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

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Date:   Sat, 21 Dec 2019 15:22:35 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        kexec@...ts.infradead.org
Subject: Re: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer
 implementation (writer)

Hi John,

Sorry for the delay.

I don't have an overall understanding of the patch(-set) yet, so I limit
to a couple of general questions about the memory barriers introduced by
the path.  Please see inline comments.


> +	*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?


> +	do {
> +		next_lpos = get_next_lpos(data_ring, begin_lpos, size);
> +
> +		if (!data_push_tail(rb, data_ring,
> +				    next_lpos - DATA_SIZE(data_ring))) {
> +			/* Failed to allocate, specify a data-less block. */
> +			blk_lpos->begin = INVALID_LPOS;
> +			blk_lpos->next = INVALID_LPOS;
> +			return NULL;
> +		}
> +	} while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos,
> +					  next_lpos));
> +
> +	/*
> +	 * No barrier is needed here. The data validity is defined by
> +	 * the state of the associated descriptor. They are marked as
> +	 * invalid at the moment. And only the winner of the above
> +	 * cmpxchg() could write here.
> +	 */

The (successful) CMPXCHG provides a full barrier.  This comment suggests
that that could be somehow relaxed?  Or the comment could be improved?

(The patch introduces a number of CMPXCHG: similar questions would apply
to those other instances...)

Thanks,
  Andrea

P. S.  Please use my @gmail.com address for future communications.