Messages in this thread | | | Date | Mon, 2 Nov 2020 07:25:58 +0000 | From | Christoph Hellwig <> | Subject | Re: [GIT PULL] dma-mapping fix for 5.10 |
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On Sat, Oct 31, 2020 at 12:50:44PM -0700, Linus Torvalds wrote: > So this is just a stylistic nit, and has no impact on this pull (which > I've done). But looking at the patch, it triggers one of my "this is > wrong" patterns.
Adding the author and maintainer of that code so that they can sort it out.
> > In particular, this: > > u64 dma_start = 0; > ... > for (dma_start = ~0ULL; r->size; r++) { > > is actually completely bogus in theory, and it's a horribly horribly > bad pattern to have. > > The thing that I hate about that parttern is "~0ULL", which is simply wrong. > > The correct pattern for "all bits set" is ~0. NOTHING ELSE. No extra > letters at the end. > > Why? Because using an unsigned type is wrong, and will not extend the > bits up to a potentially bigger size. > > So adding that "ULL" is not just three extra characters to type, it > actually _detracts_ from the code and makes it more fragile and > potentially wrong. > > It so happens, that yes, in the kernel, "ull" us 64-bit, and you get > the right results. So the code _works_. But it's wrong, and it now > requires that the types match exactly (ie it would not be broken if > somebody ever were to say "I want to use use 128-bit dma addresses and > u128"). > > Another example is using "~0ul", which would give different results on > a 32-bit kernel and a 64-bit kernel. Again: DON'T DO THAT. > > I repeat: the right thing to do for "all bits set" is just a plain ~0 > or -1. Either of those are fine (technically assumes a 2's complement > machine, but let's just be honest: that's a perfectly fine assumption, > and -1 might be preferred by some because it makes that sign extension > behavior of the integer constant more obvious). > > Don't try to do anything clever or anything else, because it's going > to be strictly worse. > > The old code that that patch removed was "technically correct", but > just pointless, and actually shows the problem: > > for (dma_start = ~(dma_addr_t)0; r->size; r++) { > > the above is indeed a correct way to say "I want all bits set in a > dma_addr_t", but while correct, it is - once again - strictly inferior > to just using "~0". > > Why? Because "~0" works regardless of type. IOW, exactly *because* > people used the wrong pattern for "all bits set", that patch was now > (a) bigger than necessary and (b) much more ilkely to cause bugs (ie I > could have imagined people changing just the type of the variable > without changing the initialization). > > So in that tiny three-line patch there were actually several examples > of why "~0" is the right pattern to use for "all bits set". Because it > JustWorks(tm) in ways other patterns do not. > > And if you have a compiler that complains about assigning -1 or ~0 to > an unsigned variable, get rid of that piece of garbage. You're almost > certainly either using some warning flag that you shouldn't be using, > or the compiler writer didn't know what they were doing. > > Linus > _______________________________________________ > iommu mailing list > iommu@lists.linux-foundation.org > https://lists.linuxfoundation.org/mailman/listinfo/iommu ---end quoted text---
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