > > Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
> > > > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > > > Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> > > > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > > > Link: https://lore.kernel.org/r/20250410225623.152616-2-fujita.tomonori@gmail.com
> > > > ---
> > > >  include/linux/kernel.h |  2 ++
> > > >  kernel/sched/core.c    | 62 ++++++++++++++++++++++++++++--------------
> > > >  2 files changed, 43 insertions(+), 21 deletions(-)
> > > > 
> > > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > > > index be2e8c0a187e..086ee1dc447e 100644
> > > > --- a/include/linux/kernel.h
> > > > +++ b/include/linux/kernel.h
> > > > @@ -87,6 +87,7 @@ extern int dynamic_might_resched(void);
> > > >  #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
> > > >  extern void __might_resched(const char *file, int line, unsigned int offsets);
> > > >  extern void __might_sleep(const char *file, int line);
> > > > +extern void __might_sleep_precision(const char *file, int len, int line);
> > > 
> > > Ugh.
> > > 
> > > Firstly, '_precision' is really ambiguous in this context and suggests 
> > > 'precise sleep' or something like that, which this is not about at all. 
> > > So the naming here is all sorts of bad already.
> > > 
> > 
> > I accept this is not a good naming.
> > 
> > > But more importantly, this is really a Rust problem. Does Rust really 
> > > have no NUL-terminated strings? It should hide them in shame and 
> > 
> > You can create NUL-terminated strings in Rust of course, but in this
> > case, because we want to use the "#[trace_caller]" attribute [1], which
> > allows might_sleep() in Rust to be defined as a function, and can use
> > Location::caller() to get the caller file and line number information,
> > and `Location` type yet doesn't return a Nul-terminated string literal,
> > so we have to work this around.
> > 
> > > construct proper, robust strings, instead of spreading this disease to 
> > > the rest of the kernel, IMHO ...
> > > 
> > > Rust is supposed to be about increased security, right? How does extra, 
> > > nonsensical complexity for simple concepts such as strings achieve 
> > > that? If the Rust runtime wants to hook into debug facilities of the 
> > > Linux kernel then I have bad news: almost all strings used by kernel 
> > > debugging facilities are NUL-terminated.
> > 
> > This is more of a special case because `Location` is used (i.e. file
> > name is the string literal). For things like user-defined string, we use
> > the macro c_str!(), which generates NUL-terminated strings. For example,
> > lockdep class names.
> > 
> > > 
> > > So I really don't like this patch. Is there no other way to do this?
> > > 
> > 
> 
> Trying to see if we can make some forward-progress on this one,
> considering:
> 
> 1. #[track_caller] is really a desired feature to be used for Rust's
>    might_sleep(), Alice's reply [3] also explains a bit more on the
>    "why" part.
> 
> 2. To achieve #1, we will need to handle the file name returned by
>    Rust's `Location` struct, especially Location::file() will return a
>    string literal without a tailing NUL.
> 
> 3. Other than the current approach proposed by this patch, if the
>    existing might_sleep() functionality does not couple (task) state
>    inquiries with debug printing, we can maybe avoid printing the
>    non-NUL-terminated string in C's __might_sleep*() function by
>    printing Location::file() in Rust code:
> 
>     #[track_caller]
>     fn might_sleep() {
>         let loc = Location::caller();
> 
> 	if (__might_sleep_is_violated()) {
> 	    pr_err!("BUG: sleeping function called from invalid context at {loc}\n");
> 	    
> 	    ...
> 	}
>     }
> 
>     but this essentially would add more changes into C code compared to
>     the current patch.
> 
> 4. This is only a special case where we need the "debug information"
>    provided by Rust, so this won't happen a lot; and printing a
>    non-NUL-terminated string is already supported by printk already, so
>    we reuse what kernel already has here.
> 
> Given the above, I think the current patch is the best solution.
> 

Ingo,

Alice made some progress on providing the NUL-terminated string for `Location`
[4] [5], which means in the future, we can avoid the __might_sleep_precision()
workaround here, and yet remain the benefit of `#[track_caller]` (Thanks
Alice!). This also means we'd better keep the workaround right now, because that
keeps the same interface if we have NUL-terminated string from
`Location::file()`. And we can revert this workaround easily when the feature is
available in Rust. So I think we should take this.

Moving forwards, let me know if you need me to resend the pull request (there
are also a very trivial improvements in it as well), and I could rename
__might_sleep_precision() to something else (like __might_sleep_nonnulfilename()
or anything) in the resend. Thoughts?

[4]: https://github.com/rust-lang/libs-team/issues/466#issuecomment-2914476468
[5]: https://github.com/rust-lang/rust/issues/141727

Regards,
Boqun

> [3]: https://lore.kernel.org/lkml/aB3I62o8hWSULGBm@google.com/
> 
> Regards,
> Boqun
> 
> > There's a `c_str` [2] macro which could generates a NUL-terminated
> > string, but using that will requires might_sleep() defined as a macro as
> > well. Given that might_sleep() is the user interface that most users
> > will use, and how it handles string literal for file names is an
> > implementation detail, so I figured it's better we resolve in the
> > current way.
> > 
> > [1]: https://rustc-dev-guide.rust-lang.org/backend/implicit-caller-location.html
> > [2]: https://rust.docs.kernel.org/kernel/macro.c_str.html
> > 
> > Regards,
> > Boqun
> > 
> > > Thanks,
> > > 
> > > 	Ingo