Mark Kettenis wrote: > > From: "Ted Unangst" <t...@tedunangst.com> > > Date: Tue, 16 May 2017 18:27:13 -0400 > > > > Mark Kettenis wrote: > > > We can just call copyin(9) since it already is atomic. But check > > > whether the userland futex is properly aligned and return EFAULT if it > > > isn't such that this system call behaves like it does on strict > > > alignment architectures. > > > > hmm. do we want this? i understand the appeal, but due to differing > > compilers/etc, some structs that are carefully packed on some platforms may > > not be aligned on i386. however, they would be correctly aligned where > > required. > > If you want atomicity on amd64/i386, things must not cross a > cache-line boundary. The alignment check is a bit stronger than that > but simpler to implement and ensures consistency across platforms. > > > are we trying to prevent a problem that doesn't exist? > > Maybe. The i386 ABI requires 32-bit integers to be naturally aligned. > It's 64-bit integers where it has a relaxed requirement. So unless > your structs are __packed the alignment check should never fail. > Expecting atomic access to a memeber of a __packed struct is a bug.
Sounds reasonable to me.
