On Mon, 25 Nov 2024, Maciej W. Rozycki wrote: > On Sat, 23 Nov 2024, Maciej W. Rozycki wrote: > > > I found no previous bug report, so this is now PR target/117759[1]. An > > obvious bug fix has been posted for review as well[2]. > > I gave this issue some thought as I travelled across Europe over the > weekend and I have now withdrawn the proposed fix for the PR. > > The full `-msafe-bwa' feature is required for a fix, as the data race > comes from *outside* rather than from any lack of atomicity of the store > to the object itself. As the previous value of the object itself is > discarded, a plain RMW sequence is already atomic, because the RM part > only applies to data outside the object accessed. However a similar RMW > sequence made to update data located within the same longword *outside* > the atomic object concerned will clobber it.
Note this is why the C++ memory model constrains what "outside" parts of a memory location may be accessed by such a RMW sequence to avoid producing data-races for data that isn't accessed by the program. I don't think you can reliably fix this without enforcing atomic sequences on every access that isn't constrained to memory within itself. Richard. > I've posted more details in the PR. > Maciej > -- Richard Biener <rguent...@suse.de> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg, Germany; GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
