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SubjectRe: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer)
On Fri, Jan 03, 2020 at 11:24:20AM +0100, Petr Mladek wrote:
> On Mon 2019-12-23 17:01:00, John Ogness wrote:
> > Hi Andrea,
> >
> > On 2019-12-21, Andrea Parri <parri.andrea@gmail.com> wrote:
> > >> + *desc_out = READ_ONCE(*desc);
> > >> +
> > >> + /* Load data before re-checking state. */
> > >> + smp_rmb(); /* matches LMM_REF(desc_reserve:A) */
> > >
> > > I looked for a matching WRITE_ONCE() or some other type of marked write,
> > > but I could not find it. What is the rationale? Or what did I miss?
>
> Good question. READ_ONCE() looks superfluous here because it is
> surrounded by two read barriers. In each case, there is no
> corresponding WRITE_ONCE().
>
> Note that we are copying the entire struct prb_desc here. All values
> are written only when state_val is in desc_reserved state. It happens
> between two full write barriers:
>
> + A writer is allowed to modify the descriptor after successful
> cmpxchg in desc_reserve(), see LMM_TAG(desc_reserve:A).
>
> + The writer must not touch the descriptor after changing
> state_var to committed state, see
> LMM_TAG(prb_commit:A) in prb_commit().
>
> These barriers are mentioned in the comments for the two
> read barriers here.

Thanks for these remarks. As usual, I'd recommend to (try to) map those
comments into litmus tests and check with the LKMM simulator.


> BTW: Documentation/memory-barriers.txt describes various aspects of
> the memory barriers. It describes implicit barriers provided
> by spin locks, mutexes, semaphores, and various scheduler-related
> operations.
>
> But I can't find any explanation of the various variants of the atomic
> operations: acquire, release, fetch, return, try, relaxed. I can find
> some clues here and there but it is hard to get the picture.

Documentation/atomic_t.txt could serve this purpose. Please have a look
there and let me know if you have any comments.

Thanks,
Andrea

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