[gdal-dev] Motion: approve RFC 101 "Raster dataset read-only thread-safety"

[Heiskanen Mika (FMI)]

  *
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).

Perhaps I am missing something, but why not use read locks for const methods and write locks for non-const methods so that readers would not block each other so much?

-Mika Heiskanen-

________________________________
From: gdal-dev <gdal-dev-bounces at lists.osgeo.org> on behalf of Rahkonen Jukka via gdal-dev <gdal-dev at lists.osgeo.org>
Sent: Thursday, September 12, 2024 09:27
To: Even Rouault <even.rouault at spatialys.com>; gdal-dev at lists.osgeo.org <gdal-dev at lists.osgeo.org>
Subject: Re: [gdal-dev] Motion: approve RFC 101 "Raster dataset read-only thread-safety"

+1

-Jukka Rahkonen-

-----Alkuperäinen viesti-----
Lähettäjä: gdal-dev <gdal-dev-bounces at lists.osgeo.org> Puolesta Even Rouault via gdal-dev
Lähetetty: keskiviikko 11. syyskuuta 2024 22.27
Vastaanottaja: gdal-dev at lists.osgeo.org
Aihe: [gdal-dev] Motion: approve RFC 101 "Raster dataset read-only thread-safety"

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

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