Re: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer)

[Andrea Parri <parri.andrea@gmail.com>]

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