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Thread-local contributions

Thread-local contributions. K. Rustan M. Leino and Peter Müller Microsoft Research. IFIP WG 2.3 meeting 48 22 July 2008 Cambridge, UK. Concurrency framework for verification. Each memory location has a possibly fractional permission for each thread

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Thread-local contributions

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  1. Thread-localcontributions K. Rustan M. Leino andPeter Müller Microsoft Research IFIP WG 2.3 meeting 4822 July 2008Cambridge, UK

  2. Concurrency framework for verification • Each memory location has a possibly fractional permission for each thread • Permission = 1  thread can read and write • Permission > 0  thread can read • Can handle readers-writers locks • Locking order can be changed • Can handle algorithms that rebalance trees where each node is locked • Monitors have monitor invariants • Invariant is admissible if monitor has appropriate read permission • Example: invariant CanRead(x)  x < 100; • History constraints (two-state invariants) • Not worked out in detail • Could be used to allow monitor invariants to mention stable conditions, even without read permission • Can aid in rely / guarantee reasoning • Thread contributions…

  3. Owicki and Gries, 1976 { x = 0 } begin var y := 0, z := 0; resource r(x,y,z) invariant x=y+z; cobegin {y=0} lock r do x := x + 1; y := 1 end {y=1} || {z=0} lock r do x := x + 1; z := 1 end {z=1} coend {y=1  z=1  x=y+z} end { x = 2 }

  4. More threads { x = 0 } begin var y: array [0,N) ofint, a: array [0,N) of C; foreachiin [0,N) do y[ i ] := 0; end; share r(x,y) invariant x = ( iin [0,N) :: y[ i ]); foreachiin [0,N) do a[ i ] := new C; a[ i ].k := i; fork a[ i ]; end; foreachiin [0,N) do join a[ i ]; { y[ i ] = 1 } end; unshare r(x,y); end { x = N } class C { var k: int; run() ensures y[k] = old(y)[k] + 1; { lock r do x := x + 1; y[k] := y[k] + 1 end } }

  5. More threads begin var r := new R(); var a: array [0,N) of C; share r; foreachiin [0,N) do a[ i ] := new C; a[ i ].r := r; fork a[ i ]; end; foreachiin [0,N) do join a[ i ]; { a[ i ].y = 1 } end; unshare r; { r.x = N } end class R { var x: int; monitorinvariant x = ( iin [0,N) :: a[ i ].y); } class C { var r: R, y: int := 0; run() ensures y = old(y) + 1; { lock r dor.x := r.x + 1; y := y + 1 end } }

  6. Idea • Programs can only refer to global state and thread-local state of the current thread • Invariants that tie together thread-local state of several threads comes from the methodology

  7. Thread contributions varyglobal: int; for each thread th: varyth: int; class R { var x: int; contributionvar y: int; monitorinvariant x = y; } class C { var r: R; run() ensuresr.y = old(r.y) + 1; { lock r do r.x := r.x + 1; contribute 1; end } } system invariant:yglobal = ( active threads th :: yth) yglobal commutative, associative ytid yglobal := yglobal + 1; ytid := ytid + 1;

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