Date: Fri, 26 Aug 2022 21:40:19 +0000 From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 265974] SMR has several missing barriers Message-ID: <bug-265974-227-9Pgv5Z0OfB@https.bugs.freebsd.org/bugzilla/> In-Reply-To: <bug-265974-227@https.bugs.freebsd.org/bugzilla/> References: <bug-265974-227@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D265974 --- Comment #6 from Pierre Habouzit <pierre@habouzit.net> --- (In reply to Mark Johnston from comment #4) > The suggested patch adds two barriers to smr_poll_scan() and none to smr_= poll(), is that intentional? Yes, I cover this in my (2) paragraph: > 2. similarly smr_poll_scan() needs a full barrier after the scan _before_= it updates the global rd_seq, this is about serializing the fast path of s= mr_poll with CPUs that weren't in a critical section (while the one on entr= y of smr_poll() is about synchronizing with the CPUs inside an active SMR c= ritical section and was demonstrated in (1)). fundamentally the first barrier is about sequencing the "scan" operation wi= th CPUs actively inside an active SMR section (and basically pairs with their smr_enter()), but the second barrier is about all the sections we ignored because they weren't active and pairs with the last `smr_leave()` that that core executed. Writing the litmus test to exhibit this problem was more work than I was willing to put in. For full disclosure I am the author of XNU's smr.[hc] an= d am reporting this bug as a courtesy given that FreeBSD was the inspiration. In= XNU I'm definitely going to put both barriers (the current open source drop doe= sn't have it but as I was reasoning through this code recently I convinced myself they were needed). --=20 You are receiving this mail because: You are the assignee for the bug.=
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