+1 KurtS On Wed, Sep 11, 2024 at 12:27 PM Even Rouault via gdal-dev < gdal-dev@lists.osgeo.org> wrote:
> Hi, > > I move to approve RFC 101 "Raster dataset read-only thread-safety": > https://github.com/OSGeo/gdal/pull/10676 > > Starting with my +1, > > The candidate implementation is available in > https://github.com/OSGeo/gdal/pull/10746. While fine tuning it, I > realized there were subtleties, related to the fact we use thread-local > datasets under the hood, for methods returning a non-"primitive" type, > such as a "const char*" whose lifetime is tied to a dataset/band. If > we'd get it from a thread-local dataset, and that one would be later > evicted from the cache (not very common, but could happen if one would > open tons of thread-safe datasets at the same time), then you could have > use-after-free issues. For such methods (GetMetadata(), > GetMetadataItem(), GetProjectionRef(), etc. as well as GetColorTable() > which returns a GDALColorTable pointer, or GetSpatialRef() which returns > a OGRSpatialReference*), I've preferred to protect their access with a > mutex around the "prototype" dataset passed when constructing > GDALMultiThreadedDataset, whose lifetime is at least as long as > GDALMultiThreadedDataset (cf commit > > https://github.com/OSGeo/gdal/pull/10746/commits/849e9cea711efd30c47fd90a7a6b71a75611c1a5#diff-687008dd6e3d5ac8ad05568272746ed75b84de50bde508f78f2d79a6842825c7L428) > > . Normally such methods aren't called in user code repeatedly, so there > should be any noticeable lock contention in practice. The main objective > of the RFC which is to be able to issue RasterIO() requests in parallel > isn't affected by that. Ah, and one thing I realized is that > OGRSpatialReference isn't thread-safe, so I've also added an optional > SetThreadSafe() on it, to also use a per-instance mutex in > multi-threaded scenarios (cf commit > > https://github.com/OSGeo/gdal/pull/10746/commits/c7e1862273dd018e58a01f25b21fdff6dbfdd1cd). > > Multi-threading is hard... > > Even > > -- > http://www.spatialys.com > My software is free, but my time generally not. > > _______________________________________________ > gdal-dev mailing list > gdal-dev@lists.osgeo.org > https://lists.osgeo.org/mailman/listinfo/gdal-dev >
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