Re: [Qemu-devel] [PATCH v6 07/73] cpu: make per-CPU locks an alias of the BQL in TCG rr mode

[Richard Henderson]

On 1/29/19 4:47 PM, Emilio G. Cota wrote:
> Before we can switch from the BQL to per-CPU locks in
> the CPU loop, we have to accommodate the fact that TCG
> rr mode (i.e. !MTTCG) cannot work with separate per-vCPU
> locks. That would lead to deadlock since we need a single
> lock/condvar pair on which to wait for events that affect
> any vCPU, e.g. in qemu_tcg_rr_wait_io_event.
> 
> At the same time, we are moving towards an interface where
> the BQL and CPU locks are independent, and the only requirement
> is that the locking order is respected, i.e. the BQL is
> acquired first if both locks have to be held at the same time.
> 
> In this patch we make the BQL a recursive lock under the hood.
> This allows us to (1) keep the BQL and CPU locks interfaces
> separate, and (2) use a single lock for all vCPUs in TCG rr mode.
> 
> Note that the BQL's API (qemu_mutex_lock/unlock_iothread) remains
> non-recursive.
> 
> Signed-off-by: Emilio G. Cota <address@hidden>
> ---
>  include/qom/cpu.h |  2 +-
>  cpus-common.c     |  2 +-
>  cpus.c            | 90 +++++++++++++++++++++++++++++++++++++++++------
>  qom/cpu.c         |  3 +-
>  stubs/cpu-lock.c  |  6 ++--
>  5 files changed, 86 insertions(+), 17 deletions(-)

Reviewed-by: Richard Henderson <address@hidden>


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