On Tue, Jul 15, 2025 at 05:44:13PM -0400, Mark Johnston wrote: > On Tue, Jul 15, 2025 at 11:04:57PM +0300, Konstantin Belousov wrote: > > On Tue, Jul 15, 2025 at 01:24:36PM -0400, Mark Johnston wrote: > > > On Tue, Jul 15, 2025 at 09:40:18AM -0700, Gleb Smirnoff wrote: > > > > On Tue, Jul 15, 2025 at 03:19:04PM +0000, Mark Johnston wrote: > > > > M> The branch main has been updated by markj: > > > > M> > > > > M> URL: > > > > https://cgit.FreeBSD.org/src/commit/?id=fad79db405052f3faad7184ea2c8bfe9f92a700d > > > > M> > > > > M> commit fad79db405052f3faad7184ea2c8bfe9f92a700d > > > > M> Author: Mark Johnston <ma...@freebsd.org> > > > > M> AuthorDate: 2025-07-15 15:16:40 +0000 > > > > M> Commit: Mark Johnston <ma...@freebsd.org> > > > > M> CommitDate: 2025-07-15 15:16:40 +0000 > > > > M> > > > > M> vm_pageout: Remove a volatile qualifier from some vm_domain > > > > members > > > > M> > > > > M> These are always accessed using atomic(9) intrinsics, so do not > > > > need the > > > > M> qualifier. No functional change intended. > > > > M> > > > > M> Reviewed by: alc, kib > > > > M> MFC after: 2 weeks > > > > M> Sponsored by: Modirum MDPay > > > > M> Sponsored by: Klara, Inc. > > > > M> Differential Revision: https://reviews.freebsd.org/D51322 > > > > > > > > What's the benefit of removing the qualifiers? They act as > > > > documentation > > > > and they match atomic(9) prototypes. To me this looks like removing a > > > > const qualifier with a reasoning that we use the variable only as an > > > > argument to functions that have const qualifier. > > > > > > Note that I updated the comments as well to indicate that accesses to > > > the fields should be done through atomic(9), so the documentation is > > > preserved. > > > > > > The reason atomic(9) prototypes include the volatile qualifier is to > > > permit their use with volatile-qualified variables without a cast, not > > > because the interface expects only volatile-qualified parameters. > > > > > > More generally, I believe that new code should always avoid using > > > volatile to provide any kind of synchronization, ignoring the case of > > > accesses to memory mapped with non-default attributes. atomic(9) > > > intrinsics should be used where they are needed, and some comments > > > should explain the synchronization protocol if it's not obvious. > > > > Hmm, when I wrote atomic_load/store(), volatile casts were used to utilize > > compiler-specific semantic of volatile accesses, that happens to match > > what loads and stores should do (access that place now). I.e. it is not > > for the allowance to use volatile-qualified locations, but to provide > > the C11-compatible semantic. > > My understanding is that C11 atomic ops take volatile-qualified > parameters for the reason I gave above. There is a note in the standard > which suggests this: > > NOTE Many operations are volatile-qualifed. The ‘‘volatile as device > register’’ semantics have not changed in the standard. This qualifcation > means that volatility is preserved when applying these operations to > volatile objects. > > I see similar hints here, but I don't know what source this is based on: > https://en.cppreference.com/w/c/atomic/atomic_load.html
I answered more to the Gleb' question, about making atomically accessed locations volatile. Yes, volatile (and const for load since some time) are used to allow also accessing volatile-qualified atomics. But it is not to provide atomicity properties through volatile properties. > > > After stating that, it is clear why qualifying the vars with volatile > > is not what we want: the semantic of volatile is compiler-dependent, > > and it only happens to match what is really used for code. Atomics > > loads and stores do provide the primitive ops we need, and hide the > > compiler-specific implementation under. > > > > I.e., volatile should *not* be used.
