runtime: revert "avoid lock acquisition after uring init" (#7849)

This reverts commit ff9681b3c6.

tokio/Cargo.toml

@@ -85,7 +85,7 @@ sync = []
 test-util = ["rt", "sync", "time"]
 time = []
 # Unstable feature. Requires `--cfg tokio_unstable` to enable.
-io-uring = ["dep:io-uring", "libc", "mio/os-poll", "mio/os-ext", "dep:slab", "once_cell"]
+io-uring = ["dep:io-uring", "libc", "mio/os-poll", "mio/os-ext", "dep:slab"]
 # Unstable feature. Requires `--cfg tokio_unstable` to enable.
 taskdump = ["dep:backtrace"]
 
@@ -115,7 +115,6 @@ libc = { version = "0.2.168", optional = true }
 mio = { version = "1.0.1", default-features = false, features = ["os-poll", "os-ext"], optional = true }
 slab = { version = "0.4.9", optional = true }
 backtrace = { version = "0.3.58", optional = true }
-once_cell = { version = "1.21.3", optional = true }
 
 [target.'cfg(unix)'.dependencies]
 libc = { version = "0.2.168", optional = true }

tokio/src/runtime/io/driver.rs

@@ -5,8 +5,7 @@ cfg_signal_internal_and_unix! {
 cfg_io_uring! {
     mod uring;
     use uring::UringContext;
-    // TODO: Replace with std after https://github.com/rust-lang/rust/issues/109737
-    use once_cell::sync::OnceCell;
+    use crate::loom::sync::atomic::AtomicUsize;
 }
 
 use crate::io::interest::Interest;
@@ -68,7 +67,7 @@ pub(crate) struct Handle {
         feature = "fs",
         target_os = "linux",
     ))]
-    pub(crate) uring_probe: OnceCell<Option<io_uring::Probe>>,
+    pub(crate) uring_state: AtomicUsize,
 }
 
 #[derive(Debug)]
@@ -151,7 +150,7 @@ impl Driver {
                 feature = "fs",
                 target_os = "linux",
             ))]
-            uring_probe: OnceCell::new(),
+            uring_state: AtomicUsize::new(0),
         };
 
         Ok((driver, handle))

tokio/src/runtime/io/driver/uring.rs

@@ -2,6 +2,7 @@ use io_uring::{squeue::Entry, IoUring, Probe};
 use mio::unix::SourceFd;
 use slab::Slab;
 
+use crate::loom::sync::atomic::Ordering;
 use crate::runtime::driver::op::{Cancellable, Lifecycle};
 use crate::{io::Interest, loom::sync::Mutex};
 
@@ -12,8 +13,32 @@ use std::{io, mem, task::Waker};
 
 const DEFAULT_RING_SIZE: u32 = 256;
 
+#[repr(usize)]
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+enum State {
+    Uninitialized = 0,
+    Initialized = 1,
+    Unsupported = 2,
+}
+
+impl State {
+    fn as_usize(&self) -> usize {
+        *self as usize
+    }
+
+    fn from_usize(value: usize) -> Self {
+        match value {
+            0 => State::Uninitialized,
+            1 => State::Initialized,
+            2 => State::Unsupported,
+            _ => unreachable!("invalid Uring state: {}", value),
+        }
+    }
+}
+
 pub(crate) struct UringContext {
     pub(crate) uring: Option<io_uring::IoUring>,
+    pub(crate) probe: io_uring::Probe,
     pub(crate) ops: slab::Slab<Lifecycle>,
 }
 
@@ -22,6 +47,7 @@ impl UringContext {
         Self {
             ops: Slab::new(),
             uring: None,
+            probe: Probe::new(),
         }
     }
 
@@ -33,12 +59,16 @@ impl UringContext {
         self.uring.as_mut().expect("io_uring not initialized")
     }
 
