Re: [PATCH 3/8] uprobes: protected uprobe lifetime with SRCU

[Andrii Nakryiko]

On Thu, Aug 1, 2024 at 5:23 AM Liao, Chang <liaochang1@xxxxxxxxxx> wrote:
>
>
>
> 在 2024/8/1 5:42, Andrii Nakryiko 写道:
> > To avoid unnecessarily taking a (brief) refcount on uprobe during
> > breakpoint handling in handle_swbp for entry uprobes, make find_uprobe()
> > not take refcount, but protect the lifetime of a uprobe instance with
> > RCU. This improves scalability, as refcount gets quite expensive due to
> > cache line bouncing between multiple CPUs.
> >
> > Specifically, we utilize our own uprobe-specific SRCU instance for this
> > RCU protection. put_uprobe() will delay actual kfree() using call_srcu().
> >
> > For now, uretprobe and single-stepping handling will still acquire
> > refcount as necessary. We'll address these issues in follow up patches
> > by making them use SRCU with timeout.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> >  kernel/events/uprobes.c | 93 ++++++++++++++++++++++++-----------------
> >  1 file changed, 55 insertions(+), 38 deletions(-)
> >

[...]

> >
> > @@ -2258,12 +2275,12 @@ static void handle_swbp(struct pt_regs *regs)
> >       if (arch_uprobe_skip_sstep(&uprobe->arch, regs))
> >               goto out;
> >
> > -     if (!pre_ssout(uprobe, regs, bp_vaddr))
> > -             return;
> > +     if (pre_ssout(uprobe, regs, bp_vaddr))
> > +             goto out;
> >
>
> Regardless what pre_ssout() returns, it always reach the label 'out', so the
> if block is unnecessary.

yep, I know, but I felt like

if (something is wrong)
    goto out;

pattern was important to keep for each possible failing step for consistency.

so unless this is a big deal, I'd keep it as is, as in the future
there might be some other steps after pre_ssout() before returning, so
this is a bit more "composable"


>
>
> > -     /* arch_uprobe_skip_sstep() succeeded, or restart if can't singlestep */
> >  out:
> > -     put_uprobe(uprobe);
> > +     /* arch_uprobe_skip_sstep() succeeded, or restart if can't singlestep */
> > +     srcu_read_unlock(&uprobes_srcu, srcu_idx);
> >  }
> >
> >  /*
>
> --
> BR
> Liao, Chang