Date: Fri, 12 Mar 2021 16:14:51 -0500 From: Dominic Chen <d.c.ddcc@...il.com> To: Rich Felker <dalias@...c.org> Cc: fweimer@...hat.com, musl@...ts.openwall.com Subject: Re: Incorrect thread TID caching On 2/17/2021 4:07 PM, Rich Felker wrote: >> Whether it's inlined isn't really a big deal; this is not a hot path. >> It's more just a matter of how it needs to be split up at the source >> level, and it seems to be messy whichever way we choose. >> >> Trying to avoid calling __clone doesn't seem like such a good idea, >> since the child has to run on a new stack -- if we did avoid it we'd >> need a new way to switch stacks. The generic __unmapself has a hack >> to do this already that we could reuse without needing new >> arch-specific glue though. >> >> I'll keep trying things and see if I come up with something not too >> unreasonable. >> >> Attached is a draft of how clone() *could* work without refactoring >> the pre/post logic from _Fork to use __clone. I don't particularly >> like it, and CRTJMP abuse like this (which __unmapself also does, as >> noted above) is not valid for FDPIC archs (it actually expects a code >> address not a function pointer). I'll try a version the other way and >> see how it looks. Sorry for the delay. I did end up needing robust mutex functionality, and ended up using a variant of your patch to fix-up. There were two issues that I noticed: 1. Prior to CRTJMP, the 'arg' argument needs to be moved into the first argument register for the local calling convention. I spent a little time trying to get an architecture-neutral version working using a wrapper function, but gave up and used some inline assembly to populate %rdi on x86_64. 2. If a thread exits without unlocking a robust mutex, the program will subsequently SIGSEGV since the thread's memory region has been unmapped, but a pointer to the robust mutex remains on the internal linked list. Not sure where the fault lies here, but just thought I'd mention it. Thanks, Dominic
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