[PATCH v3 5/7] rust: percpu: Support non-zeroable types for DynamicPerCpu

[Mitchell Levy <levymitchell0@gmail.com>]

Add functionality to `PerCpuPtr` to compute pointers to per-CPU variable
slots on other CPUs. Use this facility to initialize per-CPU variables
on all possible CPUs when a dynamic per-CPU variable is created with a
non-zeroable type. Since `RefCell` and other `Cell`-like types fall into
this category, `impl CheckedPerCpu` on `DynamicPerCpu` for these
`InteriorMutable` types since they can now be used. Add examples of
these usages to `samples/rust/rust_percpu.rs`.

Signed-off-by: Mitchell Levy <levymitchell0@gmail.com>
---
 rust/helpers/percpu.c         |  5 +++
 rust/kernel/percpu.rs         | 15 +++++++
 rust/kernel/percpu/dynamic.rs | 40 +++++++++++++++++
 samples/rust/rust_percpu.rs   | 99 ++++++++++++++++++++++++++++++++++++++++---
 4 files changed, 152 insertions(+), 7 deletions(-)

diff --git a/rust/helpers/percpu.c b/rust/helpers/percpu.c
index 8cc01d094752..8d83b6b86106 100644
--- a/rust/helpers/percpu.c
+++ b/rust/helpers/percpu.c
@@ -8,6 +8,11 @@ void __percpu *rust_helper_alloc_percpu(size_t sz, size_t align)
 	return __alloc_percpu(sz, align);
 }
 
+void *rust_helper_per_cpu_ptr(void __percpu *ptr, unsigned int cpu)
+{
+	return per_cpu_ptr(ptr, cpu);
+}
+
 void rust_helper_on_each_cpu(smp_call_func_t func, void *info, int wait)
 {
 	on_each_cpu(func, info, wait);
diff --git a/rust/kernel/percpu.rs b/rust/kernel/percpu.rs
index 35afcdba3ccd..c68c7520b67f 100644
--- a/rust/kernel/percpu.rs
+++ b/rust/kernel/percpu.rs
@@ -14,6 +14,7 @@
 use bindings::{alloc_percpu, free_percpu};
 
 use crate::alloc::Flags;
+use crate::cpu::CpuId;
 use crate::percpu::cpu_guard::CpuGuard;
 use crate::prelude::*;
 use crate::sync::Arc;
@@ -115,6 +116,20 @@ pub fn get_ptr(&self) -> *mut MaybeUninit<T> {
         // the invariant that self.0 is a valid offset into the per-CPU area.
         (this_cpu_area).wrapping_add(self.0 as usize).cast()
     }
+
+    /// Get a `*mut MaybeUninit<T>` to the per-CPU variable on the CPU represented by `cpu`. Note
+    /// that without some kind of synchronization, use of the returned pointer may cause a data
+    /// race. It is the caller's responsibility to use the returned pointer in a reasonable way.
+    ///
+    /// # Safety
+    /// - The returned pointer is valid only if `self` is (that is, it points to a live allocation
+    ///   correctly sized and aligned to hold a `T`)
+    /// - The returned pointer is valid only if the bit corresponding to `cpu` is set in
+    ///   `Cpumask::possible()`.
+    pub unsafe fn get_remote_ptr(&self, cpu: CpuId) -> *mut MaybeUninit<T> {
+        // SAFETY: The requirements of this function ensure this call is safe.
+        unsafe { bindings::per_cpu_ptr(self.0.cast(), cpu.as_u32()) }.cast()
+    }
 }
 
 // SAFETY: Sending a `PerCpuPtr<T>` to another thread is safe because as soon as it's sent, the
diff --git a/rust/kernel/percpu/dynamic.rs b/rust/kernel/percpu/dynamic.rs
index ce95e420f943..64f04cef3705 100644
--- a/rust/kernel/percpu/dynamic.rs
+++ b/rust/kernel/percpu/dynamic.rs
@@ -3,6 +3,8 @@
 
 use super::*;
 
+use crate::cpumask::Cpumask;
+
 /// Represents a dynamic allocation of a per-CPU variable via alloc_percpu. Calls free_percpu when
 /// dropped.
 pub struct PerCpuAllocation<T>(PerCpuPtr<T>);
@@ -74,6 +76,36 @@ pub fn new_zero(flags: Flags) -> Option<Self> {
     }
 }
 
