From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Lobakin Subject: Re: [PATCH v2 2/6] bitops: always define asm-generic non-atomic bitops Date: Mon, 13 Jun 2022 16:19:47 +0200 Message-ID: <20220613141947.1176100-1-alexandr.lobakin@intel.com> References: <20220610113427.908751-1-alexandr.lobakin@intel.com> <20220610113427.908751-3-alexandr.lobakin@intel.com> <22042c14bc6a437d9c6b235fbfa32c8a@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1655130093; x=1686666093; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version:content-transfer-encoding; bh=kRJcWACmO/lwAN79tjxhatvQLKzpV/YPdBZo5g5JtF4=; b=Ilxw8Xw2W7MusTNqmJ+s0Uz+bLzQzZ3jhilka9H0oN4yHst6U+MfWoKB ywRhtSZMzgu7n/E8qH3qsXUF+snSIOalZY/VcEirFrW+tWov2qD73wnsX TOcmB6fK/mcFh4BIsfi1o3FtWgVt/OTiJHpVXK7FQuYb2GjC0ouv0hbvU ot/V/fz2M7Q5yefBLLZt98Zd6j1oNKEa/6eMpnYEnnRDXYu7Ix2X6KxGj tEizP98o3RZI/R+MM1vCv5pZCzggcGKz6X0xZYEp4TxQgrEqCRss6OCrn pxmnEeDpO0ygQDFtbBqgCoUMtWaA8ZmjpShIOnsdKAZslBVdI1LtAZGxA A==; In-Reply-To: List-ID: To: Marco Elver Cc: Alexander Lobakin , Tony Luck , Andy Shevchenko , Arnd Bergmann , Yury Norov , Mark Rutland , Matt Turner , Brian Cain , Geert Uytterhoeven , Yoshinori Sato , Rich Felker , "David S. Miller" , Kees Cook , "Peter Zijlstra (Intel)" , Borislav Petkov , Greg Kroah-Hartman , "linux-alpha@vger.kernel.org" , "linux-hexagon@vger.kernel.org" From: Marco Elver Date: Fri, 10 Jun 2022 18:32:36 +0200 > On Fri, 10 Jun 2022 at 18:02, Luck, Tony wrote: > > > > > > +/** > > > > + * generic_test_bit - Determine whether a bit is set > > > > + * @nr: bit number to test > > > > + * @addr: Address to start counting from > > > > + */ > > > > > > Shouldn't we add in this or in separate patch a big NOTE to explain that this > > > is actually atomic and must be kept as a such? > > > > "atomic" isn't really the right word. The volatile access makes sure that the > > compiler does the test at the point that the source code asked, and doesn't > > move it before/after other operations. > > It's listed in Documentation/atomic_bitops.txt. Oh, so my memory was actually correct that I saw it in the docs somewhere. WDYT, should I mention this here in the code (block comment) as well that it's atomic and must not lose `volatile` as Andy suggested or it's sufficient to have it in the docs (+ it's not underscored)? > > It is as "atomic" as READ_ONCE() or atomic_read() is. Though you are > right that the "atomicity" of reading one bit is almost a given, > because we can't really read half a bit. > The main thing is that the compiler keeps it "atomic" and e.g. doesn't > fuse the load with another or elide it completely, and then transforms > the code in concurrency-unfriendly ways. > > Like READ_ONCE() and friends, test_bit(), unlike non-atomic bitops, > may also be used to dependency-order some subsequent marked (viz. > atomic) operations. > > > But there is no such thing as an atomic test_bit() operation: > > > > if (test_bit(5, addr)) { > > /* some other CPU nukes bit 5 */ > > > > /* I know it was set when I looked, but now, could be anything */ > > The operation itself is atomic, because reading half a bit is > impossible. Whether or not that bit is modified concurrently is a > different problem. > > Thanks, > -- Marco Thanks, Olek