From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793632091; b=hE3xfAOnHQPTodZ2O/gfbRy5vqPtUNFbeMLFT6H1MxM0ubrtc/WCCc3IkCk7cRqH6V/2 aZTfHedUOWM0PpeSF2nXgPS9BbO3z4+jNTfysHIkqiqDR7hySp/jBEOyUcrFHScLWjmLz NcKsoQYMDoFTJtAtvJJT/vHmwdmgfqSI8zIEshwEgFYH/o8pDzdrWXGVB1jHMnSc84iiJ sgG447w8c549YHHmyJHlsEjse748ztJnggBygSGKswFj02Rxb17l688PfQvfbhSw6bLk6 jqy15cKGWq9YByC/k7XfAicFpDnJlGdQ26O31myC5lciA0nutqEP5aCU3Ho3un/lcVQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793632091; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=nrM/uUf4HdTSRJMfQyJdPJ//6muAeY8Jt8qA4ElcX4M=; b=REuQnvKqC4J8+Ubfl2FcQ//qXiry7ozu3CiY7BzWgKOFy5ZU6HPK/p2Gq+GcPOlwg1E9 ov9xSrzGD4H5NAV3/ew1KvQ7YvH7CekoVhPefXg3CDpeNt1XwYTA/cqKT092mVN4Fi/vq SkmuQG6wq+1aagFjywZ8ej3aG1aqQAUOnb1bn31Bw6eD80fz56ecM7wPx6jSaCU40ktUa aH3FsNcsayEBBz/b5MSsba7oDKLKI3Y9xm6G/VxXjcL9//m1OscH3yNXHRoF0J8d/nN54 5DVlrTz3phHPwHQMrBp+MuJ5fj1HbTPLA5vsLHrx/vK04nEHVvNtMvTsX7QdC/Y2BDw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=oBRRGcw8; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8B78C60A44ED; Mon, 2 Nov 2026 15:08:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=oBRRGcw8 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3B30B1F00A18; Mon, 2 Nov 2026 15:08:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793632091; bh=nrM/uUf4HdTSRJMfQyJdPJ//6muAeY8Jt8qA4ElcX4M=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=oBRRGcw8ROwqhrET8KcqKuAjekSVKjxNq9Q0Q751SFL+LSUrJpxsC913yy5VOwUXi bdKiVpK9mZxauiFIwdUcDRbZIFP9OvJ9dyjD+Tglv5N2t5iGLI29Ay1L/Bq3DHVs5o XAvycYvaGF8N5poqs68eHyLCLfiztRIU+ndpeFQT8WjtDdDrNMieh9PNCRgnjXrwVI F++7JSqENNOsj+IuiSaWxFDvRby3VhBmPllhcuRHbt7Vun0AJi+JeLtjQyObSl7Iz4 9OTpDV3EeUJErTLY6313UhvMhpXmtLKHpq04j3nyRdu4kbeeEo1viU0hwy9Akmn/VK iWKHKet3HOhkA== From: Philipp Stanner To: Danilo Krummrich , Alice Ryhl , Sumit Semwal , =?UTF-8?q?Christian=20K=C3=B6nig?= , Philipp Stanner , Miguel Ojeda , Boqun Feng , Gary Guo , =?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?= , Benno Lossin , Andreas Hindborg , Trevor Gross , Daniel Almeida , Tamir Duberstein , Alexandre Courbot , =?UTF-8?q?Onur=20=C3=96zkan?= Cc: linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH 2/2] rust: DmaFence: Replace call_rcu() with synchronize_rcu() Date: Mon, 2 Nov 2026 16:06:18 +0100 Message-ID: <20261102150617.527772-4-phasta@kernel.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261102150617.527772-2-phasta@kernel.org> References: <20261102150617.527772-2-phasta@kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The C dma_fence contract demands that a fence's data disappears no earlier than 1 RCU grace period after the DriverFence was signaled. So far, we achieved this with dropping the DriverFence's data in a deferred manner with a manual, raw binding call to call_rcu(). However, this call_rcu() does not solve one problem: It is conceivable that some driver data must not drop in atomic context; but call_rcu()'s payload *can* be executed in atomic context. Moreover, the current developments in drm::JobQueue, where a DriverFence is always coupled 1:1 with a Job, allows for dropping job and fence through a work item. The undesirable blocking behavior of synchronize_rcu() would, thus, not be annoying any thread anymore. The direct users of DmaFence could achieve the same by dropping their DriverFences through work items. The only conceivable other solution, queue_rcu_work(), would, for many users, schedule a work_item from another work_item, which seems undesirable. Additionally, using the existing synchronize_rcu() abstraction has the advantage of us getting rid of a raw bindings call. Replace DriverFence::drop()'s call_rcu() with synchronize_rcu(). Signed-off-by: Philipp Stanner --- rust/kernel/dma_buf/dma_fence.rs | 89 ++++++++++---------------------- 1 file changed, 26 insertions(+), 63 deletions(-) diff --git a/rust/kernel/dma_buf/dma_fence.rs b/rust/kernel/dma_buf/dma_fence.rs index 3022bdb5f7c3..ad9cebf8428c 100644 --- a/rust/kernel/dma_buf/dma_fence.rs +++ b/rust/kernel/dma_buf/dma_fence.rs @@ -42,7 +42,10 @@ Atomic, Relaxed, // }, - rcu::rcu_barrier, // + rcu::{ + rcu_barrier, + synchronize_rcu, // + }, // }, // }; @@ -65,7 +68,7 @@ pub struct FenceContext { /// The sequence number for the next fence created. seqno: Atomic, // The name parameters can be accessed by the dma_fence backend_ops. UAF - // errors are prevented by the `call_rcu()` in `drop_driver_fence_data()`. + // errors are prevented by the `synchronize_rcu()` in `drop_driver_fence_data()`. /// The name of the driver this FenceContext's fences belong to. driver_name: CString, /// The name of the timeline this FenceContext's fences belong to. @@ -142,7 +145,6 @@ pub fn new_fence_allocation( data: T::FenceDataType, ) -> Result> { let fence_data = DriverFenceData { - rcu_head: Default::default(), // `inner` remains uninitialized until a `DriverFence` takes over. inner: Fence { inner: Opaque::uninit(), @@ -600,8 +602,6 @@ struct DriverFenceData<'a, T: Send + Sync + FenceContextOps> { // necessary