Re: [PATCH v3 00/35] Compiler-Based Capability- and Locking-Analysis

[Marco Elver <elver@google.com>]

On Fri, 19 Sept 2025 at 16:08, Christoph Hellwig <hch@lst.de> wrote:
>
> On Thu, Sep 18, 2025 at 10:45:55AM -0700, Nathan Chancellor wrote:
> > On Thu, Sep 18, 2025 at 04:15:11PM +0200, Christoph Hellwig wrote:
> > > On Thu, Sep 18, 2025 at 03:59:11PM +0200, Marco Elver wrote:
> > > > A Clang version that supports `-Wthread-safety-pointer` and the new
> > > > alias-analysis of capability pointers is required (from this version
> > > > onwards):
> > > >
> > > >   https://github.com/llvm/llvm-project/commit/b4c98fcbe1504841203e610c351a3227f36c92a4 [3]
> > >
> > > There's no chance to make say x86 pre-built binaries for that available?
> >
> > I can use my existing kernel.org LLVM [1] build infrastructure to
> > generate prebuilt x86 binaries. Just give me a bit to build and upload
> > them. You may not be the only developer or maintainer who may want to
> > play with this.
>
> That did work, thanks.
>
> I started to play around with that.  For the nvme code adding the
> annotations was very simply, and I also started adding trivial
> __guarded_by which instantly found issues.
>
> For XFS it was a lot more work and I still see tons of compiler
> warnings, which I'm not entirely sure how to address.  Right now I
> see three major classes:
>
> 1) locks held over loop iterations like:
>
> fs/xfs/xfs_extent_busy.c:573:26: warning: expecting spinlock 'xfs_group_hold(busyp->group)..xg_busy_extents->eb_lock' to be held at start of each loop [-Wthread-safety-analysis]
>   573 |                 struct xfs_group        *xg = xfs_group_hold(busyp->group);
>       |                                               ^
> fs/xfs/xfs_extent_busy.c:577:3: note: spinlock acquired here
>   577 |                 spin_lock(&eb->eb_lock);
>       |                 ^
>
> This is perfectly find code and needs some annotations, but I can't find
> any good example.

This is an interesting one, and might be a bug in the alias analysis I
recently implemented in Clang. I'll try to figure out a fix.