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. > > 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 > >
