Re: Thread Safety Analysis and the Linux kernel

Marco Elver
2025-06-05T12:44:44Z

On Mon, 5 May 2025 at 15:44, Marco Elver <elver@google.com> wrote:
> On Wed, 5 Mar 2025 at 12:47, Marco Elver <elver@google.com> wrote:
> [...]
> > 1. Re-entrant acquires: rcu_read_lock(), preempt_disable(), etc. are
> > all re-entrant locks. My proposal is to introduce an attribute that
> > can be added to "ACQUIRE(..)" annotated functions to indicate they are
> > re-entrant. Release-count needs to then match acquire-count to fully
> > release a capability.
>
> FTR, reentrant locking support is taken care of:
> https://github.com/llvm/llvm-project/pull/137133 (should land soon).
>
> But I think we need at least #2 (basic alias analysis) as well for
> things to become tolerable for the kernel side. And that one looks a
> bit harder to implement in Clang, so it'll be a while.
[..]
> > 2. Basic alias analysis, e.g. when storing a pointer to a lock in a
> > function-local variable. [Complaint in:
> > https://lore.kernel.org/all/569186c5-8663-43df-a01c-d543f57ce5ca@kernel.org/
> > -- quote: "Fix the analyzer instead."]

There is hope: https://github.com/llvm/llvm-project/pull/142955 - but
it turned out to be "too easy to be true", so who knows if that'll
stick.

> > 3. Ability to refer to locks in returned reference/pointer. For example:
> > struct foo *ret_lock_struct(void) ACQUIRE(return->somelock);
> > struct foo *try_ret_lock_struct(void) TRY_ACQUIRE(1,
> > return->somelock); // locked if non-NULL
> > I expect this also requires basic alias analysis to work so that
> > assigning the returned pointer to a function-local variable and then
> > later use in an unlock function works as expected.
> >
> > More complex requests:
> >
> > 4. The ability to deal with conditional locking with return values
> > that are not just true/non-zero and false/zero. A concrete case here
> > is that a lot of functions return error codes, and if the error code
> > is < 0, no lock is taken. If the error code is >= 0, the lock is
> > taken. [ Source:
> > https://lore.kernel.org/all/569186c5-8663-43df-a01c-d543f57ce5ca@kernel.org/
> > ]
> >
> > 5. Better control-flow handling. Basic understanding of conditional
> > locking, which is explicitly ruled out in:
> > https://clang.llvm.org/docs/ThreadSafetyAnalysis.html#conditional-locks
> > - however, if there's some way to even get basic support, would vastly
> > improve things for the kernel.
> …