Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, May 14, 2020 at 03:35:58PM +0200, Marco Elver wrote:
> On Thu, 14 May 2020 at 13:05, Will Deacon <will@kernel.org> wrote:
> >
> > Hi Marco,
> >
> > On Thu, May 14, 2020 at 09:31:49AM +0200, Marco Elver wrote:
> > > Ouch. With the __{READ,WRITE}_ONCE requirement, we're going to need
> > > Clang 11 though.
> > >
> > > Because without the data_race() around __*_ONCE,
> > > arch_atomic_{read,set} will be broken for KCSAN, but we can't have
> > > data_race() because it would still add
> > > kcsan_{enable,disable}_current() calls to __no_sanitize functions (if
> > > compilation unit is instrumented). We can't make arch_atomic functions
> > > __no_sanitize_or_inline, because even in code that we want to
> > > sanitize, they should remain __always_inline (so they work properly in
> > > __no_sanitize functions). Therefore, Clang 11 with support for
> > > distinguishing volatiles will be the compiler that will satisfy all
> > > the constraints.
> > >
> > > If this is what we want, let me prepare a series on top of
> > > -tip/locking/kcsan with all the things I think we need.
> >
> > Stepping back a second, the locking/kcsan branch is at least functional at
> > the moment by virtue of KCSAN_SANITIZE := n being used liberally in
> > arch/x86/. However, I still think we want to do better than that because (a)
> > it would be good to get more x86 coverage and (b) enabling this for arm64,
> > where objtool is not yet available, will be fragile if we have to whitelist
> > object files. There's also a fair bit of arm64 low-level code spread around
> > drivers/, so it feels like we'd end up with a really bad case of whack-a-mole.
> >
> > Talking off-list, Clang >= 7 is pretty reasonable wrt inlining decisions
> > and the behaviour for __always_inline is:
> >
> >   * An __always_inline function inlined into a __no_sanitize function is
> >     not instrumented
> >   * An __always_inline function inlined into an instrumented function is
> >     instrumented
> >   * You can't mark a function as both __always_inline __no_sanitize, because
> >     __no_sanitize functions are never inlined
> >
> > GCC, on the other hand, may still inline __no_sanitize functions and then
> > subsequently instrument them.
> >
> > So if were willing to make KCSAN depend on Clang >= 7, then we could:
> >
> >   - Remove the data_race() from __{READ,WRITE}_ONCE()
> >   - Wrap arch_atomic*() in data_race() when called from the instrumented
> >     atomic wrappers
> >
> > At which point, I *think* everything works as expected. READ_ONCE_NOCHECK()
> > won't generate any surprises, and Peter can happily use arch_atomic()
> > from non-instrumented code.
> >
> > Thoughts? I don't see the need to support buggy compilers when enabling
> > a new debug feature.
> 
> This is also a reply to
> https://lkml.kernel.org/r/20200514122038.GH3001@hirez.programming.kicks-ass.net
> -- the problem with __READ_ONCE would be solved with what Will
> proposed above.
> 
> Let me try to spell out the requirements I see so far (this is for
> KCSAN only though -- other sanitizers might be similar):
> 
>   1. __no_kcsan functions should not call anything, not even
> kcsan_{enable,disable}_current(), when using __{READ,WRITE}_ONCE.
> [Requires leaving data_race() off of these.]
> 
>   2. __always_inline functions inlined into __no_sanitize function is
> not instrumented. [Has always been satisfied by GCC and Clang.]
> 
>   3. __always_inline functions inlined into instrumented function is
> instrumented. [Has always been satisfied by GCC and Clang.]
> 
>   4. __no_kcsan functions should never be spuriously inlined into
> instrumented functions, causing the accesses of the __no_kcsan
> function to be instrumented. [Satisfied by Clang >= 7. All GCC
> versions are broken.]
> 
>   5. we should not break atomic_{read,set} for KCSAN. [Because of #1,
> we'd need to add data_race() around the arch-calls in
> atomic_{read,set}; or rely on Clang 11's -tsan-distinguish-volatile
> support (GCC 11 might get this as well).]
> 
>   6. never emit __tsan_func_{entry,exit}. [Clang supports disabling
> this, GCC doesn't.]
> 
>   7. kernel is supported by compiler. [Clang >= 9 seems to build -tip
> for me, anything below complains about lack of asm goto. GCC trivial.]
> 
> So, because of #4 & #6 & #7 we're down to Clang >= 9. Because of #5
> we'll have to make a choice between Clang >= 9 or Clang >= 11
> (released in ~June). In an ideal world we might even fix GCC in
> future.
> 
> That's not even considering the problems around UBSan and KASAN. But
> maybe one step at a time?
> 
> Any preferences?

I am already having to choose where I run KCSAN based on what compiler
is available, so I cannot argue too hard against a dependency on a
specific compiler.  I reserve the right to ask for help installing it,
if need be though.  ;-)

							Thanx, Paul