Messages in this thread | | | Date | Tue, 9 Nov 2021 20:22:30 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH 1/7] static_call: get rid of static_call_cond() |
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On Tue, Nov 09, 2021 at 07:41:03PM +0100, Ard Biesheuvel wrote: > On Tue, 9 Nov 2021 at 19:38, Peter Zijlstra <peterz@infradead.org> 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.
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