linux-kernel - rseq + membarrier programming model

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Date:   Mon, 13 Dec 2021 19:47:57 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     linux-api@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Jann Horn <jannh@...gle.com>, libc-alpha@...rceware.org,
        linux-kernel@...r.kernel.org
Subject: rseq + membarrier programming model

I've been studying Jann Horn's biased locking example:

  Re: [PATCH 0/4 POC] Allow executing code and syscalls in another address space
  <https://lore.kernel.org/linux-api/CAG48ez02UDn_yeLuLF4c=kX0=h2Qq8Fdb0cer1yN8atbXSNjkQ@mail.gmail.com/>

It uses MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ as part of the biased lock
revocation.

How does the this code know that the process has called
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ?  Could it fall back to
MEMBARRIER_CMD_GLOBAL instead?  Why is it that MEMBARRIER_CMD_GLOBAL
does not require registration (the broader/more expensive barrier), but
the more restricted versions do?

Or put differently, why wouldn't we request
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ unconditionally at
process start in glibc, once we start biased locking in a few places?

Thanks,
Florian