Messages in this thread |  | | Date | Tue, 04 Oct 2016 14:28:00 -0400 | From | bdegraaf@codeauro ... | Subject | Re: [RFC] arm64: Enforce observed order for spinlock and data |
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On 2016-10-04 13:53, bdegraaf@codeaurora.org wrote: > On 2016-10-04 06:12, Mark Rutland wrote: >> On Mon, Oct 03, 2016 at 03:20:57PM -0400, bdegraaf@codeaurora.org >> wrote: >>> On 2016-10-01 14:11, Mark Rutland wrote: >>> >Hi Brent, >>> > >>> >Evidently my questions weren't sufficiently clear; even with your >>> >answers it's not clear to me what precise issue you're attempting to >>> >solve. I've tried to be more specific this time. >>> > >>> >At a high-level, can you clarify whether you're attempting to solve is: >>> > >>> >(a) a functional correctness issue (e.g. data corruption) >>> >(b) a performance issue >>> > >>> >And whether this was seen in practice, or found through code >>> >inspection? >> >>> Thinking about this, as the reader/writer code has no known "abuse" >>> case, I'll remove it from the patchset, then provide a v2 patchset >>> with a detailed explanation for the lockref problem using the commits >>> you provided as an example, as well as performance consideration. >> >> If there's a functional problem, let's consider that in isolation >> first. >> Once we understand that, then we can consider doing what is optimal. >> >> As should be obvious from the above, I'm confused because this patch >> conflates functional details with performance optimisations which (to >> me) sound architecturally dubious. >> >> I completely agree with Peter that if the problem lies with lockref, >> it >> should be solved in the lockref code. >> >> Thanks, >> Mark. > > After looking at this, the problem is not with the lockref code per se: > it is > a problem with arch_spin_value_unlocked(). In the out-of-order case, > arch_spin_value_unlocked() can return TRUE for a spinlock that is in > fact > locked but the lock is not observable yet via an ordinary load. Other > than > ensuring order on the locking side (as the prior patch did), there is a > way > to make arch_spin_value_unlock's TRUE return value deterministic, but > it > requires that it does a write-back to the lock to ensure we didn't > observe > the unlocked value while another agent was in process of writing back a > locked value. > > Brent
Scratch that--things get complicated as the lock itself gets "cloned," which could happen during the out-of-order window. I'll post back later after I've analyzed it fully.
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