Messages in this thread | | | Date | Fri, 6 Dec 2019 18:37:05 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] futex: Support smaller futexes of one byte or two byte size. |
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On Fri, Dec 06, 2019 at 04:31:29PM +0100, Peter Zijlstra wrote: > > + case FUTEX_WAKE: > > + case FUTEX_REQUEUE: > > + /* > > + * these instructions work with sized mutexes, but you > > + * don't need to pass the size. we could silently > > + * ignore the size argument, but the code won't verify > > + * that the correct size is used, so it's preferable > > + * to make that clear to the caller. > > + * > > + * for requeue the meaning would also be ambiguous: do > > + * both of them have to be the same size or not? they > > + * don't, and that's clearer when you just don't pass > > + * a size argument. > > + */ > > + return -EINVAL; > > Took me a while to figure out this relies on FUTEX_NONE to avoid the > alignment tests.
And thikning more on that, I really _realy_ hate this.
You're basically saying WAKE is size-less, but that means we must consider what it means to have a u32 WAIT on @ptr, and a u8 WAKE on @ptr+1. If the wake really is size-less that should match.
I'd be much happier with requiring strict sizing. Because conversely, what happens if you have a u32-WAIT at @ptr paired with a u8-WAKE at @ptr? If we demand strict size we can say that should not match. This does however mean we should include the size in the hash-match function.
Your Changelog did not consider these implications at all.
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