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May 20, 2023 •452 likes •807 views

Frightening small children and disconcerting grown-ups: Concurrency in the Linux kernel Jade Alglave, Luc Maranget, Paul McKenney, Andrea Parri and Alan Stern Concurrency in Linux can be a contentious topic Model URL SPARC [McKenney, 2001,

  1. Frightening small children and disconcerting grown-ups: Concurrency in the Linux kernel Jade Alglave, Luc Maranget, Paul McKenney, Andrea Parri and Alan Stern

  2. Concurrency in Linux can be a contentious topic Model URL SPARC [McKenney, 2001, Spraul, 2001] LK [Vaddagiri, 2005] LK [McKenney, 2007, Alglave et al., 2013] LK [Corbet, 2008] LK [Olsa, 2009] LK [Heo, 2010] LK/Itanium [Blanchard, 2011, Miller, 2011] LK [Corbet, 2012] Itanium [McKenney, 2013b, Luck, 2013a, Luck, 2013b, Zijlstra, 2013] Intel [McKenney, 2013a, Kleen, 2013] LK/C11 [Corbet, 2014b, Corbet, 2014c] LK [Corbet, 2014a] Alpha [Torvalds, 2015] LK [Feng, 2015] ARM64 [Deacon, 2015a] LK [Deacon, 2015b] MIPS [Yegoshin, 2016a, Yegoshin, 2016b, Yegoshin, 2016c, Torvalds, 2016b] PowerPC [Feng, 2016a, Feng, 2016b, Ellerman, 2016] ARM64 [Deacon, 2016] LK/C11 [Corbet, 2016] LK [Molnar, 2017] Jade Alglave LKMM 2 / 30

  3. “Still confusion situation all round”[sic] [Ziljstra, 2013] Jade Alglave LKMM 3 / 30

  4. Existing documentation I [Howells et al., 2017] lists what orderings are guaranteed; I [Miller, 2017] summarises semantics of read-modify-writes; I [McKenney, 2017a] documents ways of avoiding counterproductive optimisations. Jade Alglave LKMM 4 / 30

  5. But [Gorman, 2013] If Documentation/memory-barriers.txt could not be used to frighten small children before, it certainly can now. Jade Alglave LKMM 5 / 30

  6. Also [Howells et al., 2017]: This document is not a specification; it is intentionally (for the sake of brevity) and unintentionally (due to being human) incomplete. [. . . ] in case of any doubt (and there are many) please ask. Jade Alglave LKMM 6 / 30

  7. And anyway [Torvalds, 2012] With specs, there really *are* people who spend years discussing what the meaning of the word ”access” is or similar [. . . ]. Combine that with a big spec that is 500+ pages in size and then try to apply that all to a project that is 15 million lines of code and sometimes *knowingly* has to do things that it simply knows are outside the spec [. . . ]” Jade Alglave LKMM 7 / 30

  8. What we o ff er I a formal consistency model I written in the cat language I thus executable within the herd tool Jade Alglave LKMM 8 / 30

  9. “[I]t is your kernel, so what is your preference?” [McKenney, 2016a] Jade Alglave LKMM 9 / 30

  10. A common denominator of hardware models? [Torvalds, 2016a] Weak memory ordering is [. . . ] hard to think about [. . . ] So the memory ordering rules should [. . . ] absolutely be as tight as at all humanly possible, given real hardware constraints. Jade Alglave LKMM 10 / 30

  11. Not an envelope for the architectures it supports? [Molnar, 2013] it’s not true that Linux has to o ff er a barrier and locking model that panders to the weakest (and craziest!) memory ordering model amongst all the possible Linux platforms—theoretical or real metal. Instead what we want to do is to consciously, intelligently pick a sane, maintainable memory model and o ff er primitives for that—at least as far as generic code is concerned. Each architecture can map those primitives to the best of its abilities. Jade Alglave LKMM 11 / 30

  12. The LK should have a model of its own [Torvalds, 2012] I do not believe for a second that we can actually use the C11 memory model in the kernel [. . . ] We will continue to have to do things that are “outside the specs” [. . . ] with models that C11 simply doesn’t cover. Jade Alglave LKMM 12 / 30

  13. Core model Jade Alglave LKMM 13 / 30

  14. An example from PeterZ https://www.spinics.net/lists/kernel/msg2421883.html Jade Alglave LKMM 14 / 30

  15. PeterZ: Forbidden a: W[once]x=1 c: W[once]y=1 e: R[acquire]z=1 fr mb po mb fr rf b: R[once]y=0 d: W[release]z=1 f: R[once]x=0 Jade Alglave LKMM 15 / 30

  16. RCU Jade Alglave LKMM 16 / 30

  17. Fundamental law of RCU [McKenney et al., 2013] Read-side critical sections cannot span grace periods. Jade Alglave LKMM 18 / 30

  18. RCU-MP: Forbidden g: F[rcu-lock] po c: W[once]x=1 po a: R[once]y=1 fr po k: F[sync-rcu] rf po b: R[once]x=0 po d: W[once]y=1 j: F[rcu-unlock] Jade Alglave LKMM 17 / 30

