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use explicit memory order for rcu reader/write exclusion

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This commit is contained in:
Paul Davis 2023-02-20 12:40:09 -07:00
parent d601e78e06
commit fe17b98079
1 changed files with 11 additions and 7 deletions
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18
libs/pbd/pbd/rcu.h
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@ -74,14 +74,18 @@ public:
/* Keep count of any readers in this section of code, so writers can /* Keep count of any readers in this section of code, so writers can
* wait until managed_object is no longer in use after an atomic exchange * wait until managed_object is no longer in use after an atomic exchange
* before dropping it. * before dropping it.
*
* rg: this is not great, 3 consecutive full compiler and hardware
* memory barterers. For an edge-case lock that is not usually contended.
* consider reverting f87de76b9fc8b3a5a.
*/ */
_active_reads++; /* no reads or writes below this atomic store (e.g. the copying
* of *managed_object) can move before this "barrier".
*/
_active_reads.fetch_add (1, std::memory_order_release);
rv = *managed_object; rv = *managed_object;
_active_reads--; /* no reads or writes below this atomic store (e.g. the copying
* of *managed_object) can move before this "barrier", and this
* also synchronizes with a memory_order_acquire load when
* testing for active readers (see below).
*/
_active_reads.fetch_sub (1, std::memory_order_release);
return rv; return rv;
} }
@ -99,7 +103,7 @@ protected:
std::atomic<PtrToSharedPtr> managed_object; std::atomic<PtrToSharedPtr> managed_object;
inline bool active_read () const { inline bool active_read () const {
return _active_reads.load() != 0; return _active_reads.load (std::memory_order_acquire) != 0;
} }
private: private: