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s t a t i c bool [ ] flags = new bool [ 2 ] j s t a t i c int t u r n ej
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void E n t e r C r it i c a l Region ( i nt i ) I I i w i l l only ever b e e or 1 { int j = 1 ij flags [ i ] = truej w h i l e ( fl a g s [ j ] ) { if ( t u rn { flags [ i ] = falsej wh i l e ( t u r n = = j ) 1 * b u s y w a i t * I j flags [ i ] = truej } } } v o i d Le aveC r it i c a lRegion ( i nt i ) { turn 1 flags [ i ]
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II t h e ot her t h read ' s i n d e x I I n o t e ou r interest I I wait u n t i l t h e ot h e r i s not inte rested j) I I not o u r t u r n , we m u s t b a c k off a n d wa it
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I I give away t h e t u r n I I a n d exit t h e region
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A common problem with this code is that the inner loop in E nt e r C r it
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ant This means the compiler could hoist the read outside of the loop, leading to a thread busy spinning forever Marking t u r n as v o l a t i l e is sufficient to avoid this problem Similarly, a smart compiler may deduce that i could never equal 1 - i and, therefore, the flags element read in the loop is never written to inside the loop body Once again, the compiler may hoist the read outside of the loop and cause an infinite spinning situation So we need to mark flags as volat ile too Notice some other issues if we weren't to mark things as v o l a t i l e The write of fa l s e to f l a g s [ i ] , just before spinning on turn, could move after the reads and be coalesced with the write of t r u e to f l a g s [ i ] The result would be that we never give away our flag, causing our partner thread to spin forever waiting to see our flag become f a l s e A more fundamental problem i s that, without v o l a t i l e s, the fast-path of E n t e rC r i t i c a l Region causes no fence Imagining the caller loads a vari able immediately after entering the region, this load could be moved before the write to f l a g s [ i ] and before the read of f l a g s [ 1 - i ] , since stores can pass loads This has the effect of removing mutual exclusion: the variables read inside the critical region could be changing concurrently out from underneath us, which could be disastrous
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This chapter covered a lot We began by reviewing instruction reordering and its subtle implications to concurrent programs Processors and some programming models (eg, in the case of NET) make strong guarantees about which operations can freely reorder, making it at least feasible for real human beings to program in a lock free way We then saw the basic mech anisms that can be used for atomic memory operations and how fences limit processors and compilers from reordering certain instructions Finally, we concluded with some examples of safe lock free techniques They were not exhaustive, but at least provide a useful starting point Up next: we'll take a closer look at the types of hazards concurrency can cause
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