Re: [PATCH net-next v2 4/7] net: openvswitch: fix load tearing with u64_stats

On Mon, 26 Jan 2026 19:35:38 +0100
Eric Dumazet [off-list ref] wrote:

On Mon, Jan 26, 2026 at 7:29 PM David Laight
[off-list ref] wrote:

quoted

On Sat, 24 Jan 2026 00:21:36 +0800
David Yang [off-list ref] wrote:
 

quoted

On 64bit arches, struct u64_stats_sync is empty and provides no help
against load/store tearing. struct copying should not be considered
tear-free. Use u64_stats_reads() instead.  

Except that the compiler doesn't ever generate 'tearing accesses' for
aligned 64bit accesses on any 64bit architecture.
Similarly memcpy() won't generate problematic accesses.

The problem is purely theoretical - the C language lets the compiler
split accesses, but it doesn't.  

Yeah, although we still have races that KCSAN can detect.

data_race() or READ_ONCE() would be necessary to avoid noisy KCSAN reports.

While many KCSAN reports are boring, some of them point to real bugs.

Could something be put in u64_stats_fetch_begin() to stop KCSAN bleating?

With the way READ_ONCE and WRITE_ONCE now get enforced I do wonder if
some data shouldn't just be marked 'volatile'?
That would give 'non-tearing' accesses, but without the inter-cpu
ordering that (I think) READ/WRITE_ONCE also give.
For stats counters that is enough.

	David