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Tue, 9 Nov 2021 20:22:30 +0100 (CET) Date: Tue, 9 Nov 2021 20:22:30 +0100 From: Peter Zijlstra To: Ard Biesheuvel Cc: Linux Kernel Mailing List , Linux ARM , Josh Poimboeuf , Jason Baron , Steven Rostedt , Mark Rutland , Kees Cook Subject: Re: [RFC PATCH 1/7] static_call: get rid of static_call_cond() Message-ID: References: <20211109164549.1724710-1-ardb@kernel.org> <20211109164549.1724710-2-ardb@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Nov 09, 2021 at 07:41:03PM +0100, Ard Biesheuvel wrote: > On Tue, 9 Nov 2021 at 19:38, Peter Zijlstra wrote: > > > > On Tue, Nov 09, 2021 at 05:45:43PM +0100, Ard Biesheuvel wrote: > > > The main reason for the existence of static_call_cond() seems to be that > > > in theory, when using the generic implementation of static calls, it may > > > be possible for a compiler to elide the indirect call entirely if the > > > target is NULL, while still guaranteeing that all side effects of > > > argument evaluation occur as expected. > > > > > > This is rather optimistic: as documented by an existing code comment, > > > both GCC and Clang (version 10) get this wrong, and even if they ever > > > get it right, this is far too subtle to rely on for a code path that is > > > expected to be used only by the 'remaining' architectures once all the > > > best supported ones implement either the out-of-line or inline optimized > > > variety of static calls. > > > > > > Given that having static_call_cond() clutters up the API, and puts the > > > burden on the caller to go and check what kind of static call they are > > > dealing with, let's just get rid of the distinction. > > > > No, static_call_cond() signifies the function can be NULL. Both gcc and > > clang generate correct (but wildly ineffecient) code for this. Without > > static_call_cond() the generic implementation will do a NULL deref. > > > > That is, static_call_cond() does properly encapuslate: > > > > func = READ_ONCE(key.func); > > if (func) > > func(ARGS); > > > > You can't take that out. > > I actually address that in the patch. > > AIUI, the compiler generates an indirect call to __static_call_nop(), > right? So why not simply set .func to the address of > __static_call_nop() when NULL is passed to update / the initializer? Ooh, lemme go have a proper look then. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel