2024年9月29日 07:55，Boqun Feng <boqun.feng@xxxxxxxxx> wrote： > > > > 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. Yeah, barrier_data generates more instructions. > > 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
