Date: Wed, 20 May 2015 02:36:32 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Refactoring atomics as llsc? On Wed, May 20, 2015 at 08:33:23AM +0300, Timo Teras wrote: > On Wed, 20 May 2015 01:11:08 -0400 > Rich Felker <dalias@...c.org> wrote: > > > Of course the big outlier is x86, which is not llsc based but has > > actual atomic primitives at the instruction level. If we defined the > > sc() primitive to take 3 args instead of 2 (address, old value from > > ll, new value to conditionally store; most archs would ignore the old > > value argument) then we could model x86 with ll being a plain load and > > sc being cmpxchg to allow any new custom primitives to work using > > cmpxchg. Then we would just continue providing custom versions of all > > the old a_* ops (a_cas, a_fetch_add, a_inc, a_dec, a_and, a_or, > > a_swap) to take advantage of the x86 instructions. These versions > > could probably be shared by all x86 variants (i386, x86_64, x32) since > > they're operating on 32-bit values and the asm should be the same. > > I wonder if calling that kind of emulation ll()/sc() would be > misleading. load-linked store-conditional has stronger guarantees. sc > will fail if the cache-line was invalidated in-between, thread was > pre-empted etc. > > Using cmpxchg can be used to emulate it only when the user is aware of > ABA problem (some other thread may have changed the value behind us > multiple times). Such emulation is of course ok for a_fetch_add, etc. > But one needs to be more careful if using pointers (and trying to make > sure the same pointer was not first removed and later re-added). > > And if you want to optimize the above mentioned cases, one really needs > to know if it's true ll+sc, or write the synchronization differently. > In these cases the algorithm is often implemented twice with the > different available atomics. And yes, an alternative would be not to provide fake ll/sc for archs without it but instead to have the existing generic cas-based implementations to be used when ll/sc is not available. Then we'd have 2 generic implementations of everything instead of just one, but it would probably be cleaner. Rich
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