Date: Wed, 20 Apr 2022 19:56:17 -0400 From: Rich Felker <dalias@...c.org> To: Leonid Shamis <leonid.shamis@...il.com> Cc: musl@...ts.openwall.com Subject: Re: robust list not issuing FUTEX_WAKE On Wed, Apr 20, 2022 at 04:46:23PM -0700, Leonid Shamis wrote: > Hey Musl Folks, > > I'm trying to understand the futex robust list and it's not quite working > how I would expect it to from a reading of the man pages. > > In a minimal example, I'm getting the futex changed to FUTEX_OWNER_DIED > instead of FUTEX_OWNER_DIED|tid and I'm not getting a FUTEX_WAKE event. > Any idea why this might be? > > Happy to share the minimal example. Can you clarify if you're trying to use robust mutexes under musl or roll your own thing using the kernel robust list directly? I don't think I'm understanding what you're confused about. If the owner of a robust mutex dies, its tid can no longer be in the futex word because that tid is immediately available to be assigned to a new task, and the next thread trying to lock the mutex would not be able to distinguish whether it's owned by a new thread with that tid, or available. So as far as the kernel is concerned, FUTEX_OWNER_DIED is more of a value than a flag. It's what the futex word gets set to when the owner dies with it held. In musl, we use FUTEX_OWNER_DIED as a flag as well for a robust mutex whose old owner died and who has a new owner but hasn't yet called pthread_mutex_consistent (in which case it would become unrecoverable on unlock). Rich
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