Re: [PATCH 3/3] tools: memory-model: Improve data-race detection

From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: LKMM Maintainers -- Akira Yokosawa <akiyks@gmail.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Daniel Lustig <dlustig@nvidia.com>,
	David Howells <dhowells@redhat.com>,
	Jade Alglave <j.alglave@ucl.ac.uk>,
	Luc Maranget <luc.maranget@inria.fr>,
	Nicholas Piggin <npiggin@gmail.com>,
	"Paul E. McKenney" <paulmck@linux.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/3] tools: memory-model: Improve data-race detection
Date: Fri, 21 Jun 2019 10:41:29 +0200	[thread overview]
Message-ID: <20190621084129.GA6827@andrea> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1906201153470.1512-100000@iolanthe.rowland.org>

On Thu, Jun 20, 2019 at 11:55:58AM -0400, Alan Stern wrote:
> Herbert Xu recently reported a problem concerning RCU and compiler
> barriers.  In the course of discussing the problem, he put forth a
> litmus test which illustrated a serious defect in the Linux Kernel
> Memory Model's data-race-detection code.
> 
> The defect was that the LKMM assumed visibility and executes-before
> ordering of plain accesses had to be mediated by marked accesses.  In
> Herbert's litmus test this wasn't so, and the LKMM claimed the litmus
> test was allowed and contained a data race although neither is true.
> 
> In fact, plain accesses can be ordered by fences even in the absence
> of marked accesses.  In most cases this doesn't matter, because most
> fences only order accesses within a single thread.  But the rcu-fence
> relation is different; it can order (and induce visibility between)
> accesses in different threads -- events which otherwise might be
> concurrent.  This makes it relevant to data-race detection.
> 
> This patch makes two changes to the memory model to incorporate the
> new insight:
> 
> 	If a store is separated by a fence from another access,
> 	the store is necessarily visible to the other access (as
> 	reflected in the ww-vis and wr-vis relations).  Similarly,
> 	if a load is separated by a fence from another access then
> 	the load necessarily executes before the other access (as
> 	reflected in the rw-xbstar relation).
> 
> 	If a store is separated by a strong fence from a marked access
> 	then it is necessarily visible to any access that executes
> 	after the marked access (as reflected in the ww-vis and wr-vis
> 	relations).
> 
> With these changes, the LKMM gives the desired result for Herbert's
> litmus test and other related ones.
> 
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> Reported-by: Herbert Xu <herbert@gondor.apana.org.au>

For the entire series:

Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com>

Two nits, but up to Paul AFAIAC:

 - This is a first time for "tools: memory-model:" in Subject; we were
   kind of converging to "tools/memory-model:"...

 - The report preceded the patch; we might as well reflect this in the
   order of the tags.

Thanks,

  Andrea