linux-kernel - About compiler memory barrier for atomic_set/atomic

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <256e8ee2-a23c-28e9-3988-8b77307c001a@intel.com>
Date:   Tue, 3 Sep 2019 21:23:41 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     peterz@...radead.org, linux-kernel@...r.kernel.org
Cc:     "He, Min" <min.he@...el.com>, "Zhao, Yakui" <yakui.zhao@...el.com>
Subject: About compiler memory barrier for atomic_set/atomic_read on x86

Hi Peter,
There is one question regarding following commit:

commit 69d927bba39517d0980462efc051875b7f4db185
Author: Peter Zijlstra <peterz@...radead.org>
Date:   Wed Apr 24 13:38:23 2019 +0200

     x86/atomic: Fix smp_mb__{before,after}_atomic()

     Recent probing at the Linux Kernel Memory Model uncovered a
     'surprise'. Strongly ordered architectures where the atomic RmW
     primitive implies full memory ordering and
     smp_mb__{before,after}_atomic() are a simple barrier() (such as x86)

This change made atomic RmW operations include compiler barrier. And 
made __smp_mb__before_atomic/__smp_mb__after_atomic not include compiler
barrier any more for x86.

We face the issue to handle atomic_set/atomic_read which is mapped to
WRITE_ONCE/READ_ONCE on x86. These two functions don't include compiler
barrier actually (if operator size is less than 8 bytes).

Before the commit 69d927bba39517d0980462efc051875b7f4db185, we could use
__smp_mb__before_atomic/__smp_mb__after_atomic together with these two
functions to make sure the memory order. It can't work after the commit 
69d927bba39517d0980462efc051875b7f4db185. I am wandering whether
we should make atomic_set/atomic_read also include compiler memory
barrier on x86? Thanks.

Regards
Yin, Fengwei