Messages in this thread | | | Date | Thu, 22 Feb 2018 08:54:25 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [QUESTION] srcu: Remove the SCAN2 state |
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On Thu, Feb 22, 2018 at 02:05:18PM +0900, Byungchul Park wrote: > On 2/22/2018 11:11 AM, Paul E. McKenney wrote: > >On Thu, Feb 22, 2018 at 08:57:27AM +0900, Byungchul Park wrote: > >>Hello, > >> > >>I'm sorry for bothering you, and I seem to be obviously missing > >>something, but I'm really wondering why we check try_check_zero() > >>again in the state, SCAN1, for the previous srcu_idx. > >> > >>I mean, since we've already checked try_check_zero() in the previous > >>grace period and gotten 'true' as a return value, all readers who see > >>the flipped idx via srcu_flip() won't update the src_{lock,unlock}_count > >>for the previous idx until it gets flipped back again. > >> > >>Is there any reasons we check try_check_zero() again in the state, SCAN1? > >>Is there any problems if the following patch's applied? > > > >Indeed there are! Removing the second scan exposes us to a nasty race > >condition where a reader is preempted (or interrupted or whatever) just > > Indeed! I missed the cases. It should be as it is. > > Thanks a lot for pointing it out.
Heh! Everyone I know, myself included, who has written such an algorithm has had this bug in their initial version. In one case, the algorithm was published in a high-end journal and the bug not spotted for more than a decade. I suppose I could brag about Mathieu's and my offerings having been corrected before we published, but the fact remains that an earlier publication of mine gave the aforementioned algorithm from the high-end journal as an alternative implementation, and I did not spot the bug. Nor did any of my co-authors. ;-)
Thanx, Paul
> >after fetching its counter. A detailed explanation for an essentially > > -- > Thanks, > Byungchul >
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