+    pub(crate) fn is_opcode_supported(&self, opcode: u8) -> bool {
+        self.probe.is_supported(opcode)
+    }
+
     /// Perform `io_uring_setup` system call, and Returns true if this
     /// actually initialized the io_uring.
     ///
     /// If the machine doesn't support io_uring, then this will return an
     /// `ENOSYS` error.
-    pub(crate) fn try_init(&mut self, probe: &mut Probe) -> io::Result<bool> {
+    pub(crate) fn try_init(&mut self) -> io::Result<bool> {
         if self.uring.is_some() {
             // Already initialized.
             return Ok(false);
@@ -46,7 +76,7 @@ impl UringContext {
 
         let uring = IoUring::new(DEFAULT_RING_SIZE)?;
 
-        match uring.submitter().register_probe(probe) {
+        match uring.submitter().register_probe(&mut self.probe) {
             Ok(_) => {}
             Err(e) if e.raw_os_error() == Some(libc::EINVAL) => {
                 // The kernel does not support IORING_REGISTER_PROBE.
@@ -164,6 +194,10 @@ impl Handle {
         &self.uring_context
     }
 
+    fn set_uring_state(&self, state: State) {
+        self.uring_state.store(state.as_usize(), Ordering::Release);
+    }
+
     /// Check if the io_uring context is initialized. If not, it will try to initialize it.
     /// Then, check if the provided opcode is supported.
     ///
@@ -173,34 +207,41 @@ impl Handle {
     /// this returns `Ok(false)`.
     /// An error is returned if an io_uring syscall returns an unexpected error value.
     pub(crate) fn check_and_init(&self, opcode: u8) -> io::Result<bool> {
-        let probe = self.uring_probe.get_or_try_init(|| {
-            let mut probe = Probe::new();
-            match self.try_init(&mut probe) {
-                Ok(()) => Ok(Some(probe)),
-                // If the system doesn't support io_uring, we set the probe to `None`.
-                Err(e) if e.raw_os_error() == Some(libc::ENOSYS) => Ok(None),
+        match State::from_usize(self.uring_state.load(Ordering::Acquire)) {
+            State::Uninitialized => match self.try_init_and_check_opcode(opcode) {
+                Ok(opcode_supported) => {
+                    self.set_uring_state(State::Initialized);
+                    Ok(opcode_supported)
+                }
+                // If the system doesn't support io_uring, we set the state to Unsupported.
+                Err(e) if e.raw_os_error() == Some(libc::ENOSYS) => {
+                    self.set_uring_state(State::Unsupported);
+                    Ok(false)
+                }
                 // If we get EPERM, io-uring syscalls may be blocked (for example, by seccomp).
                 // In this case, we try to fall back to spawn_blocking for this and future operations.
                 // See also: https://github.com/tokio-rs/tokio/issues/7691
-                Err(e) if e.raw_os_error() == Some(libc::EPERM) => Ok(None),
+                Err(e) if e.raw_os_error() == Some(libc::EPERM) => {
+                    self.set_uring_state(State::Unsupported);
+                    Ok(false)
+                }
                 // For other system errors, we just return it.
                 Err(e) => Err(e),
-            }
-        })?;
-
-        Ok(probe
-            .as_ref()
-            .is_some_and(|probe| probe.is_supported(opcode)))
+            },
+            State::Unsupported => Ok(false),
+            State::Initialized => Ok(self.get_uring().lock().is_opcode_supported(opcode)),
+        }
     }
 
     /// Initialize the io_uring context if it hasn't been initialized yet.
-    fn try_init(&self, probe: &mut Probe) -> io::Result<()> {
+    /// Then, check whether the given opcode is supported.
+    fn try_init_and_check_opcode(&self, opcode: u8) -> io::Result<bool> {
         let mut guard = self.get_uring().lock();
-        if guard.try_init(probe)? {
+        if guard.try_init()? {
             self.add_uring_source(guard.ring().as_raw_fd())?;
         }
 
-        Ok(())
+        Ok(guard.is_opcode_supported(opcode))
     }
 
     /// Register an operation with the io_uring.