Re: [Xen-devel] [PATCH v3 2/2] rwlock: allow recursive read locking when already locked in write mode

[Julien Grall <julien@xxxxxxx>]

Hi Roger,

On 24/02/2020 08:44, Roger Pau Monne wrote:
> Allow a CPU already holding the lock in write mode to also lock it in
> read mode. There's no harm in allowing read locking a rwlock that's
> already owned by the caller (ie: CPU) in write mode. Allowing such
> accesses is required at least for the CPU maps use-case.
>
> In order to do this reserve 12bits of the lock, this allows to support
> up to 4096 CPUs. Also reduce the write lock mask to 2 bits: one to
> signal there are pending writers waiting on the lock and the other to
> signal the lock is owned in write mode.
>
> This reduces the maximum number of concurrent readers from 16777216 to
> 262144, I think this should still be enough, or else the lock field
> can be expanded from 32 to 64bits if all architectures support atomic
> operations on 64bit integers.
>
> Fixes: 5872c83b42c608 ('smp: convert the cpu maps lock into a rw lock')
> Reported-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reported-by: Jürgen Groß <jgross@xxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Changes since v2:
>   - Use atomic_and to clear the writers part of the lock when
>     write-unlocking.
>   - Reduce writer-related area to 14bits.
>   - Include xen/smp.h for Arm64.
OOI, is it to use smp_processor_id()?

Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel