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Date: Wed, 20 Feb 2019 10:30:31 -0500 (EST) From: Alan Stern <stern@...land.harvard.edu> To: Will Deacon <will.deacon@....com> cc: Andrea Parri <andrea.parri@...rulasolutions.com>, Peter Zijlstra <peterz@...radead.org>, "Paul E. McKenney" <paulmck@...ux.ibm.com>, <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>, Nicholas Piggin <npiggin@...il.com>, David Howells <dhowells@...hat.com>, Jade Alglave <j.alglave@....ac.uk>, Luc Maranget <luc.maranget@...ia.fr>, Akira Yokosawa <akiyks@...il.com>, Daniel Lustig <dlustig@...dia.com> Subject: Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo On Wed, 20 Feb 2019, Will Deacon wrote: > Whilst I completely agree that relying on the ordering provided by "dep ; > rfi" is subtle and error prone, having it forbid the outcome above appeals > to a hardware-based mindset of how memory ordering works. In the kernel > community, I would posit that the majority of developers are writing code > with the underlying hardware in mind and so allowing behaviours in the > memory model which are counter to how a real machine operates is likely to > make things more confusing, rather than simplifying them! > > IIRC, herd has a feature where you can "flag" the result of a litmus test > to highlight certain internal constraint violations (e.g. warning that a > data race is present in a concurrent C11 program). How about we preserve > the existing semantics, but flag any use of "dep; rfi" to indicate that > the ordering guarantees being relied upon are subtle and error-prone, and > therefore should only be considered for fast-path code? Unfortunately, herd can't really tell whether a particular ordering is being _used_; it can only tell when the ordering is present. Therefore such a flag would be prone to false positives. Alan
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