----- On Dec 13, 2021, at 1:47 PM, Florian Weimer fweimer@xxxxxxxxxx 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@xxxxxxxxxxxxxx/> > > 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
