Messages in this thread | | | Date | Mon, 15 May 2023 12:00:54 +0200 | From | Petr Tesařík <> | Subject | Re: [PATCH v2 RESEND 7/7] swiotlb: per-device flag if there are dynamically allocated buffers |
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On Mon, 15 May 2023 10:48:47 +0200 Petr Tesařík <petr@tesarici.cz> wrote:
> Hi Catalin, > > On Sun, 14 May 2023 19:54:27 +0100 > Catalin Marinas <catalin.marinas@arm.com> wrote: >[...] > > Now, thinking about the list_head access and the flag ordering, since it > > doesn't matter, you might as well not bother with the flag at all and > > rely on list_add() and list_empty() ordering vs the hypothetical 'blah' > > access. Both of these use READ/WRITE_ONCE() for setting > > dma_io_tlb_dyn_slots.next. You only need an smp_wmb() after the > > list_add() and an smp_rmb() before a list_empty() check in ^^^^^^^^^ Got it, finally. Well, that's exactly something I don't want to do. For example, on arm64 (seeing your email address), smp_rmb() translates to a "dsb ld" instruction. I would expect that this is more expensive than a "ldar", generated by smp_load_acquire().
I mean, for devices that do not need swiotlb, the flag never changes from zero, so reading it must be as cheap as possible.
Petr T
> > is_swiotlb_buffer(), no dma_iotlb_have_dyn variable. > > Wait, let me check that I understand you right. Do you suggest that I > convert dma_io_tlb_dyn_slots to a lockless list and get rid of the > spinlock? > > I'm sure it can be done for list_add() and list_del(). I'll have > to think about list_move().
Hm, even the documentation of llist_empty() says that it is "not guaranteed to be accurate or up to date". If it could be, I'm quite sure the authors would have gladly implemented it as such.
Petr T
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