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Date: Fri, 23 Jul 2021 14:11:38 -0400 From: Alan Stern <stern@...land.harvard.edu> To: "Paul E. McKenney" <paulmck@...nel.org> Cc: linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org, kernel-team@...com, mingo@...nel.org, parri.andrea@...il.com, will@...nel.org, peterz@...radead.org, boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com, Manfred Spraul <manfred@...orfullife.com> Subject: Re: [PATCH memory-model 2/4] tools/memory-model: Add example for heuristic lockless reads On Fri, Jul 23, 2021 at 10:30:10AM -0700, Paul E. McKenney wrote: > On Fri, Jul 23, 2021 at 12:59:47PM -0400, Alan Stern wrote: > > On Fri, Jul 23, 2021 at 09:24:31AM -0700, Paul E. McKenney wrote: > > > On Thu, Jul 22, 2021 at 10:08:46PM -0400, Alan Stern wrote: > > > > > > > + void do_something_locked(struct foo *fp) > > > > > + { > > > > > + bool gf = true; > > > > > + > > > > > + /* IMPORTANT: Heuristic plus spin_lock()! */ > > > > > + if (!data_race(global_flag)) { > > > > > + spin_lock(&fp->f_lock); > > > > > + if (!smp_load_acquire(&global_flag)) { > > > > > > > + void begin_global(void) > > > > > + { > > > > > + int i; > > > > > + > > > > > + spin_lock(&global_lock); > > > > > + WRITE_ONCE(global_flag, true); > > > > > > > > Why does this need to be WRITE_ONCE? It still races with the first read > > > > of global_flag above. > > > > > > But also with the smp_load_acquire() of global_flag, right? > > > > What I'm curious about is why, given these two races, you notate one of > > them by changing a normal write to WRITE_ONCE and you notate the other > > by changing a normal read to a data_race() read. Why not handle them > > both the same way? > > Because the code can tolerate the first read returning complete nonsense, > but needs the value from the second read to be exact at that point in > time. In other words, if the second read races with the WRITE_ONCE, it needs to get either the value before the write or the value after the write; nothing else will do because it isn't a heuristic here. Fair point. > (If the value changes immediately after being read, the fact that > ->f_lock is held prevents begin_global() from completing.) This seems like something worth explaining in the document. That "IMPORTANT" comment doesn't really get the full point across. Alan
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