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Re: [PATCH v3 1/6] xen: add reference counter support
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 16 Mar 2023 15:03:42 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Delivery-date: Thu, 16 Mar 2023 14:03:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 16.03.2023 14:54, Roger Pau Monné wrote:
> On Tue, Mar 14, 2023 at 08:56:29PM +0000, Volodymyr Babchuk wrote:
>> --- /dev/null
>> +++ b/xen/include/xen/refcnt.h
>> @@ -0,0 +1,59 @@
>
> This seems to be missing some kind of license, can we have an SPDX tag
> at least?
Not "at least", but strictly that way for any new code. Patches to convert
various existing code to SPDX are already pending iirc.
>> +#ifndef __XEN_REFCNT_H__
>> +#define __XEN_REFCNT_H__
>> +
>> +#include <asm/atomic.h>
>> +#include <asm/bug.h>
>> +
>> +#define REFCNT_SATURATED (INT_MIN / 2)
>> +
>> +typedef struct {
>> + atomic_t refcnt;
>> +} refcnt_t;
>> +
>> +static inline void refcnt_init(refcnt_t *refcnt)
>> +{
>> + atomic_set(&refcnt->refcnt, 1);
>> +}
>> +
>> +static inline int refcnt_read(refcnt_t *refcnt)
>
> const.
>
>> +{
>> + return atomic_read(&refcnt->refcnt);
>> +}
>> +
>> +static inline void refcnt_get(refcnt_t *refcnt)
>> +{
>> + int old = atomic_add_unless(&refcnt->refcnt, 1, 0);
>> +
>> + if ( unlikely(old < 0) || unlikely (old + 1 < 0) )
> ^ extra space
>
> You want a single unlikely for both conditions.
Are you sure? My experience was generally the other way around: likely()
and unlikely() become ineffectual as soon as the compiler needs more
than one branch for the inner construct (ie generally for and && or ||).
Jan
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