Date: Thu, 7 Dec 2023 10:10:23 +0100
From: Jan Kara <jack@...e.cz>
To: Yury Norov <yury.norov@...il.com>
Cc: Jan Kara <jack@...e.cz>, 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,
	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

On Tue 05-12-23 21:22:59, Yury Norov wrote:
> On Mon, Dec 04, 2023 at 07:51:01PM +0100, Jan Kara wrote:
> > > 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.
> 
> Here is some examples when concurrent operations with plain find_bit()
> are acceptable:
> 
>  - two threads running find_*_bit(): safe wrt ffs(0) and returns correct
>    value, because underlying bitmap is unchanged;
>  - find_next_bit() in parallel with set or clear_bit(), when modifying
>    a bit prior to the start bit to search: safe and correct;
>  - find_first_bit() in parallel with set_bit(): safe, but may return wrong
>    bit number;
>  - find_first_zero_bit() in parallel with clear_bit(): same as above.
> 
> In last 2 cases find_bit() may not return a correct bit number, but
> it may be OK if caller requires any (not exactly first) set or clear
> bit, correspondingly.
> 
> In such cases, KCSAN may be safely silenced.

True - but these are special cases. In particular the case in xas_find_chunk()
is not any of these special cases. It is using find_next_bit() which is can
be racing with clear_bit(). So what are your plans for such usecase?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

Powered by blists - more mailing lists