Received-SPF: pass (google.com: domain of linux-wireless+bounces-869-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Date: Sat, 16 Dec 2023 13:48:18 -0800
From: Yury Norov <yury.norov@gmail.com>
To: linux-kernel@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"James E.J. Bottomley" <jejb@linux.ibm.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	"Md. Haris Iqbal" <haris.iqbal@ionos.com>,
	Akinobu Mita <akinobu.mita@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Bjorn Andersson <andersson@kernel.org>,
	Borislav Petkov <bp@alien8.de>, Chaitanya Kulkarni <kch@nvidia.com>,
	Christian Brauner <brauner@kernel.org>,
	Damien Le Moal <damien.lemoal@opensource.wdc.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Disseldorp <ddiss@suse.de>,
	Edward Cree <ecree.xilinx@gmail.com>,
	Eric Dumazet <edumazet@google.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Gregory Greenman <gregory.greenman@intel.com>,
	Hans Verkuil <hverkuil@xs4all.nl>,
	Hans de Goede <hdegoede@redhat.com>,
	Hugh Dickins <hughd@google.com>, Ingo Molnar <mingo@redhat.com>,
	Jakub Kicinski <kuba@kernel.org>, Jaroslav Kysela <perex@perex.cz>,
	Jason Gunthorpe <jgg@ziepe.ca>, Jens Axboe <axboe@kernel.dk>,
	Jiri Pirko <jiri@resnulli.us>, Jiri Slaby <jirislaby@kernel.org>,
	Kalle Valo <kvalo@kernel.org>, Karsten Graul <kgraul@linux.ibm.com>,
	Karsten Keil <isdn@linux-pingi.de>,
	Kees Cook <keescook@chromium.org>,
	Leon Romanovsky <leon@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Martin Habets <habetsm.xilinx@gmail.com>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Michal Simek <monstr@monstr.eu>,
	Nicholas Piggin <npiggin@gmail.com>,
	Oliver Neukum <oneukum@suse.com>, Paolo Abeni <pabeni@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ping-Ke Shih <pkshih@realtek.com>, Rich Felker <dalias@libc.org>,
	Rob Herring <robh@kernel.org>, Robin Murphy <robin.murphy@arm.com>,
	Sean Christopherson <seanjc@google.com>,
	Shuai Xue <xueshuai@linux.alibaba.com>,
	Stanislaw Gruszka <stf_xl@wp.pl>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Valentin Schneider <vschneid@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wenjia Zhang <wenjia@linux.ibm.com>, Will Deacon <will@kernel.org>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	GR-QLogic-Storage-Upstream@marvell.com, alsa-devel@alsa-project.org,
	ath10k@lists.infradead.org, dmaengine@vger.kernel.org,
	iommu@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-arm-msm@vger.kernel.org,
	linux-block@vger.kernel.org, linux-bluetooth@vger.kernel.org,
	linux-hyperv@vger.kernel.org, linux-m68k@lists.linux-m68k.org,
	linux-media@vger.kernel.org, linux-mips@vger.kernel.org,
	linux-net-drivers@amd.com, linux-pci@vger.kernel.org,
	linux-rdma@vger.kernel.org, linux-s390@vger.kernel.org,
	linux-scsi@vger.kernel.org, linux-serial@vger.kernel.org,
	linux-sh@vger.kernel.org, linux-sound@vger.kernel.org,
	linux-usb@vger.kernel.org, linux-wireless@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, mpi3mr-linuxdrv.pdl@broadcom.com,
	netdev@vger.kernel.org, sparclinux@vger.kernel.org, x86@kernel.org
Cc: Jan Kara <jack@suse.cz>,
	Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>,
	Matthew Wilcox <willy@infradead.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>,
	Alexey Klimov <klimov.linux@gmail.com>,
	Bart Van Assche <bvanassche@acm.org>,
	Sergey Shtylyov <s.shtylyov@omp.ru>
Subject: Re: [PATCH v3 00/35] bitops: add atomic find_bit() operations
Message-ID: <ZX4bIisLzpW8c4WM@yury-ThinkPad>
References: <20231212022749.625238-1-yury.norov@gmail.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20231212022749.625238-1-yury.norov@gmail.com>

On Mon, Dec 11, 2023 at 06:27:14PM -0800, 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);
> 
> Obviously, the less routine code people have to write themself, the
> less probability to make a mistake.
> 
> Those are not only handy helpers but also resolve a non-trivial
> issue of using non-atomic find_bit() together with atomic
> test_and_{set,clear)_bit().
> 
> The trick is that find_bit() implies that the bitmap is a regular
> non-volatile piece of memory, and compiler is allowed to use such
> optimization techniques like re-fetching memory instead of caching it.
> 
> For example, find_first_bit() is implemented like this:
> 
>       for (idx = 0; idx * BITS_PER_LONG < sz; idx++) {
>               val = addr[idx];
>               if (val) {
>                       sz = min(idx * BITS_PER_LONG + __ffs(val), sz);
>                       break;
>               }
>       }
> 
> On register-memory architectures, like x86, compiler may decide to
> access memory twice - first time to compare against 0, and second time
> to fetch its value to pass it to __ffs().
> 
> When running find_first_bit() on volatile memory, the memory may get
> changed in-between, and for instance, it may lead to passing 0 to
> __ffs(), which is undefined. This is a potentially dangerous call.
> 
> find_and_clear_bit() as a wrapper around test_and_clear_bit()
> naturally treats underlying bitmap as a volatile memory and prevents
> compiler from such optimizations.
> 
> Now that KCSAN is catching exactly this type of situations and warns on
> undercover memory modifications. We can use it to reveal improper usage
> of find_bit(), and convert it to atomic find_and_*_bit() as appropriate.
> 
> In some cases concurrent operations with plain find_bit() are acceptable.
> For example:
> 
>  - 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 the first) set or clear
> bit, correspondingly.
> 
> In such cases, KCSAN may be safely silenced with data_race(). But in most
> cases where KCSAN detects concurrency people should carefully review their
> code and likely protect critical sections or switch to atomic
> find_and_bit(), as appropriate.
> 
> The 1st patch of the series adds the following atomic primitives:
> 
> 	find_and_set_bit(addr, nbits);
> 	find_and_set_next_bit(addr, nbits, start);
> 	...
> 
> Here find_and_{set,clear} part refers to the corresponding
> test_and_{set,clear}_bit function. Suffixes like _wrap or _lock
> derive their semantics from corresponding find() or test() functions.
> 
> For brevity, the naming omits the fact that we search for zero bit in
> find_and_set, and correspondingly search for set bit in find_and_clear
> functions.
> 
> The patch also adds iterators with atomic semantics, like
> for_each_test_and_set_bit(). Here, the naming rule is to simply prefix
> corresponding atomic operation with 'for_each'.
> 
> In [1] Jan 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.
> 
> 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.
> 
> The 1st patch of this series adds atomic find_and_bit() API, 2nd adds
> a basic test for new API, and all the following patches spread it over
> the kernel.
> 
> They can be applied separately from each other on per-subsystems basis,
> or I can pull them in bitmap tree, as appropriate.
> 
> [1] https://lore.kernel.org/lkml/634f5fdf-e236-42cf-be8d-48a581c21660@alu.unizg.hr/T/#m3e7341eb3571753f3acf8fe166f3fb5b2c12e615
 
Thank you all for reviews and comments. Now moving the series to
bitmap-for-next for testing.

Thanks,
Yury