Add Rust version of might_sleep()

Hi,

This is a v4 of Tomo's previous version:

v3: https://lore.kernel.org/rust-for-linux/[email protected]/
v2: https://lore.kernel.org/rust-for-linux/[email protected]/

Given Ingo's feedback [1], and the ongoing work at Rust side [2] (thanks Alice!), I think it's better that we just start to use Location::file_with_nul() if available, otherwise use a "this is not supported" as file names for might_sleep() debug output, this should be sufficient since might_sleep() only uses the file names for debug output, and it will also trigger a stack trace when error, so some callsite information can be gather from there too.

Having might_sleep() will unblock the support of read_poll_timeout() [3], and then wait_gfw_boot_completion() [4] in nova. So hopefully this will allow use moving forward.

I will a PR to tip including this updated version if no one objects. Thanks.

Changes since v3:

* Changed the name of Kconfig per Miguel's feedback.
* Added comments on the unstable feature file_with_nul per Miguel's feedback.
* Added an example for file_from_location() per Miguel's feedback.
* Changed the return type from kernel::str::CStr to core::ffi::CStr to align with Tamir's effort [5].
* Applied the documentation change for might_sleep() from Tomo.

[1]: https://lore.kernel.org/rust-for-linux/[email protected]/
[2]: https://github.com/rust-lang/rust/issues/141727
[3]: https://lore.kernel.org/lkml/[email protected]/
[4]: https://lore.kernel.org/rust-for-linux/[email protected]/
[5]: https://lore.kernel.org/rust-for-linux/[email protected]/

Regards,
Boqun

Boqun Feng (1):
rust: Introduce file_from_location()

FUJITA Tomonori (1):
rust: task: Add Rust version of might_sleep()

 init/Kconfig        |  3 +++
 rust/helpers/task.c |  6 ++++++
 rust/kernel/lib.rs  | 43 +++++++++++++++++++++++++++++++++++++++++++
 rust/kernel/task.rs | 24 ++++++++++++++++++++++++
 4 files changed, 76 insertions(+)

2.39.5 (Apple Git-154)

Most of kernel debugging facilities take a nul-terminated string for file names for a callsite (generated from __FILE__), however the Rust courterpart, Location, would return a Rust string (not nul-terminated) from method .file(). And such a string cannot be passed to C debugging function directly.

There is ongoing work to support a Location::file_with_nul() [1], which returns a nul-terminated string from a Location. Since it's still working in progress, and it will take some time before the feature finally gets stabilized and the kernel's minimal rustc version might also take a while to bump to a version that at least has that feature, introduce a file_from_location() function, which returns a warning string if Location::file_with_nul() is not available.

This should work in most cases because as for now the known usage of Location::file_with_nul() is only in debugging code (e.g. might_sleep()) and there might be other information reported by the debugging code that could help locate the problematic function, so missing the file name is fine at the moment.

---
 init/Kconfig       |  3 +++
 rust/kernel/lib.rs | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

index af4c2f085455..6f4ec5633ffa 100644
@@ -142,6 +142,9 @@ config RUSTC_HAS_SPAN_FILE
 config RUSTC_HAS_UNNECESSARY_TRANSMUTES
 	def_bool RUSTC_VERSION >= 108800
 
+config RUSTC_HAS_FILE_WITH_NUL
+	def_bool RUSTC_VERSION >= 108900
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))

2.39.5 (Apple Git-154)

"is available."

Fixed.

Make sense, applied.

Thanks! I pushed the updated version at:

https://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git/log/?h=rust-sched-v2

"-v2" because it's going to be a v2 of the pull request.

Regards,
Boqun

From: FUJITA Tomonori <[email protected]>

Add a helper function equivalent to the C's might_sleep(), which serves as a debugging aid and a potential scheduling point.

Note that this function can only be used in a nonatomic context.

This will be used by Rust version of read_poll_timeout().

[boqun: Use file_from_location() to get a C string instead of changing __might_sleep()]

---
 rust/helpers/task.c |  6 ++++++
 rust/kernel/task.rs | 24 ++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

index 31c33ea2dce6..2c85bbc2727e 100644
@@ -1,7 +1,13 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#include <linux/kernel.h>
 #include <linux/sched/task.h>
 
+void rust_helper_might_resched(void)
+{
+	might_resched();
+}
+
 struct task_struct *rust_helper_get_current(void)
 {
 	return current;

index 834368313088..7d0935bc325c 100644
@@ -400,3 +400,27 @@ fn eq(&self, other: &Kuid) -> bool {
 }
 
 impl Eq for Kuid {}
+
+/// Annotation for functions that can sleep.
+///
+/// Equivalent to the C side [`might_sleep()`], this function serves as
+/// a debugging aid and a potential scheduling point.
+///
+/// This function can only be used in a nonatomic context.
+///
+/// [`might_sleep()`]: https://docs.kernel.org/driver-api/basics.html#c.might_sleep
+#[track_caller]
+#[inline]
+pub fn might_sleep() {
+    #[cfg(CONFIG_DEBUG_ATOMIC_SLEEP)]
+    {
+        let loc = core::panic::Location::caller();
+        let file = kernel::file_from_location(loc);
+
+        // SAFETY: `file.as_ptr()` is valid for reading and guaranteed to be nul-terminated.
+        unsafe { crate::bindings::__might_sleep(file.as_ptr().cast(), loc.line() as i32) }
+    }
+
+    // SAFETY: Always safe to call.
+    unsafe { crate::bindings::might_resched() }
+}

2.39.5 (Apple Git-154)