One often cannot allocate in the kernel with the desired flags, most notably in atomic context. Pre-allocating the memory is the preferred solution in such situations.
Add support for xa_reserve() in the Rust abstractions of xarray. Create a Reservation object similar to a lock-guard, that can be dropped once the reservation is no longer needed or once the index was stored to. Signed-off-by: Philipp Stanner <[email protected]> --- Please regard this more as an RFC. I need pre-allocating in XArray for DmaFence. How exactly we achieve this is open for discussion. P. --- rust/helpers/xarray.c | 10 ++++++++++ rust/kernel/xarray.rs | 45 ++++++++++++++++++++++++++++++++++++++++++- 2 files changed, 54 insertions(+), 1 deletion(-) diff --git a/rust/helpers/xarray.c b/rust/helpers/xarray.c index 08979b304341..1504ab242949 100644 --- a/rust/helpers/xarray.c +++ b/rust/helpers/xarray.c @@ -26,3 +26,13 @@ __rust_helper void rust_helper_xa_unlock(struct xarray *xa) { return xa_unlock(xa); } + +__rust_helper int rust_helper_xa_reserve(struct xarray *xa, unsigned long index, gfp_t flags) +{ + return xa_reserve(xa, index, flags); +} + +__rust_helper void rust_helper_xa_release(struct xarray *xa, unsigned long index) +{ + xa_release(xa, index); +} diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs index 987c9c0c2198..fa5d47935f0a 100644 --- a/rust/kernel/xarray.rs +++ b/rust/kernel/xarray.rs @@ -8,7 +8,10 @@ alloc, bindings, build_assert::build_assert, - error::{Error, Result}, + error::{ + Error, + Result, // + }, ffi::c_void, types::{ ForeignOwnable, @@ -147,6 +150,46 @@ pub fn lock(&self) -> Guard<'_, T> { _not_send: NotThreadSafe, } } + + /// Reserve memory in the XArray at the appropriate index. If this call + /// succeeds, later storing at the index will not require an allocation. + /// + /// Loading from reserved entries will return `None`. + pub fn reserve(&self, index: usize, gfp: alloc::Flags) -> Result<Reservation<'_, T>> { + // SAFETY: + // - `self.xa.xa` is always valid by the type invariant. + // - The caller holds the lock. + // + // INVARIANT: `new` came from `T::into_foreign`. + let ret = unsafe { bindings::xa_reserve(self.xa.get(), index, gfp.as_raw()) }; + match ret { + 0 => Ok(Reservation { xa: self, index }), + _ => Err(Error::from_errno(ret)), + } + } +} + +/// An index reservation object. +/// +/// As long as you keep this object alive, your reservation will stay valid. If +/// someone has stored to this index in the meantime, dropping your reservation +/// does nothing, which is why you can always drop this reservation object once +/// you performed your store operation. +/// +/// Refer to the main C xarray documentation for more details. +pub struct Reservation<'a, T: ForeignOwnable> { + xa: &'a XArray<T>, + index: usize, +} + +impl<T: ForeignOwnable> Drop for Reservation<'_, T> { + fn drop(&mut self) { + // SAFETY: + // - `self.xa` is always valid by the type invariant. + // - If `self.id` is not used or has been stored to by a racing party, + // `xa_release()` will do nothing. + unsafe { bindings::xa_release(self.xa.xa.get(), self.index) }; + } } /// A lock guard. -- 2.55.0
