Re: [PATCH v3 tip/perf/core 4/4] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution

[Peter Zijlstra]

On Thu, Oct 10, 2024 at 01:56:44PM -0700, Andrii Nakryiko wrote:

> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>

I'm fairly sure I've suggested much the same :-)

> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> ---
>  kernel/events/uprobes.c | 50 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 50 insertions(+)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index fa1024aad6c4..9dc6e78975c9 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -2047,6 +2047,52 @@ static int is_trap_at_addr(struct mm_struct *mm, unsigned long vaddr)
>  	return is_trap_insn(&opcode);
>  }
>  
> +static struct uprobe *find_active_uprobe_speculative(unsigned long bp_vaddr)
> +{
> +	struct mm_struct *mm = current->mm;
> +	struct uprobe *uprobe = NULL;
> +	struct vm_area_struct *vma;
> +	struct file *vm_file;
> +	struct inode *vm_inode;
> +	unsigned long vm_pgoff, vm_start;
> +	loff_t offset;
> +	long seq;
> +
> +	guard(rcu)();
> +
> +	if (!mmap_lock_speculation_start(mm, &seq))
> +		return NULL;

So traditional seqcount assumed non-preemptible lock sides and would
spin-wait for the LSB to clear, but for PREEMPT_RT we added preemptible
seqcount support and that takes the lock to wait, which in this case is
exactly the same as returning NULL and doing the lookup holding
mmap_lock, so yeah.

> +
> +	vma = vma_lookup(mm, bp_vaddr);
> +	if (!vma)
> +		return NULL;
> +
> +	/* vm_file memory can be reused for another instance of struct file,

Comment style nit.

> +	 * but can't be freed from under us, so it's safe to read fields from
> +	 * it, even if the values are some garbage values; ultimately
> +	 * find_uprobe_rcu() + mmap_lock_speculation_end() check will ensure
> +	 * that whatever we speculatively found is correct
> +	 */
> +	vm_file = READ_ONCE(vma->vm_file);
> +	if (!vm_file)
> +		return NULL;
> +
> +	vm_pgoff = data_race(vma->vm_pgoff);
> +	vm_start = data_race(vma->vm_start);
> +	vm_inode = data_race(vm_file->f_inode);

So... seqcount has kcsan annotations other than data_race(). I suppose
this works, but it all feels like a bad copy with random changes.

> +
> +	offset = (loff_t)(vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vm_start);
> +	uprobe = find_uprobe_rcu(vm_inode, offset);
> +	if (!uprobe)
> +		return NULL;
> +
> +	/* now double check that nothing about MM changed */
> +	if (!mmap_lock_speculation_end(mm, seq))
> +		return NULL;

Typically seqcount does a re-try here.

> +
> +	return uprobe;
> +}