Re: [PATCH v7 3/4] rust: sync: add SRCU abstraction

["Onur Özkan" <work@onurozkan.dev>]

On Fri, 29 May 2026 13:41:10 +0100
Gary Guo <gary@garyguo.net> wrote:

> On Fri May 29, 2026 at 1:29 PM BST, Onur Özkan wrote:
> > On Fri, 29 May 2026 13:07:18 +0100
> > Gary Guo <gary@garyguo.net> wrote:
> >
> >> On Fri May 29, 2026 at 7:57 AM BST, Onur Özkan wrote:
> >> >> >> > +#[pinned_drop]
> >> >> >> > +impl PinnedDrop for Srcu {
> >> >> >> > +    fn drop(self: Pin<&mut Self>) {
> >> >> >> > +        let ptr = self.inner.get();
> >> >> >> > +
> >> >> >> > +        // SAFETY: By the type invariants, `self` contains a valid and pinned `struct srcu_struct`
> >> >> >> > +        // and `srcu_readers_active()` only checks the active reader count.
> >> >> >> > +        if unsafe { bindings::srcu_readers_active(ptr) } {
> >> >> >> > +            crate::pr_warn!(
> >> >> >> > +                "Leaked `Guard` detected while dropping SRCU; drop will block forever.\n"
> >> >> >> > +            );
> >> >> 
> >> >> I think this could be a `warn_on` similar to how cleanup_srcu_struct handle the
> >> >> condition.
> >> >
> >> > We also call cleanup_srcu_struct below. The idea was to provide additional
> >> > information, we don't need to call warn_on twice.
> >> 
> >> If the code blocks on `synchronize_srcu` then there's no call to
> >> `cleanup_srcu_struct`.
> >
> >
> > Ah right. I can do that in this case but honestly it's still more informative
> > with the current way. It explicitly tells you what the problem is.
> 
> While the error message itself is not that informative (there's some potential
> to improve this, see
> https://lore.kernel.org/all/DI1IQE7MDV4O.5B2DVIXMX2OT@garyguo.net/), the stack
> trace produced by a `warn_on` would have all the information and is more useful
> to troubleshoot than a `pr_warn` which doesn't tell you which `Srcu` being
> dropped is causing the issue.

Cool, I will use it in the next version. Thanks!

> 
> >> >> >
> >> >> > Actually, now I am now thinking about whether we can come up with a better
> >> >> > approach when we detect leaked guards. Initially I came up with the
> >> >> > synchronize_srcu() solution because it would handle leaked guards automatically
> >> >> > without requiring any additional checks. But now that we can actually detect
> >> >> > whether guards are leaked the question becomes:
> >> >> >
> >> >> > 	"Is there a better option than effectively sleeping forever when leaked
> >> >> > 	 guards are detected?"
> >> >> >
> >> >> > I have no plans for tomorrow other than finalizing this series including the
> >> >> > question above.
> >> >> 
> >> >> The best solution is to proceed cleanups anyway, given Rust rules ensure that
> >> >> these are actual leaks and not just srcu read-side critical section that failed
> >> >> to synchronize with the destruction of SRCU.
> >> >> 
> >> >> This obviously require changes to the SRCU code though.
> >> >
> >> >
> >> > The issue is difficult to fix purely from the C side. Once drop returns Rust
> >> > is free to destroy srcu_struct. If srcu still has pending callback associated
> >> > with that srcu_struct, for example from a future call_srcu() wrapper then
> >> > returning from drop while readers are active can turn into a UAF. There is also
> >> > no way to handle callbacks in a reasonable way in cleanup logic while there are
> >> > active readers.
> >> 
> >> Callbacks should be flushed during the drop due to srcu_barrier. Am I missing
> >> something?
> >
> > No. Callbacks can only be invoked once the grace period has completed [1], which
> > can never happen while there is an active reader.
> >
> > [1]: https://elixir.bootlin.com/linux/v7.1-rc5/source/kernel/rcu/srcutree.c#L1452-L1454
> 
> Well, then srcu_barrier will not return. When `srcu_barrier` returns all
> in-flight SRCU callbacks must have been executed.

Yes, or there weren't any callbacks posted.

> 
> Best,
> Gary
> 
> >
> >> 
> >> I'm pretty sure that, if we disregard potential misuses from C side, removing
> >> all "leak it" paths would be fine and won't leak to UAF if all users are from
> >> Rust side.
> >> 
> >> To be very clear, I am not advocating to actually implement this way. I agree
> >> with your conclusion below that this is broken code and a warning + blocking is
> >> good enough. This is really just my thoughts on your "is there a better option"
> >> question, and I think it's better in ideal world, but I think blocking is a
> >> good pragmatic choice.
> >
> > I see. Maybe I should have phrased the question like "Is there a better option
> > with similar complexity" to be more clear.