+impl<T: Clone> DynamicPerCpu<T> {
+    /// Allocates a new per-CPU variable
+    ///
+    /// # Arguments
+    /// * `val` - The initial value of the per-CPU variable on all CPUs.
+    /// * `flags` - Flags used to allocate an `Arc` that keeps track of the underlying
+    ///   `PerCpuAllocation`.
+    pub fn new_with(val: T, flags: Flags) -> Option<Self> {
+        let alloc: PerCpuAllocation<T> = PerCpuAllocation::new_uninit()?;
+        let ptr = alloc.0;
+
+        for cpu in Cpumask::possible().iter() {
+            // SAFETY: `ptr` is a valid allocation, and `cpu` appears in `Cpumask::possible()`
+            let remote_ptr = unsafe { ptr.get_remote_ptr(cpu) };
+            // SAFETY: Each CPU's slot corresponding to `ptr` is currently uninitialized, and no
+            // one else has a reference to it. Therefore, we can freely write to it without
+            // worrying about the need to drop what was there or whether we're racing with someone
+            // else. `PerCpuPtr::get_remote_ptr` guarantees that the pointer is valid since we
+            // derived it from a valid allocation and `cpu`.
+            unsafe {
+                (*remote_ptr).write(val.clone());
+            }
+        }
+
+        let arc = Arc::new(alloc, flags).ok()?;
+
+        Some(Self { alloc: arc })
+    }
+}
+
 impl<T> PerCpu<T> for DynamicPerCpu<T> {
     unsafe fn get_mut(&mut self, guard: CpuGuard) -> PerCpuToken<'_, T> {
         // SAFETY: The requirements of `PerCpu::get_mut` and this type's invariant ensure that the
@@ -81,3 +113,11 @@ unsafe fn get_mut(&mut self, guard: CpuGuard) -> PerCpuToken<'_, T> {
         unsafe { PerCpuToken::new(guard, &self.alloc.0) }
     }
 }
+
+impl<T: InteriorMutable> CheckedPerCpu<T> for DynamicPerCpu<T> {
+    fn get(&mut self, guard: CpuGuard) -> CheckedPerCpuToken<'_, T> {
+        // SAFETY: By the invariant of `DynamicPerCpu`, the memory location in each CPU's
+        // per-CPU area corresponding to this variable has been initialized.
+        unsafe { CheckedPerCpuToken::new(guard, &self.alloc.0) }
+    }
+}
diff --git a/samples/rust/rust_percpu.rs b/samples/rust/rust_percpu.rs
index 98ca1c781b6b..06b322019134 100644
--- a/samples/rust/rust_percpu.rs
+++ b/samples/rust/rust_percpu.rs
@@ -130,13 +130,72 @@ fn init(_module: &'static ThisModule) -> Result<Self, Error> {
 
         // SAFETY: No prerequisites for on_each_cpu.
         unsafe {
-            on_each_cpu(Some(inc_percpu), (&raw mut test).cast(), 0);
-            on_each_cpu(Some(inc_percpu), (&raw mut test).cast(), 0);
-            on_each_cpu(Some(inc_percpu), (&raw mut test).cast(), 0);
-            on_each_cpu(Some(inc_percpu), (&raw mut test).cast(), 1);
-            on_each_cpu(Some(check_percpu), (&raw mut test).cast(), 1);
+            on_each_cpu(Some(inc_percpu_u64), (&raw mut test).cast(), 0);
+            on_each_cpu(Some(inc_percpu_u64), (&raw mut test).cast(), 0);
+            on_each_cpu(Some(inc_percpu_u64), (&raw mut test).cast(), 0);
+            on_each_cpu(Some(inc_percpu_u64), (&raw mut test).cast(), 1);
+            on_each_cpu(Some(check_percpu_u64), (&raw mut test).cast(), 1);
         }
 