so that the C backend can free the allocation (coming from our // Rust code) with kfree_rcu(). inner: Fence, - /// Callback head for dropping this in a deferred manner through RCU. - rcu_head: bindings::callback_head, /// Reference to access the FenceContext. fctx: &'a FenceContext, /// The API user's data. It is essential that the data only performs @@ -644,6 +644,10 @@ struct DriverFenceData<'a, T: Send + Sync + FenceContextOps> { /// /// In the unlikely case of such violations, a panic is thrown. /// +/// For requirements imposed by the C backend, a [`DriverFence`] might perform +/// a call to [`rcu::synchronize_rcu`] when it drops. Thus, avoid dropping a +/// [`DriverFence`] in atomic context. +/// /// # Examples /// /// ``` @@ -940,15 +944,15 @@ fn drop(&mut self) { // dma_fence_is_signaled_locked() because the C backend wants to get rid // of the latter. - // SAFETY: `guard` is valid until the `call_rcu()` below. + // SAFETY: `guard` is valid. let signaled: bool = unsafe { bindings::dma_fence_test_signaled_flag(guard.as_raw()) }; if !signaled { pr_err!("DriverFence drops unsignaled. Danger of memory corruption!\n"); - // SAFETY: `guard` is valid until the `call_rcu()` below. The fence - // must not have been signaled yet, which we check directly above. + // SAFETY: `guard` is valid. The fence must not have been signaled + // yet, which we check directly above. unsafe { bindings::dma_fence_set_error(guard.as_raw(), ECANCELED.to_errno()) }; - // SAFETY: `guard` is valid until the `call_rcu()` below. The lock - // must be held, which we acquired above. + // SAFETY: `guard` is valid. The lock must be held, which we + // acquired above. unsafe { bindings::dma_fence_signal_locked(guard.as_raw()) }; // SAFETY: `self.data` is valid because `self` is valid. @@ -973,59 +977,18 @@ fn drop(&mut self) { return; } - // SAFETY: Valid because `self` is valid. - let rcu_head_ptr = unsafe { &raw mut (*self.data.as_ptr()).rcu_head }; + // Make sure none of the fence backend_ops can access data anymore. + // + // TODO: + // This would not be necessary if the C dma_fence backend were using a + // spinlock to properly synchronize its signaled state. Fix it in C and + // then remove synchronize_rcu(). + synchronize_rcu(); - // SAFETY: `call_rcu()` is always safe to be called. `rcu_head_ptr` was - // created validly above. The module must perform a `synchronize_rcu()` - // or `rcu_barrier()` call to guard against module unload. - unsafe { bindings::call_rcu(rcu_head_ptr, Some(drop_driver_fence_data::)) }; + // SAFETY: Valid because `self` is valid. + unsafe { drop_in_place(&raw mut self.data) }; + + // SAFETY: The `synchronize_rcu()` above ensures all accessors are gone. + unsafe { bindings::dma_fence_put(self.as_raw()) }; } } - -// TODO: -// The entire call_rcu() mechanism in the drop above and the code below would be -// unnecessary if C's dma_fence_signal() could be reworked in a way that after it -// ran, the caller knows that no fence_ops callbacks can be running anymore. -// In other words, if the dma_fence backend would use its spinlock for full -// synchronization. -// -// Then we could move the drop_in_place() and dma_fence_put() upwards into the -// drop() implementation and call it a day. - -/// Finally really drop this `DriverFence` -/// -/// # Safety -/// -/// `head` references the `rcu_head` field of an `DriverFenceData`. All -/// accessors to that `DriverFenceData` must be gone by now. This must be -/// ensured by signalling the associated `DriverFence` and then waiting -/// for a grace period until calling this function here. -unsafe extern "C" fn drop_driver_fence_data( - head: *mut bindings::callback_head, -) { - // SAFETY: Caller provides a pointer to the `rcu_head` field of a `DriverFenceData`. - let fence_data = unsafe { container_of!(head, DriverFenceData<'_, T>, rcu_head) }; - - // SAFETY: `fence_data` was created validly above. All the fence's data will - // only drop below, but the raw pointer to the raw C `dma_fence` remains - // valid because the reference count is only decremented at the end of the - // function. - let fence = unsafe { (*fence_data).inner.inner.get() }; - - // SAFETY: `fence_data` was created validly above. The user has already - // dropped the only conventional accessor to the user data, the `DriverFence`, - // one grace period ago. All accessors are gone now. - unsafe { drop_in_place(&raw mut (*fence_data).data) }; - - // The inner `Fence` explicitly does not get dropped because there may be - // many more users / consumers, each holding their own reference. - - // SAFETY: Once a `DriverFence` is initialized, the inner `fence` is valid - // and initialized. It is valid until the refcount drops to 0, which can - // earliest happen once we drop the `DriverFence`'s reference here. - unsafe { bindings::dma_fence_put(fence) }; - - // The actual memory the data associated with a `DriverFence` lives in - // gets freed by the C dma_fence backend once the fence's refcount reaches 0. -} -- 2.56.0