> > diff --git a/arch/powerpc/platforms/powernv/npu-dma.c > > b/arch/powerpc/platforms/powernv/npu-dma.c > > index 0a253b64ac5f..2fed4b116e19 100644 > > --- a/arch/powerpc/platforms/powernv/npu-dma.c > > +++ b/arch/powerpc/platforms/powernv/npu-dma.c > > @@ -726,7 +749,7 @@ struct npu_context *pnv_npu2_init_context(struct > > pci_dev *gpdev, > > if (WARN_ON(of_property_read_u32(nvlink_dn, "ibm,npu-link-index", > > &nvlink_index))) > > return ERR_PTR(-ENODEV); > > - npu_context->npdev[npu->index][nvlink_index] = npdev; > > + WRITE_ONCE(npu_context->npdev[npu->index][nvlink_index], npdev); > > When you're publishing a struct via a pointer you would typically have a > barrier between the stores that set the fields of the struct, and the > store that publishes the struct. Otherwise the reader can see a > partially setup struct.
npdev is just a pointer to a pci_dev setup by the PCI code. We assign it to npdev[][] to indicate to the mmu notifiers that an invalidation should also be sent to this nvlink. However I don't think there is any ordering required wrt to the rest of the npu_context setup - so long as when the notifiers deference npu_context->npdev[i][j] it either sees a valid non-NULL pointer or NULL assigned to npdev[][] everything should be ok. > I think here the npdev was setup somewhere else, and maybe there has > been an intervening barrier, but it's not clear. A comment at least > would be good. Yes it has been. I will submit a v3 with some more comments incorporating the above. Unless you think my argument is wrong? > In general I feel like a spinlock or two could significantly simply the > locking/ordering in this code, and given we're doing MMIOs anyway would > not affect performance. Indeed, I am working on a patch to add a spinlock around the allocation of the npu_context as pnv_npu2_destroy_context() needs to be serialised with respect to pnv_npu2_init_context() on the same mm_struct. I'd incorrectly assumed the driver would do this serialisation but apparently it can't so we need to ensure kref_get(npu_context) can't race with it's destruction (concurrent init_context() is protected by mmap_sem). I could fold that (unposted) patch into this one if you think that would be better? Thanks! - Alistair > </rant> > > cheers >
