Messages in this thread | | | Date | Mon, 13 Dec 2021 14:31:44 -0500 (EST) | From | Mathieu Desnoyers <> | Subject | Re: rseq + membarrier programming model |
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----- On Dec 13, 2021, at 1:47 PM, Florian Weimer fweimer@redhat.com wrote:
> 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.
By the way, there might be something good to salvage from this prototype I did a while back:
https://github.com/compudj/rseq-test/blob/adapt-lock/test-rseq-adaptative-lock.c
The neat trick there is to use a combination of the Zero Flag and rbx==0/1 to detect whether the rseq critical section was aborted before or after successful execution of the CAS. This allows the rseq c.s. to cover an entire loop, which contains a CAS instruction, without requiring that the critical section ends with a "commit" instruction.
Some characteristics of this prototype:
- Don't busy-wait in user-space if the lock owner belongs to the same CPU where the waiter executes. Immediately use futex. - Adaptative busy-wait delay (per-lock). - If busy-spinning is preempted, it jumps to abort immediately when resumed. Therefore, the loop count for adaptative busy-spinning is very precise.
Of course, much more work would be needed, but I suspect a few ideas there can be useful.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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