  19. Validating the model Jade Alglave LKMM 21 / 30

  20. Experimentally Model Power8 ARMv8 ARMv7 X86 C11 LB Allow 0/15G 0/10G 0/3.0G 0/33G Allow LB+ctrl+mb Forbid 0/17G 0/5.1G 0/4.4G 0/18G Allow WRC Allow 741k/7.7G 13k/5.2G 0/1.6G 0/17G Allow WRC+wmb+acq Allow 0/7.5G 0/4.6G 0/1.6G 0/16G Forbid WRC+po-rel+rmb Forbid 0/7.7G 0/5.3G 0/1.6G 0/17G Forbid SB Allow 4.4G/15G 2.4G/10G 429M/3.0G 765M/33G Allow SB+mbs Forbid 0/15G 0/10G 0/3.0G 0/33G Forbid MP Allow 57M/15G 104M/10G 3.0M/3.0G 0/33G Allow MP+wmb+rmb Forbid 0/15G 0/9.4G 0/2.6G 0/33G Forbid PeterZ-No-Synchro Allow 26M/4.6G 3.6M/900M 10k/380M 351k/7.2G Allow PeterZ Forbid 0/8.7G 0/2.5G 0/2.2G 0/9.1G Allow RCU-deferred-free Forbid 0/256M 0/576M 0/15M 0/25M — RCU-MP Forbid 0/672M 0/336M 0/336M 0/336M — RWC Allow 88M/11G 94M/4.8G 7.5M/1.6G 5.6M/17G Allow RWC+mbs Forbid 0/8.7G 0/2.5G 0/2.2G 0/9.1G Allow Jade Alglave LKMM 22 / 30

  21. Socially Five maintainers have agreed to chaperon our model. https://www.spinics.net/lists/kernel/msg2421883.html Jade Alglave LKMM 23 / 30

  22. Issues that our work helped address LK issue URL locking on ARM64 [Deacon, 2015a] ambiguities in [Howells et al., 2017] [McKenney, 2016c] ambiguities in RCU documentation [McKenney, 2016b] CPU hotplug [Zijlstra, 2016] assumption about lock-unlock [McKenney, 2017b] semantics of spin unlock wait [Torvalds, 2017] As well as: I updates to documentation [McKenney, 2016b, McKenney, 2016c, Howells et al., 2017]; I model to be pushed to LK release candidate anon. Jade Alglave LKMM 30 / 30

  23. Authors’ repo Paul’s tree Ingo Molnar’s tree Linus Torvalds’s tree aim: 2018-01-18 2018-01-31 v4.17 merge window April 2018

  24. Alglave, J., Kroening, D., and Tautschnig, M. (2013). Partial orders for e ffi cient Bounded Model Checking of concurrent software. In Computer Aided Verification (CAV), volume 8044 of LNCS, pages 141–157. Springer. Blanchard, A. (2011). RE: [PATCH] smp call function many SMP race. https://lkml.org/lkml/2011/1/11/489 . Corbet, J. (2008). The lockless page cache. https://lwn.net/Articles/291826/ . Corbet, J. (2012). ACCESS ONCE(). https://lwn.net/Articles/508991/ . Corbet, J. (2014a). ACCESS ONCE() and compiler bugs. https://lwn.net/Articles/624126/ . Jade Alglave LKMM 30 / 30

  25. Corbet, J. (2014b). C11 atomic variables and the kernel. https://lwn.net/Articles/586838/ . Corbet, J. (2014c). C11 atomics part 2: “consume” semantics. https://lwn.net/Articles/588300/ . Corbet, J. (2016). Time to move to C11 atomics? https://lwn.net/Articles/691128/ . Deacon, W. (2015a). [PATCH] arm64: spinlock: serialise spin unlock wait against concurrent lockers. https: //marc.info/?l=linux-arm-kernel&m=144862480822027 . Deacon, W. (2015b). Re: [PATCH] arm64: spinlock: serialise spin unlock wait against concurrent lockers. Jade Alglave LKMM 30 / 30

  26. https: //marc.info/?l=linux-arm-kernel&m=144898777124295 . Deacon, W. (2016). [PATCH v2 1/3] arm64: spinlock: order spin { is locked, unlock wait } against local locks. http://lists.infradead.org/pipermail/ linux-arm-kernel/2016-June/434765.html . Desnoyers, M., McKenney, P. E., Stern, A. S., Dagenais, M. R., and Walpole, J. (2012). User-level implementations of Read-Copy Update. IEEE Trans. Parallel Distrib. Syst., 23(2):375–382. Ellerman, M. (2016). [PATCH v3] powerpc: spinlock: Fix spin unlock wait(). https: //marc.info/?l=linux-kernel&m=146521336230748&w=2 . Feng, B. (2015). Re: [PATCH 4/4] locking: Introduce smp cond acquire(). Jade Alglave LKMM 30 / 30

  27. https: //marc.info/?l=linux-kernel&m=144723482232285 . Feng, B. (2016a). [PATCH v2] powerpc: spinlock: Fix spin unlock wait(). https: //marc.info/?l=linux-kernel&m=146492558531292&w=2 . Feng, B. (2016b). [PATCH v4] powerpc: spinlock: Fix spin unlock wait(). https://marc.info/?l=linuxppc-embedded&m= 146553051027169&w=2 . Gorman, M. (2013). LWN Quotes of the week, 2013-12-11. http://lwn.net/Articles/575835/ . Heo, T. (2010). [PATCH 3/4] scheduler: replace migration thread with cpuhog. Jade Alglave LKMM 30 / 30

  28. https: //marc.info/?l=linux-kernel&m=126806371630498 . Howells, D., McKenney, P. E., Deacon, W., and Zijlstra, P. (2017). Linux kernel memory barriers. https://www.kernel.org/doc/Documentation/ memory-barriers.txt . Kleen, A. (2013). Re: [patch v6 4/5] MCS lock: Barrier corrections. https://marc.info/?l=linux-mm&m=138619237606428. Luck, T. (2013a). RE: Does Itanium permit speculative stores? https: //marc.info/?l=linux-kernel&m=138427925823852 . Luck, T. (2013b). RE: Does Itanium permit speculative stores? Jade Alglave LKMM 30 / 30

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