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-prev] [thread-next>] [day] [month] [year] [list]
Date:   Wed, 27 Oct 2021 14:22:08 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-m68k@...ts.linux-m68k.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        william.kucharski@...cle.com,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        schmitzmic@...il.com, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, weixugc@...gle.com,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [RFC 1/8] mm: add overflow and underflow checks for page->_refcount

> I found some atomic_add/dec are replaced with atomic_add/dec_return,

I am going to replace -return variants with -fetch variants, potentially -fetch

> those helpers with return value imply a full memory barrier around it, but
> others without return value do not. Do you have any numbers to show
> the impact? Maybe atomic_add/dec_return_relaxed can help this.

The generic variant uses  arch_cmpxchg() for all atomic variants
without any extra barriers. Therefore, on platforms that use generic
implementations there won't be performance differences except for an
extra branch that checks results when VM_BUG_ON is enabled.

On x86 the difference between the two is the following

atomic_add:
   lock add %eax,(%rsi)

atomic_fetch_add:
   lock xadd %eax,(%rsi)

atomic_fetch_add_relaxed:
   lock xadd %eax,(%rsi)

No differences between relaxed and non relaxed variants. However, we
used lock xadd instead of lock add. I am not sure if the performance
difference is going to be different.

Pasha

Powered by blists - more mailing lists