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bionic: add __system_property_area_serial()

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Adds a new _internal_ function. Provide a global serial number to
support more efficient private caching algorithms. This allows
to skip re-running the __system_property_find() call on misses until
there is a global change in the properties. This call is a read
barrier, the property data to be read following this call will be
read sequentially and up to date.

(Cherry pick from bfd65279a5)

Bug: 19544788
Change-Id: I58e6a92baa0f3e8e7b9ec79b10af6d56407dab48
This commit is contained in:
Mark Salyzyn
2015-04-24 09:31:32 -07:00
parent 452742d2bf
commit 123927dffc
2 changed files with 28 additions and 0 deletions
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10
libc/bionic/system_properties.cpp
View File

@@ -598,6 +598,16 @@ int __system_property_area_init()
return map_prop_area_rw(); return map_prop_area_rw();
} }
unsigned int __system_property_area_serial()
{
prop_area *pa = __system_property_area__;
if (!pa) {
return -1;
}
// Make sure this read fulfilled before __system_property_serial
return atomic_load_explicit(&(pa->serial), memory_order_acquire);
}
const prop_info *__system_property_find(const char *name) const prop_info *__system_property_find(const char *name)
{ {
if (__predict_false(compat_mode)) { if (__predict_false(compat_mode)) {

18
libc/include/sys/_system_properties.h
View File

@@ -98,6 +98,24 @@ int __system_property_set_filename(const char *filename);
*/ */
int __system_property_area_init(); int __system_property_area_init();
/* Read the global serial number of the system properties
**
** Called to predict if a series of cached __system_property_find
** objects will have seen __system_property_serial values change.
** But also aids the converse, as changes in the global serial can
** also be used to predict if a failed __system_property_find
** could in-turn now find an new object; thus preventing the
** cycles of effort to poll __system_property_find.
**
** Called at the beginning of a cache cycle to signal _any_ possible
** changes have occurred since last. If there is, check each individual
** __system_property_serial to confirm dirty, or __system_property_find
** to check if the property now exists.
**
** Returns the serial number on success, -1 on error.
*/
unsigned int __system_property_area_serial();
/* Add a new system property. Can only be done by a single /* Add a new system property. Can only be done by a single
** process that has write access to the property area, and ** process that has write access to the property area, and
** that process must handle sequencing to ensure the property ** that process must handle sequencing to ensure the property