+        let mut checked: DynamicPerCpu<RefCell<u64>> =
+            DynamicPerCpu::new_with(RefCell::new(100), GFP_KERNEL).unwrap();
+
+        // SAFETY: No prerequisites for on_each_cpu.
+        unsafe {
+            on_each_cpu(Some(inc_percpu_refcell_u64), (&raw mut checked).cast(), 0);
+            on_each_cpu(Some(inc_percpu_refcell_u64), (&raw mut checked).cast(), 0);
+            on_each_cpu(Some(inc_percpu_refcell_u64), (&raw mut checked).cast(), 0);
+            on_each_cpu(Some(inc_percpu_refcell_u64), (&raw mut checked).cast(), 1);
+            on_each_cpu(Some(check_percpu_refcell_u64), (&raw mut checked).cast(), 1);
+        }
+
+        checked.get(CpuGuard::new()).with(|val: &RefCell<u64>| {
+            assert!(*val.borrow() == 104);
+
+            let mut checked_native = 0;
+            *val.borrow_mut() = 0;
+
+            checked_native += 1;
+            *val.borrow_mut() += 1;
+            pr_info!(
+                "Checked native: {}, *checked: {}\n",
+                checked_native,
+                val.borrow()
+            );
+            assert!(checked_native == *val.borrow() && checked_native == 1);
+
+            checked_native = checked_native.wrapping_add((-1i64) as u64);
+            val.replace_with(|old: &mut u64| old.wrapping_add((-1i64) as u64));
+            pr_info!(
+                "Checked native: {}, *checked: {}\n",
+                checked_native,
+                val.borrow()
+            );
+            assert!(checked_native == *val.borrow() && checked_native == 0);
+
+            checked_native = checked_native.wrapping_add((-1i64) as u64);
+            val.replace_with(|old: &mut u64| old.wrapping_add((-1i64) as u64));
+            pr_info!(
+                "Checked native: {}, *checked: {}\n",
+                checked_native,
+                val.borrow()
+            );
+            assert!(checked_native == *val.borrow() && checked_native == (-1i64) as u64);
+
+            checked_native = 0;
+            *val.borrow_mut() = 0;
+
+            checked_native = checked_native.wrapping_sub(1);
+            val.replace_with(|old: &mut u64| old.wrapping_sub(1));
+            pr_info!(
+                "Checked native: {}, *checked: {}\n",
+                checked_native,
+                val.borrow()
+            );
+            assert!(checked_native == *val.borrow() && checked_native == (-1i64) as u64);
+            assert!(checked_native == *val.borrow() && checked_native == u64::MAX);
+        });
+
         pr_info!("rust dynamic percpu test done\n");
 
         // Return Err to unload the module
@@ -144,7 +203,7 @@ fn init(_module: &'static ThisModule) -> Result<Self, Error> {
     }
 }
 
-extern "C" fn inc_percpu(info: *mut c_void) {
+extern "C" fn inc_percpu_u64(info: *mut c_void) {
     // SAFETY: We know that info is a void *const DynamicPerCpu<u64> and DynamicPerCpu<u64> is Send.
     let mut pcpu = unsafe { (*(info as *const DynamicPerCpu<u64>)).clone() };
     pr_info!("Incrementing on {}\n", CpuId::current().as_u32());
@@ -153,7 +212,7 @@ extern "C" fn inc_percpu(info: *mut c_void) {
     unsafe { pcpu.get_mut(CpuGuard::new()) }.with(|val: &mut u64| *val += 1);
 }
 
-extern "C" fn check_percpu(info: *mut c_void) {
+extern "C" fn check_percpu_u64(info: *mut c_void) {
     // SAFETY: We know that info is a void *const DynamicPerCpu<u64> and DynamicPerCpu<u64> is Send.
     let mut pcpu = unsafe { (*(info as *const DynamicPerCpu<u64>)).clone() };
     pr_info!("Asserting on {}\n", CpuId::current().as_u32());
@@ -161,3 +220,29 @@ extern "C" fn check_percpu(info: *mut c_void) {
     // SAFETY: We don't have multiple clones of pcpu in scope
     unsafe { pcpu.get_mut(CpuGuard::new()) }.with(|val: &mut u64| assert!(*val == 4));
 }
+
+extern "C" fn inc_percpu_refcell_u64(info: *mut c_void) {
+    // SAFETY: We know that info is a void *const DynamicPerCpu<RefCell<u64>> and
+    // DynamicPerCpu<RefCell<u64>> is Send.
+    let mut pcpu = unsafe { (*(info as *const DynamicPerCpu<RefCell<u64>>)).clone() };
+    // SAFETY: smp_processor_id has no preconditions
+    pr_info!("Incrementing on {}\n", CpuId::current().as_u32());
+
+    pcpu.get(CpuGuard::new()).with(|val: &RefCell<u64>| {
+        let mut val = val.borrow_mut();
+        *val += 1;
+    });
+}
+
+extern "C" fn check_percpu_refcell_u64(info: *mut c_void) {
+    // SAFETY: We know that info is a void *const DynamicPerCpu<RefCell<u64>> and
+    // DynamicPerCpu<RefCell<u64>> is Send.
+    let mut pcpu = unsafe { (*(info as *const DynamicPerCpu<RefCell<u64>>)).clone() };
+    // SAFETY: smp_processor_id has no preconditions
+    pr_info!("Asserting on {}\n", CpuId::current().as_u32());
+
+    pcpu.get(CpuGuard::new()).with(|val: &RefCell<u64>| {
+        let val = val.borrow();
+        assert!(*val == 104);
+    });
+}

-- 
2.34.1