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Re: [PATCH] rwlock: remove unneeded subtraction
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Luca Fancellu <luca.fancellu@xxxxxxx>
- Date: Tue, 15 Feb 2022 11:54:07 +0000
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Tue, 15 Feb 2022 11:54:32 +0000
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> On 15 Feb 2022, at 09:39, Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
>
> There's no need to subtract _QR_BIAS from the lock value for storing
> in the local cnts variable in the read lock slow path: the users of
> the value in cnts only care about the writer-related bits and use a
> mask to get the value.
>
> Note that further setting of cnts in rspin_until_writer_unlock already
> do not subtract _QR_BIAS.
>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Luca Fancellu <luca.fancellu@xxxxxxx>
> ---
> xen/common/rwlock.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/xen/common/rwlock.c b/xen/common/rwlock.c
> index dadab372b5..aa15529bbe 100644
> --- a/xen/common/rwlock.c
> +++ b/xen/common/rwlock.c
> @@ -47,7 +47,7 @@ void queue_read_lock_slowpath(rwlock_t *lock)
> while ( atomic_read(&lock->cnts) & _QW_WMASK )
> cpu_relax();
>
> - cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
> + cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
> rspin_until_writer_unlock(lock, cnts);
>
> /*
> --
> 2.34.1
>
>
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