It's up to the programmer-user to impose thread-safety around an array or other data structure. The compiler doesn't know if you want to allow multiple threads to write and read multiple array members, or prevent all but one thread to read and write multiple array members, or allow multiple readers when there are no writers, etc. (And we have queues precisely for these scenarios.)
The mutation counter detects if modifications happen while iterating over a collection, modifications like adding or removing members. When this happens, it's probably a programmer error. It's a bonus if this can be detected in a multiple thread scenario. -- C. Keith Ray * https://leanpub.com/wepntk <- buy my book? * http://www.thirdfoundationsw.com/keith_ray_resume_2014_long.pdf * http://agilesolutionspace.blogspot.com/ > On Jan 8, 2018, at 6:25 PM, Fadi Botros via swift-users > <swift-users@swift.org> wrote: > > What about double checking? > > > > Something like that > > if counter != 0 { throw exc } > counter += 1 > if counter != 1 { throw exc } > counter += 1 > > I think the performance penalty is just a check and an increment > It could be disabled after optimization or in release version to increase > performance > > > من: Greg Parker <gpar...@apple.com> > إلى: Chris Lattner <clatt...@nondot.org> > نسخة كربونية: Jordan Rose <jordan_r...@apple.com>; swift-users > <swift-users@swift.org>; Fadi Botros <botros_f...@yahoo.com> > تاريخ الإرسال: الثلاثاء 9 يناير، 2018 3:47 ص > الموضوع: Re: [swift-users] بخصوص: Swift 4.0 bug in concurrent array access > > >>> On Jan 8, 2018, at 5:20 PM, Chris Lattner <clatt...@nondot.org> wrote: >>> >>>> On Jan 8, 2018, at 2:59 PM, Greg Parker via swift-users >>>> <swift-users@swift.org> wrote: >>>> >>>>> On Jan 8, 2018, at 12:51 PM, Jordan Rose via swift-users >>>>> <swift-users@swift.org> wrote: >>>>> >>>>> On Jan 8, 2018, at 11:47, Fadi Botros <botros_f...@yahoo.com> >>>>> wrote: >>>>> >>>>> 1st: How to invoke the thread sanitizer? >>>> >>>> Check out this article on developer.apple.com: >>>> https://developer.apple.com/documentation/code_diagnostics/thread_sanitizer/enabling_the_thread_sanitizer >>>> >>>>> 2nd: I think making something like ConcurrentModificationException of >>>>> Java should be a little better experience (if it will not make, or make a >>>>> small performance penalty, also if it makes performance penalty, it would >>>>> be activated only when you make a debug version or non-optimized version) >>>> >>>> I don't exactly disagree, but my understanding is that the work to do such >>>> a thing is nearly equivalent to having thread sanitizer on all the time. >>>> But maybe there's a simpler model that we could still turn on in debug >>>> builds. Can you file a bug report requesting this now that bugs.swift.org >>>> is back up? >>> >>> Something like ConcurrentModificationException ought to be cheaper than the >>> thread sanitizer. The thread sanitizer works hard to detect every >>> concurrency error. ConcurrentModificationException typically does something >>> simple and cheap that catches errors sometimes but makes no attempt to be >>> exhaustive. >>> >>> Objective-C's fast enumeration protocol includes a mechanism like this. The >>> collection can have a simple mutation counter, and the enumerator captures >>> the counter's value at the start and checks that the value is unchanged as >>> the enumeration proceeds. >> >> Does ObjC synchronize the mutation counter? My understanding is that that >> counter was used to detect mutation while iteration on a single thread, not >> across threads. > > The mutation counter is not synchronized; this is safe for correct code. An > un-synchronized counter can still sometimes catch mutations from other > threads. > > > -- > Greg Parker > gpar...@apple.com Runtime Wrangler > > > > > > > _______________________________________________ > swift-users mailing list > swift-users@swift.org > https://lists.swift.org/mailman/listinfo/swift-users
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