Messages in this thread | | | Date | Mon, 3 Feb 2020 09:29:47 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Confused about hlist_unhashed_lockless() |
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On Mon, Feb 03, 2020 at 04:02:28PM +0000, Will Deacon wrote: > On Mon, Feb 03, 2020 at 07:58:39AM -0800, Paul E. McKenney wrote: > > On Mon, Feb 03, 2020 at 03:45:54PM +0000, David Laight wrote: > > > From: Eric Dumazet > > > > Sent: 31 January 2020 18:53 > > > > > > > > On Fri, Jan 31, 2020 at 10:48 AM Eric Dumazet <edumazet@google.com> wrote: > > > > > > > > > > > > > > This is nice, now with have data_race() > > > > > > > > > > Remember these patches were sent 2 months ago, at a time we were > > > > > trying to sort out things. > > > > > > > > > > data_race() was merged a few days ago. > > > > > > > > Well, actually data_race() is not there yet anyway. > > > > > > Shouldn't it be NO_DATA_RACE() ?? > > > > No, because you use data_race() when there really are data races, but you > > want KCSAN to ignore them. For example, diagnostic code that doesn't > > participate in the actual concurrency design and that doesn't run all > > that often might use data_race(). For another example, if a developer > > knew that data races existed, but that the compiler could not reasonably > > do anything untoward with those data races, that developer might well > > choose to use data_race() instead of READ_ONCE(). Especially if the > > access in question was on a fastpath where helpful compiler optimizations > > would be prohibited by use of READ_ONCE(). > > Yes, and in this particular case I think we can remove some WRITE_ONCE()s > from the non-RCU hlist code too (similarly for hlist_nulls).
Quite possibly, but we should take them case by case. READ_ONCE() really does protect against some optimizations, while data_race() does not at all.
But yes, in some cases you want to -avoid- using READ_ONCE() and WRITE_ONCE() so that KCSAN can do its job. For example, given a per-CPU variable that is only supposed to be accessed from the corresponding CPU except for reads by diagnostic code, you should have the main algorithm use plain C-language reads and writes, and have the diagnostic code use data_race(). This allows KCSAN to correctly flag bugs that access this per-CPU variable off-CPU while leaving the diagnostic code alone.
Seem reasonable?
Thanx, Paul
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