On 25.10.2016 10:24, Kevin Wolf wrote: > Am 24.10.2016 um 20:03 hat Max Reitz geschrieben: >> On 24.10.2016 12:11, Kevin Wolf wrote: >> >> [...] >> >>> Now, the big question is how to translate this into file locking. This >>> could become a little tricky. I had a few thoughts involving another >>> lock on byte 2, but none of them actually worked out so far, because >>> what we want is essentially a lock that can be shared by readers, that >>> can also be shared by writers, but not by readers and writers at the >>> same time. >> >> You can also share it between readers and writers, as long as everyone >> can cope with volatile data. > > Sorry, that was ambiguous. I meant a file-level lock rather than the > high-level one. If we had a lock that can be shared by one or the other, > but not both, then two locks would be enough to build what we really > want. > >> I agree that it's very similar to the proposed op blocker style, but I >> can't really come up with a meaningful translation either. >> >> Maybe something like this (?): All readers who do not want the file to >> be modified grab a shared lock on byte 1. All writers who can deal with >> volatile data grab a shared lock on byte 2. Exclusive writers grab an >> exclusive lock on byte 1 and 2. Readers who can cope with volatile data >> get no lock at all. >> >> When opening, the first and second group would always have to test >> whether there is a lock on the other byte, respectively. E.g. sharing >> writers would first grab an exclusive lock on byte 1, then the shared >> lock on byte 2 and then release the exclusive lock again. >> >> Would that work? > > I'm afraid it wouldn't. If you start the sharing writer first and then > the writer-blocking reader, the writer doesn't hold a lock on byte 1 any > more,
But it holds a lock on byte 2. > so the reader can start even though someone is writing to the > image. It can't because it would try to grab an exclusive lock on byte 2 before grabbing the shared lock on byte 1. Max > On the other hand, the writer can't keep an exclusive lock > because it would block other users that can share the image. > > Kevin >
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