On Sun, Sep 29, 2024, at 6:26 AM, Alan Huang wrote: > 2024年9月28日 23:55，Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote： >> >> On 2024-09-28 17:49, Alan Stern wrote: >>> On Sat, Sep 28, 2024 at 11:32:18AM -0400, Mathieu Desnoyers wrote: >>>> On 2024-09-28 16:49, Alan Stern wrote: >>>>> On Sat, Sep 28, 2024 at 09:51:27AM -0400, Mathieu Desnoyers wrote: >>>>>> equality, which does not preserve address dependencies and allows the >>>>>> following misordering speculations: >>>>>> >>>>>> - If @b is a constant, the compiler can issue the loads which depend >>>>>> on @a before loading @a. >>>>>> - If @b is a register populated by a prior load, weakly-ordered >>>>>> CPUs can speculate loads which depend on @a before loading @a. >>>>> >>>>> It shouldn't matter whether @a and @b are constants, registers, or >>>>> anything else. All that matters is that the compiler uses the wrong >>>>> one, which allows weakly ordered CPUs to speculate loads you wouldn't >>>>> expect it to, based on the source code alone. >>>> >>>> I only partially agree here. >>>> >>>> On weakly-ordered architectures, indeed we don't care whether the >>>> issue is caused by the compiler reordering the code (constant) >>>> or the CPU speculating the load (registers). >>>> >>>> However, on strongly-ordered architectures, AFAIU, only the constant >>>> case is problematic (compiler reordering the dependent load), because >>> I thought you were trying to prevent the compiler from using one pointer >>> instead of the other, not trying to prevent it from reordering anything. >>> Isn't this the point the documentation wants to get across when it says >>> that comparing pointers can be dangerous? >> >> The motivation for introducing ptr_eq() is indeed because the >> compiler barrier is not sufficient to prevent the compiler from >> using one pointer instead of the other. > > barrier_data(&b) prevents that. > It prevents that because it acts as barrier() + OPTIMIZER_HIDE_VAR(b). I don’t see much value of using that since we can resolve the problem with OPTIMIZER_HIDE_VAR() alone. Regards, Boqun >> >> But it turns out that ptr_eq() is also a good tool to prevent the >> compiler from reordering loads in case where the comparison is >> done against a constant. >> >>>> CPU speculating the loads across the control dependency is not an >>>> issue. >>>> >>>> So am I tempted to keep examples that clearly state whether >>>> the issue is caused by compiler reordering instructions, or by >>>> CPU speculation. >>> Isn't it true that on strongly ordered CPUs, a compiler barrier is >>> sufficient to prevent the rcu_dereference() problem? So the whole idea >>> behind ptr_eq() is that it prevents the problem on all CPUs. >> >> Correct. But given that we have ptr_eq(), it's good to show how it >> equally prevents the compiler from reordering address-dependent loads >> (comparison with constant) *and* prevents the compiler from using >> one pointer rather than the other (comparison between two non-constant >> pointers) which affects speculation on weakly-ordered CPUs. >> >>> You can make your examples as specific as you like, but the fact remains >>> that ptr_eq() is meant to prevent situations where both: >>> The compiler uses the wrong pointer for a load, and >>> The CPU performs the load earlier than you want. >>> If either one of those doesn't hold then the problem won't arise. >> >> Correct. >> >> Thanks, >> >> Mathieu >> >> >> -- >> Mathieu Desnoyers >> EfficiOS Inc. >> https://www.efficios.com >> >>
