kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 110 insertions, 0 deletions

diff --git a/drivers/md/dm-vdo/funnel-queue.h b/drivers/md/dm-vdo/funnel-queue.h
new file mode 100644
index 000000000..bde0f1def
--- /dev/null
+++ b/drivers/md/dm-vdo/funnel-queue.h
@@ -0,0 +1,110 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright 2023 Red Hat
+ */
+
+#ifndef VDO_FUNNEL_QUEUE_H
+#define VDO_FUNNEL_QUEUE_H
+
+#include <linux/atomic.h>
+#include <linux/cache.h>
+
+/*
+ * A funnel queue is a simple (almost) lock-free queue that accepts entries from multiple threads
+ * (multi-producer) and delivers them to a single thread (single-consumer). "Funnel" is an attempt
+ * to evoke the image of requests from more than one producer being "funneled down" to a single
+ * consumer.
+ *
+ * This is an unsynchronized but thread-safe data structure when used as intended. There is no
+ * mechanism to ensure that only one thread is consuming from the queue. If more than one thread
+ * attempts to consume from the queue, the resulting behavior is undefined. Clients must not
+ * directly access or manipulate the internals of the queue, which are only exposed for the purpose
+ * of allowing the very simple enqueue operation to be inlined.
+ *
+ * The implementation requires that a funnel_queue_entry structure (a link pointer) is embedded in
+ * the queue entries, and pointers to those structures are used exclusively by the queue. No macros
+ * are defined to template the queue, so the offset of the funnel_queue_entry in the records placed
+ * in the queue must all be the same so the client can derive their structure pointer from the
+ * entry pointer returned by vdo_funnel_queue_poll().
+ *
+ * Callers are wholly responsible for allocating and freeing the entries. Entries may be freed as
+ * soon as they are returned since this queue is not susceptible to the "ABA problem" present in
+ * many lock-free data structures. The queue is dynamically allocated to ensure cache-line
+ * alignment, but no other dynamic allocation is used.
+ *
+ * The algorithm is not actually 100% lock-free. There is a single point in vdo_funnel_queue_put()
+ * at which a preempted producer will prevent the consumers from seeing items added to the queue by
+ * later producers, and only if the queue is short enough or the consumer fast enough for it to
+ * reach what was the end of the queue at the time of the preemption.
+ *
+ * The consumer function, vdo_funnel_queue_poll(), will return NULL when the queue is empty. To
+ * wait for data to consume, spin (if safe) or combine the queue with a struct event_count to
+ * signal the presence of new entries.
+ */
+
+/* This queue link structure must be embedded in client entries. */
+struct funnel_queue_entry {
+	/* The next (newer) entry in the queue. */
+	struct funnel_queue_entry *next;
+};
+
+/*
+ * The dynamically allocated queue structure, which is allocated on a cache line boundary so the
+ * producer and consumer fields in the structure will land on separate cache lines. This should be
+ * consider opaque but it is exposed here so vdo_funnel_queue_put() can be inlined.
+ */
+struct __aligned(L1_CACHE_BYTES) funnel_queue {
+	/*
+	 * The producers' end of the queue, an atomically exchanged pointer that will never be
+	 * NULL.
+	 */
+	struct funnel_queue_entry *newest;
+
+	/* The consumer's end of the queue, which is owned by the consumer and never NULL. */
+	struct funnel_queue_entry *oldest __aligned(L1_CACHE_BYTES);
+
+	/* A dummy entry used to provide the non-NULL invariants above. */
+	struct funnel_queue_entry stub;
+};
+
+int __must_check vdo_make_funnel_queue(struct funnel_queue **queue_ptr);
+
+void vdo_free_funnel_queue(struct funnel_queue *queue);
+
+/*
+ * Put an entry on the end of the queue.
+ *
+ * The entry pointer must be to the struct funnel_queue_entry embedded in the caller's data
+ * structure. The caller must be able to derive the address of the start of their data structure
+ * from the pointer that passed in here, so every entry in the queue must have the struct
+ * funnel_queue_entry at the same offset within the client's structure.
+ */
+static inline void vdo_funnel_queue_put(struct funnel_queue *queue,
+					struct funnel_queue_entry *entry)
+{
+	struct funnel_queue_entry *previous;
+
+	/*
+	 * Barrier requirements: All stores relating to the entry ("next" pointer, containing data
+	 * structure fields) must happen before the previous->next store making it visible to the
+	 * consumer. Also, the entry's "next" field initialization to NULL must happen before any
+	 * other producer threads can see the entry (the xchg) and try to update the "next" field.
+	 *
+	 * xchg implements a full barrier.
+	 */
+	WRITE_ONCE(entry->next, NULL);
+	previous = xchg(&queue->newest, entry);
+	/*
+	 * Preemptions between these two statements hide the rest of the queue from the consumer,
+	 * preventing consumption until the following assignment runs.
+	 */
+	WRITE_ONCE(previous->next, entry);
+}
+
+struct funnel_queue_entry *__must_check vdo_funnel_queue_poll(struct funnel_queue *queue);
+
+bool __must_check vdo_is_funnel_queue_empty(struct funnel_queue *queue);
+
+bool __must_check vdo_is_funnel_queue_idle(struct funnel_queue *queue);
+
+#endif /* VDO_FUNNEL_QUEUE_H */