From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
kernel-team@android.com, Michael Ellerman <mpe@ellerman.id.au>,
Linus Torvalds <torvalds@linux-foundation.org>,
Segher Boessenkool <segher@kernel.crashing.org>,
Christian Borntraeger <borntraeger@de.ibm.com>,
Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
Arnd Bergmann <arnd@arndb.de>,
Peter Oberparleiter <oberpar@linux.ibm.com>,
Masahiro Yamada <masahiroy@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
ying.huang@intel.com, kirill.shutemov@linux.intel.com
Subject: Re: [PATCH v2 05/10] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses
Date: Sat, 25 Jan 2020 09:27:47 +0100 [thread overview]
Message-ID: <20200125082746.GT11457@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20200123153341.19947-6-will@kernel.org>
On Thu, Jan 23, 2020 at 03:33:36PM +0000, Will Deacon wrote:
> {READ,WRITE}_ONCE() cannot guarantee atomicity for arbitrary data sizes.
> This can be surprising to callers that might incorrectly be expecting
> atomicity for accesses to aggregate structures, although there are other
> callers where tearing is actually permissable (e.g. if they are using
> something akin to sequence locking to protect the access).
>
> Linus sayeth:
>
> | We could also look at being stricter for the normal READ/WRITE_ONCE(),
> | and require that they are
> |
> | (a) regular integer types
> |
> | (b) fit in an atomic word
> |
> | We actually did (b) for a while, until we noticed that we do it on
> | loff_t's etc and relaxed the rules. But maybe we could have a
> | "non-atomic" version of READ/WRITE_ONCE() that is used for the
> | questionable cases?
>
> The slight snag is that we also have to support 64-bit accesses on 32-bit
> architectures, as these appear to be widespread and tend to work out ok
> if either the architecture supports atomic 64-bit accesses (x86, armv7)
> or if the variable being accesses represents a virtual address and
> therefore only requires 32-bit atomicity in practice.
>
> Take a step in that direction by introducing a variant of
> 'compiletime_assert_atomic_type()' and use it to check the pointer
> argument to {READ,WRITE}_ONCE(). Expose __{READ,WRITE_ONCE}() variants
> which are allowed to tear and convert the two broken callers over to the
> new macros.
The build robot is telling me we also need this for m68k; they have:
arch/m68k/include/asm/page.h:typedef struct { unsigned long pmd[16]; } pmd_t;
Commit 688272809fcce seems to suggest the below is actually wrong tho.
---
diff --git a/mm/gup.c b/mm/gup.c
index 7646bf993b25..62885dad5444 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -320,7 +320,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
* The READ_ONCE() will stabilize the pmdval in a register or
* on the stack so that it will stop changing under the code.
*/
- pmdval = READ_ONCE(*pmd);
+ pmdval = __READ_ONCE(*pmd);
if (pmd_none(pmdval))
return no_page_table(vma, flags);
if (pmd_huge(pmdval) && vma->vm_flags & VM_HUGETLB) {
@@ -345,7 +345,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
!is_pmd_migration_entry(pmdval));
if (is_pmd_migration_entry(pmdval))
pmd_migration_entry_wait(mm, pmd);
- pmdval = READ_ONCE(*pmd);
+ pmdval = __READ_ONCE(*pmd);
/*
* MADV_DONTNEED may convert the pmd to null because
* mmap_sem is held in read mode
next prev parent reply other threads:[~2020-01-25 8:28 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-23 15:33 [PATCH v2 00/10] Rework READ_ONCE() to improve codegen Will Deacon
2020-01-23 15:33 ` [PATCH v2 01/10] compiler/gcc: Emit build-time warning for GCC prior to version 4.8 Will Deacon
2020-01-23 15:33 ` [PATCH v2 02/10] netfilter: Avoid assigning 'const' pointer to non-const pointer Will Deacon
2020-01-23 19:07 ` Nick Desaulniers
2020-01-24 8:24 ` Peter Zijlstra
2020-01-24 17:20 ` Nick Desaulniers
2020-01-27 12:04 ` David Laight
2020-01-24 17:36 ` Linus Torvalds
2020-01-24 22:00 ` Peter Zijlstra
2020-01-27 12:21 ` David Laight
2020-01-23 15:33 ` [PATCH v2 03/10] fault_inject: Don't rely on "return value" from WRITE_ONCE() Will Deacon
2020-01-23 15:33 ` [PATCH v2 04/10] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() Will Deacon
2020-01-23 15:33 ` [PATCH v2 05/10] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses Will Deacon
2020-01-25 8:27 ` Peter Zijlstra [this message]
2020-01-29 10:49 ` Peter Zijlstra
2020-01-23 15:33 ` [PATCH v2 06/10] READ_ONCE: Drop pointer qualifiers when reading from scalar types Will Deacon
2020-01-23 15:33 ` [PATCH v2 07/10] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros Will Deacon
2020-01-23 15:33 ` [PATCH v2 08/10] arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros Will Deacon
2020-01-23 15:33 ` [PATCH v2 09/10] compiler/gcc: Raise minimum GCC version for kernel builds to 4.8 Will Deacon
2020-01-23 18:36 ` Nick Desaulniers
2020-01-24 8:26 ` Peter Zijlstra
2020-01-24 17:05 ` Nick Desaulniers
2020-01-24 23:29 ` Peter Zijlstra
2020-01-25 10:34 ` Michael Ellerman
2020-01-23 15:33 ` [PATCH v2 10/10] gcov: Remove old GCC 3.4 support Will Deacon
2020-01-23 18:51 ` Nick Desaulniers
2020-01-28 14:56 ` Peter Oberparleiter
2020-01-23 17:07 ` [PATCH v2 00/10] Rework READ_ONCE() to improve codegen David Laight
2020-01-23 17:16 ` Will Deacon
2020-01-23 17:32 ` David Laight
2020-01-23 18:45 ` Nick Desaulniers
2020-01-23 19:01 ` Arvind Sankar
2020-01-24 10:11 ` David Laight
2020-01-26 1:10 ` Qais Yousef
2020-01-27 7:26 ` Arnd Bergmann
2020-01-23 17:59 ` Linus Torvalds
2020-01-24 8:33 ` Peter Zijlstra
2020-01-24 10:41 ` Peter Zijlstra
2020-02-10 9:50 ` Masahiro Yamada
2020-02-10 9:59 ` Will Deacon
2020-01-31 10:20 ` David Howells
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