I'm on PTO and traveling until May 15 without my working laptop, can't read the code.
Quite possibly I am wrong, but let me try to recall what this code does... - So. uprobe_register() succeeds and changes ref_ctr from 0 to 1. - uprobe_unregister() fails but decrements ref_ctr back to zero. Because the "Revert back reference counter if instruction update failed" logic doesn't apply if is_register is true. Since uprobe_unregister() fails, this uprobe won't be removed. IIRC, we even have the warning about that. - another uprobe_register() comes and re-uses the same uprobe. In this case install_breakpoint() will do nothing, ref_ctr won't be updated (right ?) - uprobe_unregister() is called again and this time it succeeds. In this case ref_ctr is changed from 0 to -1. IIRC, we even have some warning for this case. No? Sorry, I can't check my thinking until I return. Oleg. On 05/06, Jiri Olsa wrote: > > On Mon, Apr 28, 2025 at 12:51:57PM +0200, Jiri Olsa wrote: > > On Sun, Apr 27, 2025 at 04:13:35PM +0200, Oleg Nesterov wrote: > > SNIP > > > > > > > ------------------------------------------------------------------------------- > > > OTOH, I think that the current logic is not really correct too, > > > > > > /* Revert back reference counter if instruction update failed. */ > > > if (ret < 0 && is_register && ref_ctr_updated) > > > update_ref_ctr(uprobe, mm, -1); > > > > > > I think that "Revert back reference counter" logic should not depend on > > > is_register. Otherwise we can have the unbalanced update_ref_ctr(-1) if > > > uprobe_unregister() fails, then another uprobe_register() comes at the > > > same address, and after that uprobe_unregister() succeeds. > > > > sounds good to me > > actualy after closer look, I don't see how this code could be triggered > in the first place.. any hint on how to hit such case? like: > > - ref_ctr_offset is updated > > - we fail somehow > > - "if (ret < 0 && ref_ctr_updated)" is true on the way out > > thanks, > jirka >
