Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks

From: John Hubbard <jhubbard@nvidia.com>
To: Peter Zijlstra <peterz@infradead.org>,
	Leonardo Bras <leonardo@linux.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	kvm-ppc@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-mm@kvack.org,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Paul Mackerras" <paulus@samba.org>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	"Christophe Leroy" <christophe.leroy@c-s.fr>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Mahesh Salgaonkar" <mahesh@linux.vnet.ibm.com>,
	"Reza Arbab" <arbab@linux.ibm.com>,
	"Santosh Sivaraj" <santosh@fossix.org>,
	"Balbir Singh" <bsingharora@gmail.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Mike Rapoport" <rppt@linux.ibm.com>,
	"Allison Randal" <allison@lohutok.net>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Christoph Lameter" <cl@linux.com>,
	"Logan Gunthorpe" <logang@deltatee.com>,
	"Andrey Ryabinin" <aryabinin@virtuozzo.com>,
	"Alexey Dobriyan" <adobriyan@gmail.com>,
	"Souptick Joarder" <jrdr.linux@gmail.com>,
	"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Jann Horn" <jannh@google.com>,
	"Davidlohr Bueso" <dave@stgolabs.net>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Christian Brauner" <christian.brauner@ubuntu.com>,
	"Michal Hocko" <mhocko@suse.com>,
	"Elena Reshetova" <elena.reshetova@intel.com>,
	"Roman Gushchin" <guro@fb.com>,
	"Andrea Arcangeli" <aarcange@redhat.com>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Dmitry V. Levin" <ldv@altlinux.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Song Liu" <songliubraving@fb.com>,
	"Bartlomiej Zolnierkiewicz" <b.zolnierkie@samsung.com>,
	"Ira Weiny" <ira.weiny@intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Keith Busch" <keith.busch@intel.com>,
	"Paul McKenney" <Paul.McKenney@us.ibm.com>
Subject: Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks
Date: Thu, 3 Oct 2019 13:40:38 -0700	[thread overview]
Message-ID: <c127ee98-094b-d5c0-0a42-6d01c16a72d9@nvidia.com> (raw)
In-Reply-To: <20191003115141.GJ4581@hirez.programming.kicks-ass.net>

On 10/3/19 4:51 AM, Peter Zijlstra wrote:
> On Thu, Oct 03, 2019 at 09:11:45AM +0200, Peter Zijlstra wrote:
>> On Wed, Oct 02, 2019 at 10:33:15PM -0300, Leonardo Bras wrote:
...
> 
> I'm still really confused about this barrier. It just doesn't make
> sense.
> 
> If an interrupt happens before the local_irq_disable()/save(), then it
> will discard any and all speculation that would be in progress to handle
> the exception.
> 

Hi Peter,

So, would that imply that it's correct to apply approximately the following
patch:

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 1adbb8a371c7..cf41eff37e24 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -2099,9 +2099,9 @@ INTERRUPT DISABLING FUNCTIONS
 -----------------------------
 
 Functions that disable interrupts (ACQUIRE equivalent) and enable interrupts
-(RELEASE equivalent) will act as compiler barriers only.  So if memory or I/O
-barriers are required in such a situation, they must be provided from some
-other means.
+(RELEASE equivalent) will act as full memory barriers. This is because, for
+all supported CPU architectures, interrupt arrival causes all speculative
+memory accesses to be discarded.
 
?

We're also discussing this over in [1] ("mm: don't expose non-hugetlb page to
fast gup prematurely"), so I'm adding Paul to this thread here as well.

[1] https://lore.kernel.org/r/20191002092447.GC9320@quack2.suse.cz

thanks,
-- 
John Hubbard
NVIDIA

