Re: [PATCH] hashmap: add API to disable item counting when threaded
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From: Jeff King <hidden>
Date: 2017-09-02 08:05:51
On Wed, Aug 30, 2017 at 06:59:22PM +0000, Jeff Hostetler wrote:
From: Jeff Hostetler <redacted> This is to address concerns raised by ThreadSanitizer on the mailing list about threaded unprotected R/W access to map.size with my previous "disallow rehash" change (0607e10009ee4e37cb49b4cec8d28a9dda1656a4). See: https://public-inbox.org/git/adb37b70139fd1e2bac18bfd22c8b96683ae18eb.1502780344.git.martin.agren@gmail.com/ Add API to hashmap to disable item counting and to disable automatic rehashing. Also include APIs to re-enable item counting and automatica rehashing.
It may be worth summarizing the discussion at that thread here. At first glance, it looks like this is woefully inadequate for allowing multi-threaded access. But after digging, the issue is that we're really trying to accommodate one specific callers which is doing its own per-bucket locking, and which needs the internals of the hashmap to be truly read-only. I suspect the code might be easier to follow if that pattern were pushed into its own threaded_hashmap that disabled the size and handled the mod-n locking, but I don't insist on that as a blocker to this fix. -Peff