Re: [PATCH 00/10] perf/uprobe: Optimize uprobes

[Peter Zijlstra]

On Wed, Jul 10, 2024 at 11:16:31AM +0200, Peter Zijlstra wrote:

> If it were an actual sequence count, I could make it work, but sadly,
> not. Also, vma_end_write() seems to be missing :-( If anything it could
> be used to lockdep annotate the thing.
> 
> Mooo.. I need to stare more at this to see if perhaps it can be made to
> work, but so far, no joy :/

See, this is what I want, except I can't close the race against VMA
modification because of that crazy locking scheme :/


--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -2146,11 +2146,58 @@ static int is_trap_at_addr(struct mm_str
 	return is_trap_insn(&opcode);
 }
 
-static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
+#ifndef CONFIG_PER_VMA_LOCK
+static struct uprobe *__find_active_uprobe(unsigned long bp_vaddr)
+{
+	return NULL;
+}
+#else
+static struct uprobe *__find_active_uprobe(unsigned long bp_vaddr)
 {
 	struct mm_struct *mm = current->mm;
 	struct uprobe *uprobe = NULL;
 	struct vm_area_struct *vma;
+	MA_STATE(mas, &mm->mm_mt, bp_vaddr, bp_vaddr);
+
+	guard(rcu)();
+
+again:
+	vma = mas_walk(&mas);
+	if (!vma)
+		return NULL;
+
+	/* vma_write_start() -- in progress */
+	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq))
+		return NULL;
+
+	/* 
+	 * Completely broken, because of the crazy vma locking scheme you
+	 * cannot avoid the per-vma rwlock and doing so means you're racy
+	 * against modifications.
+	 *
+	 * A simple actual seqcount would'be been cheaper and more usefull.
+	 */
+
+	if (!valid_vma(vma, false))
+		return NULL;
+
+	struct inode = file_inode(vma->vm_file);
+	loff_t offset = vaddr_to_offset(vma, bp_vaddr);
+
+	// XXX: if (vma_seq_retry(...)) goto again;
+
+	return find_uprobe(inode, offset);
+}
+#endif
+
+static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
+{
+	struct uprobe *uprobe = __find_active_uprobe(bp_vaddr)
+	struct mm_struct *mm = current->mm;
+	struct vm_area_struct *vma;
+
+	if (uprobe)
+		return uprobe;
 
 	mmap_read_lock(mm);
 	vma = vma_lookup(mm, bp_vaddr);