Re: [PATCH v2 5/5] kcsan, seqlock: Fix incorrect assumption in read_seqbegin()

[Peter Zijlstra]

On Tue, Nov 05, 2024 at 10:34:00AM +0100, Peter Zijlstra wrote:
> On Mon, Nov 04, 2024 at 04:43:09PM +0100, Marco Elver wrote:
> > During testing of the preceding changes, I noticed that in some cases,
> > current->kcsan_ctx.in_flat_atomic remained true until task exit. This is
> > obviously wrong, because _all_ accesses for the given task will be
> > treated as atomic, resulting in false negatives i.e. missed data races.
> > 
> > Debugging led to fs/dcache.c, where we can see this usage of seqlock:
> > 
> > 	struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
> > 	{
> > 		struct dentry *dentry;
> > 		unsigned seq;
> > 
> > 		do {
> > 			seq = read_seqbegin(&rename_lock);
> > 			dentry = __d_lookup(parent, name);
> > 			if (dentry)
> > 				break;
> > 		} while (read_seqretry(&rename_lock, seq));
> > 	[...]
> 
> 
> How's something like this completely untested hack?
> 
> 
> 	struct dentry *dentry;
> 
> 	read_seqcount_scope (&rename_lock) {
> 		dentry = __d_lookup(parent, name);
> 		if (dentry)
> 			break;
> 	}
> 
> 
> But perhaps naming isn't right, s/_scope/_loop/ ?

It is also confused between seqcount and seqlock. So perhaps it should
read:

	read_seqcount_loop (&rename_lock.seqcount) {
	   ...
	}

instead.