<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">What's more, volatile has very poorly defined semantics.<div>We do now expose CAS/CASP in experimental form, and those have the desirable behavior on most targets.</div><div><div><br><div> <span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; 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"><span class="Apple-style-span" style="font-family: 'Gill Sans'; ">Associate Professor</span></span><span class="Apple-style-span" style="font-family: 'Gill Sans'; "><span class="Apple-style-span" style="font-family: 'Gill Sans'; "> | Computer Science | Purdue University</span></span></font></div><div><font class="Apple-style-span" face="GillSans-Light"><span class="Apple-style-span" style="font-family: GillSans-Light; ">305 N. University Street | West Lafayette | IN 47907 | USA</span></font></div><div><font class="Apple-style-span" color="#0000FF" face="Gill Sans"><span class="Apple-style-span" style="color: rgb(0, 0, 255); font-family: 'Gill Sans'; "><span class="Apple-style-span" style="color: rgb(0, 0, 255); font-family: 'Gill Sans'; ">Office</span></span></font><font class="Apple-style-span" face="GillSans-Light"><span class="Apple-style-span" style="font-family: GillSans-Light; "><span class="Apple-style-span" style="font-family: GillSans-Light; "> +1 765 494 6001 |<span class="Apple-converted-space"> </span></span></span></font><font class="Apple-style-span" color="#0000FF" face="Gill Sans"><span class="Apple-style-span" style="color: rgb(0, 0, 255); font-family: 'Gill Sans'; "><span class="Apple-style-span" style="color: rgb(0, 0, 255); font-family: 'Gill Sans'; ">Mobile</span></span></font><font class="Apple-style-span" face="GillSans-Light"><span class="Apple-style-span" style="font-family: GillSans-Light; "><span class="Apple-style-span" style="font-family: GillSans-Light; "><span class="Apple-converted-space"> </span>+1 765 427 5484</span></span></font></div><div><font class="Apple-style-span" face="GillSans-Light"><br class="khtml-block-placeholder"></font></div></span></span></span></span></span></span></span><br class="Apple-interchange-newline"></span></div></span></div></span><br class="Apple-interchange-newline"> </div><br><div><div>On 12 Nov 2009, at 14:50, <a href="mailto:jay.krell@cornell.edu">jay.krell@cornell.edu</a> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>Yes volatile can be useful, but Modula-3 does not really expose it.<br><br> - Jay (phone)<br><br>On Nov 12, 2009, at 9:28 AM, <a href="mailto:hendrik@topoi.pooq.com">hendrik@topoi.pooq.com</a> wrote:<br><br><blockquote type="cite">On Thu, Nov 12, 2009 at 04:17:12AM +0000, Jay K wrote:<br></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">I do currently still use the critical section "throughout 'all' of AssignSlot".<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">The memory barrier doesn't replace it there. Just makes it carefully<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">ordered so other readers see things reasonably well.<br></blockquote></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">Just curious: C had "volatile" variables, meaning variables whose<br></blockquote><blockquote type="cite">writes and reads cannot be reordered or deleted by an optimiser.<br></blockquote><blockquote type="cite">They're intended for things like memory-mapped I/O. Wouldn't they be<br></blockquote><blockquote type="cite">the kinds of things we need here? How does the gcc intermediate code<br></blockquote><blockquote type="cite">represent them? By memory barriers? Or by some other means?<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite">-- hendrik<br></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">I DO suspect the locking in AssignSlot can be reduced but haven't done so yet.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Maybe it doesn't work out.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">- Jay<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">From: <a href="mailto:hosking@cs.purdue.edu">hosking@cs.purdue.edu</a><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">To: <a href="mailto:jay.krell@cornell.edu">jay.krell@cornell.edu</a><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Date: Wed, 11 Nov 2009 09:23:59 -0500<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">CC: <a href="mailto:m3devel@elegosoft.com">m3devel@elegosoft.com</a><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Subject: Re: [M3devel] FreeSlots/lookup slot locking<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">It's not the allocation I worry about.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">How do you avoid two threads getting the same slot?<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">On 11 Nov 2009, at 01:11, Jay K wrote:<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Tony:<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><blockquote type="cite">Also, I am unconvinced that the current implementation of AssignSlot can ever<br></blockquote></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><blockquote type="cite">be correct without the critical section. It requires atomic update of both<br></blockquote></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><blockquote type="cite">the slots array pointer (with the new slots) *and* the array elements. This<br></blockquote></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><blockquote type="cite">requires a proper CS instead of non-blocking synchronization.<br></blockquote></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">I don't see it.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">I could be wrong.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">The writes are done within a critical section, to avoid racing with other writers.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">Writes interact with readers in that:<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"> SUBARRAY (new_slots^, 0, n) := slots^;<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">finish writing to the array elements<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">MemoryBarrier<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"> slots := new_slots;<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">so readers don't see slots unless it has been fully initialized.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">AssignSlots should also be able to be lock free via InterlockedCompareExchangePointer.<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite">- Jay<br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><blockquote type="cite"><br></blockquote></blockquote><blockquote type="cite"><br></blockquote></div></blockquote></div><br></div></div></body></html>