Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 30 Aug 2019 08:48:49 -0700 | Subject | Re: objtool warning "uses BP as a scratch register" with clang-9 |
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On Fri, Aug 30, 2019 at 8:02 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote: > > For KASAN, the Clang threshold for inserting memset() is *2* consecutive > writes instead of 17. Isn't that likely to cause tearing-related > surprises?
Tearing isn't likely to be a problem.
It's not like memcpy() does byte-by-byte copies. If you pass it a word-aligned pointer, it will do word-aligned accesses simply for performance reasons.
Even on x86, where we use "rep movsb", we (a) tend to disable it for small copies and (b) it turns out that microcode that does the optimized movsb (which is the only case we use it) probably ends up doing atomic things anyway. Note the "probably". I don't have microcode source code, but there are other indications like "we know it doesn't take interrupts on a byte-per-byte level, only on the cacheline level".
So it's probably not an issue from a tearing standpoint - but it worries me because of "this has to be a leaf function" kind of issues where we may be using individual stores on purpose. We do have things like that.
Linus
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