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{font-size:12pt;font-family:Calibri;}</style>That is not what I meant. That is ok.<br><div dir="ltr">There is no guarantee that all of Text8.New will execute before its result is stored in the global.<br>Another thread could get back the non-NIL global, that Text8.New hasn't finished initializing.<br><br><br> - Jay<br><br><br><div><div id="ecxSkyDrivePlaceholder"></div>> To: jay.krell@cornell.edu<br>> CC: m3devel@elegosoft.com<br>> Subject: Re: [M3devel] race conditions in lock-free code...<br>> Date: Sun, 26 Aug 2012 21:23:51 -0700<br>> From: mika@async.caltech.edu<br>> <br>> <br>> Yeah it's a race condition, in theory. But is it important? If you get<br>> the wrong copy, there will be two TEXTs representing one CHAR. But no<br>> one ever said FromChar was guaranteed to return the same pointer when<br>> you call it with the same CHAR...<br>> <br>> Mika<br>> <br>> Jay K writes:<br>> >--_5f23896e-68ee-44da-82dd-311cd8c58979_<br>> >Content-Type: text/plain; charset="iso-8859-1"<br>> >Content-Transfer-Encoding: quoted-printable<br>> ><br>> >We have race conditions=2C like this=2C m3core/src/Text.m3:<br>> ><br>> ><br>> >VAR fromCharCache :=3D ARRAY CHAR OF T {NIL=2C ..}=3B (* 1-char texts *)<br>> ><br>> ><br>> >PROCEDURE FromChar (c: CHAR): T =3D<br>> > VAR buf: ARRAY [0..0] OF CHAR=3B<br>> > BEGIN<br>> > IF fromCharCache [c] =3D NIL THEN<br>> > buf [0] :=3D c=3B<br>> > fromCharCache[c] :=3D Text8.New (buf)=3B<br>> > END=3B<br>> > RETURN fromCharCache [c]<br>> > END FromChar=3B<br>> ><br>> ><br>> >It should be:<br>> ><br>> ><br>> >PROCEDURE FromChar (c: CHAR): T =3D<br>> >=0A=<br>> > VAR buf: ARRAY [0..0] OF CHAR=3B<br>> >=0A=<br>> > BEGIN<br>> >=0A=<br>> > IF fromCharCache [c] =3D NIL THEN<br>> >=0A=<br>> > buf [0] :=3D c=3B<br>> >=0A=<br>> > WITH a =3D Text8.New (buf) DO<br>> > MemoryBarrier()=3B<br>> > fromCharCache[c] :=3D a=3B<br>> > END=3B<br>> >=0A=<br>> > END=3B<br>> >=0A=<br>> > RETURN fromCharCache [c]<br>> >=0A=<br>> > END FromChar=3B<br>> >=0A=<br>> ><br>> ><br>> >to ensure that all of Text8.New() finishes before the assignment to fromCha=<br>> >rCache[c] is made.<br>> ><br>> ><br>> >Can the compiler somehow catch these?<br>> >I fear they are a small epidemic.<br>> >For a long time people didn't realize where all the compiler and processor =<br>> >could reorder.<br>> ><br>> ><br>> >Do we have the right constructs by now to fix them?<br>> >I think we do.<br>> ><br>> ><br>> > - Jay<br>> > =<br>> ><br>> >--_5f23896e-68ee-44da-82dd-311cd8c58979_<br>> >Content-Type: text/html; charset="iso-8859-1"<br>> >Content-Transfer-Encoding: quoted-printable<br>> ><br>> ><html><br>> ><head><br>> ><style><!--<br>> >.hmmessage P<br>> >{<br>> >margin:0px=3B<br>> >padding:0px<br>> >}<br>> >body.hmmessage<br>> >{<br>> >font-size: 12pt=3B<br>> >font-family:Calibri<br>> >}<br>> >--></style></head><br>> ><body class=3D'hmmessage'><div dir=3D'ltr'>We have race conditions=2C like =<br>> >this=2C m3core/src/Text.m3:<br><br><br>VAR fromCharCache :=3D ARRAY CHAR OF=<br>> > T {NIL=2C ..}=3B (* 1-char texts *)<br><br><br>PROCEDURE FromChar (c: CHAR=<br>> >): T =3D<br> =3B VAR buf: ARRAY [0..0] OF CHAR=3B<br> =3B BEGIN<br>=<br>> > =3B =3B =3B IF fromCharCache [c] =3D NIL THEN<br> =3B =<br>> >=3B =3B =3B =3B buf [0] :=3D c=3B<br> =3B =3B =3B&n=<br>> >bsp=3B =3B fromCharCache[c] :=3D Text8.New (buf)=3B<br> =3B =3B=<br>> > =3B END=3B<br> =3B =3B =3B RETURN fromCharCache [c]<br>&nb=<br>> >sp=3B END FromChar=3B<br><br><br>It should be:<br><br><br>PROCEDURE FromCha=<br>> >r (c: CHAR): T =3D<br>=0A=<br>> > =3B VAR buf: ARRAY [0..0] OF CHAR=3B<br>=0A=<br>> > =3B BEGIN<br>=0A=<br>> > =3B =3B =3B IF fromCharCache [c] =3D NIL THEN<br>=0A=<br>> > =3B =3B =3B =3B =3B buf [0] :=3D c=3B<br>=0A=<br>> > =3B =3B =3B =3B =3B WITH a =3D Text8.New (buf) DO<br>&=<br>> >nbsp=3B =3B =3B =3B =3B =3B =3B MemoryBarrier()=3B<=<br>> >br> =3B =3B =3B =3B =3B =3B =3B fromCharCache[c=<br>> >] :=3D a=3B<br> =3B =3B =3B =3B END=3B<br>=0A=<br>> > =3B =3B =3B END=3B<br>=0A=<br>> > =3B =3B =3B RETURN fromCharCache [c]<br>=0A=<br>> > =3B END FromChar=3B<br>=0A=<br>> ><br><br>to ensure that all of Text8.New() finishes before the assignment to=<br>> > fromCharCache[c] is made.<br><br><br>Can the compiler somehow catch these?=<br>> ><br>I fear they are a small epidemic.<br>For a long time people didn't real=<br>> >ize where all the compiler and processor could reorder.<br><br><br>Do we ha=<br>> >ve the right constructs by now to fix them?<br>I think we do.<br><br><br> -=<br>> > Jay<br> </div></body><br>> ></html>=<br>> ><br>> >--_5f23896e-68ee-44da-82dd-311cd8c58979_--<br></div> </div></div> </div></body>
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