uprobes: travers uprobe's consumer list locklessly under SRCU protection

uprobe->register_rwsem is one of a few big bottlenecks to scalability of
uprobes, so we need to get rid of it to improve uprobe performance and
multi-CPU scalability.

First, we turn uprobe's consumer list to a typical doubly-linked list
and utilize existing RCU-aware helpers for traversing such lists, as
well as adding and removing elements from it.

For entry uprobes we already have SRCU protection active since before
uprobe lookup. For uretprobe we keep refcount, guaranteeing that uprobe
won't go away from under us, but we add SRCU protection around consumer
list traversal.

Lastly, to keep handler_chain()'s UPROBE_HANDLER_REMOVE handling simple,
we remember whether any removal was requested during handler calls, but
then we double-check the decision under a proper register_rwsem using
consumers' filter callbacks. Handler removal is very rare, so this extra
lock won't hurt performance, overall, but we also avoid the need for any
extra protection (e.g., seqcount locks).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20240903174603.3554182-5-andrii@kernel.org

include/linux/uprobes.h

@@ -29,13 +29,21 @@ struct page;
 #define MAX_URETPROBE_DEPTH		64
 
 struct uprobe_consumer {
+	/*
+	 * handler() can return UPROBE_HANDLER_REMOVE to signal the need to
+	 * unregister uprobe for current process. If UPROBE_HANDLER_REMOVE is
+	 * returned, filter() callback has to be implemented as well and it
+	 * should return false to "confirm" the decision to uninstall uprobe
+	 * for the current process. If filter() is omitted or returns true,
+	 * UPROBE_HANDLER_REMOVE is effectively ignored.
+	 */
 	int (*handler)(struct uprobe_consumer *self, struct pt_regs *regs);
 	int (*ret_handler)(struct uprobe_consumer *self,
 				unsigned long func,
 				struct pt_regs *regs);
 	bool (*filter)(struct uprobe_consumer *self, struct mm_struct *mm);
 
-	struct uprobe_consumer *next;
+	struct list_head cons_node;
 };
 
 #ifdef CONFIG_UPROBES