| 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
| ||
|
Date: Mon, 4 Dec 2023 19:51:01 +0100
From: Jan Kara <jack@...e.cz>
To: Yury Norov <yury.norov@...il.com>
Cc: linux-kernel@...r.kernel.org,
"David S. Miller" <davem@...emloft.net>,
"H. Peter Anvin" <hpa@...or.com>,
"James E.J. Bottomley" <jejb@...ux.ibm.com>,
"K. Y. Srinivasan" <kys@...rosoft.com>,
"Md. Haris Iqbal" <haris.iqbal@...os.com>,
Akinobu Mita <akinobu.mita@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Bjorn Andersson <andersson@...nel.org>,
Borislav Petkov <bp@...en8.de>,
Chaitanya Kulkarni <kch@...dia.com>,
Christian Brauner <brauner@...nel.org>,
Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
David Disseldorp <ddiss@...e.de>,
Edward Cree <ecree.xilinx@...il.com>,
Eric Dumazet <edumazet@...gle.com>,
Fenghua Yu <fenghua.yu@...el.com>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Gregory Greenman <gregory.greenman@...el.com>,
Hans Verkuil <hverkuil@...all.nl>,
Hans de Goede <hdegoede@...hat.com>,
Hugh Dickins <hughd@...gle.com>,
Ingo Molnar <mingo@...hat.com>,
Jakub Kicinski <kuba@...nel.org>,
Jaroslav Kysela <perex@...ex.cz>,
Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
Jiri Pirko <jiri@...nulli.us>,
Jiri Slaby <jirislaby@...nel.org>,
Kalle Valo <kvalo@...nel.org>,
Karsten Graul <kgraul@...ux.ibm.com>,
Karsten Keil <isdn@...ux-pingi.de>,
Kees Cook <keescook@...omium.org>,
Leon Romanovsky <leon@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Martin Habets <habetsm.xilinx@...il.com>,
Mauro Carvalho Chehab <mchehab@...nel.org>,
Michael Ellerman <mpe@...erman.id.au>,
Michal Simek <monstr@...str.eu>,
Nicholas Piggin <npiggin@...il.com>,
Oliver Neukum <oneukum@...e.com>,
Paolo Abeni <pabeni@...hat.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Ping-Ke Shih <pkshih@...ltek.com>,
Rich Felker <dalias@...c.org>, Rob Herring <robh@...nel.org>,
Robin Murphy <robin.murphy@....com>,
Sean Christopherson <seanjc@...gle.com>,
Shuai Xue <xueshuai@...ux.alibaba.com>,
Stanislaw Gruszka <stf_xl@...pl>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
Thomas Gleixner <tglx@...utronix.de>,
Valentin Schneider <vschneid@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wenjia Zhang <wenjia@...ux.ibm.com>,
Will Deacon <will@...nel.org>,
Yoshinori Sato <ysato@...rs.sourceforge.jp>,
GR-QLogic-Storage-Upstream@...vell.com,
alsa-devel@...a-project.org, ath10k@...ts.infradead.org,
dmaengine@...r.kernel.org, iommu@...ts.linux.dev,
kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-arm-msm@...r.kernel.org, linux-block@...r.kernel.org,
linux-bluetooth@...r.kernel.org, linux-hyperv@...r.kernel.org,
linux-m68k@...ts.linux-m68k.org, linux-media@...r.kernel.org,
linux-mips@...r.kernel.org, linux-net-drivers@....com,
linux-pci@...r.kernel.org, linux-rdma@...r.kernel.org,
linux-s390@...r.kernel.org, linux-scsi@...r.kernel.org,
linux-serial@...r.kernel.org, linux-sh@...r.kernel.org,
linux-sound@...r.kernel.org, linux-usb@...r.kernel.org,
linux-wireless@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
mpi3mr-linuxdrv.pdl@...adcom.com, netdev@...r.kernel.org,
sparclinux@...r.kernel.org, x86@...nel.org,
Jan Kara <jack@...e.cz>,
Mirsad Todorovac <mirsad.todorovac@....unizg.hr>,
Matthew Wilcox <willy@...radead.org>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
Alexey Klimov <klimov.linux@...il.com>,
Bart Van Assche <bvanassche@....org>,
Sergey Shtylyov <s.shtylyov@....ru>
Subject: Re: [PATCH v2 00/35] bitops: add atomic find_bit() operations
Hello Yury!
On Sun 03-12-23 11:23:47, Yury Norov wrote:
> Add helpers around test_and_{set,clear}_bit() that allow to search for
> clear or set bits and flip them atomically.
>
> The target patterns may look like this:
>
> for (idx = 0; idx < nbits; idx++)
> if (test_and_clear_bit(idx, bitmap))
> do_something(idx);
>
> Or like this:
>
> do {
> bit = find_first_bit(bitmap, nbits);
> if (bit >= nbits)
> return nbits;
> } while (!test_and_clear_bit(bit, bitmap));
> return bit;
>
> In both cases, the opencoded loop may be converted to a single function
> or iterator call. Correspondingly:
>
> for_each_test_and_clear_bit(idx, bitmap, nbits)
> do_something(idx);
>
> Or:
> return find_and_clear_bit(bitmap, nbits);
These are fine cleanups but they actually don't address the case that has
triggered all these changes - namely the xarray use of find_next_bit() in
xas_find_chunk().
...
> This series is a result of discussion [1]. All find_bit() functions imply
> exclusive access to the bitmaps. However, KCSAN reports quite a number
> of warnings related to find_bit() API. Some of them are not pointing
> to real bugs because in many situations people intentionally allow
> concurrent bitmap operations.
>
> If so, find_bit() can be annotated such that KCSAN will ignore it:
>
> bit = data_race(find_first_bit(bitmap, nbits));
No, this is not a correct thing to do. If concurrent bitmap changes can
happen, find_first_bit() as it is currently implemented isn't ever a safe
choice because it can call __ffs(0) which is dangerous as you properly note
above. I proposed adding READ_ONCE() into find_first_bit() / find_next_bit()
implementation to fix this issue but you disliked that. So other option we
have is adding find_first_bit() and find_next_bit() variants that take
volatile 'addr' and we have to use these in code like xas_find_chunk()
which cannot be converted to your new helpers.
> This series addresses the other important case where people really need
> atomic find ops. As the following patches show, the resulting code
> looks safer and more verbose comparing to opencoded loops followed by
> atomic bit flips.
>
> In [1] Mirsad reported 2% slowdown in a single-thread search test when
> switching find_bit() function to treat bitmaps as volatile arrays. On
> the other hand, kernel robot in the same thread reported +3.7% to the
> performance of will-it-scale.per_thread_ops test.
It was actually me who reported the regression here [2] but whatever :)
[2] https://lore.kernel.org/all/20231011150252.32737-1-jack@suse.cz
> Assuming that our compilers are sane and generate better code against
> properly annotated data, the above discrepancy doesn't look weird. When
> running on non-volatile bitmaps, plain find_bit() outperforms atomic
> find_and_bit(), and vice-versa.
>
> So, all users of find_bit() API, where heavy concurrency is expected,
> are encouraged to switch to atomic find_and_bit() as appropriate.
Well, all users where any concurrency can happen should switch. Otherwise
they are prone to the (admittedly mostly theoretical) data race issue.
Honza
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
Powered by blists - more mailing lists