linux-kernel - Re: [RFC PATCH v4 4/9] printk-rb: initialize new descriptors as invalid

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Date:   Tue, 20 Aug 2019 19:16:49 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [RFC PATCH v4 4/9] printk-rb: initialize new descriptors as
 invalid

On (08/20/19 11:23), Petr Mladek wrote:
> > is no risk of the descriptor unexpectedly being determined as
> > valid due to dataring head overflowing/wrapping.
> 
> Please, provide more details about the solved race. Is it because
> some reader could have reference to an invalid (reused) descriptor?
> Can be these invalid descriptors be member of the list?

As far as I understand, such descriptors can be on the list:

	prb_reserve()
		assign_desc()
			// pick a new never used descr
			i = atomic_fetch_inc(&rb->desc_next_unused);
			d = &rb->descs[i]
			dataring_desc_init(&d->desc);
			return d
		buf = dataring_push()
			// the oldest data is reserved, but not commited
			ret = get_new_lpos()
			if (ret)
				dataring_desc_init()
				return NULL
		if (!buf)
			numlist_push()

_datablock_valid() has a "desc->begin_lpos == desc->next_lpos" check.

	-ss