wiredtiger - hazard pointers
http://www.drdobbs.com/lock-free-data-structures-with-hazard-po/184401890
memory deallocation lock-free
session.h
/*
* WT_HAZARD --
* A hazard pointer.
*/
struct __wt_hazard {
WT_PAGE *page; /* Page address */
#ifdef HAVE_DIAGNOSTIC
const char *file; /* File/line where hazard acquired */
int line;
#endif
}; struct WT_COMPILER_TYPE_ALIGN(WT_CACHE_LINE_ALIGNMENT) __wt_session_impl {
// 略...
/*
* Hazard pointers.
*
* Use the non-NULL state of the hazard field to know if the session has
* previously been initialized.
*/
#define WT_SESSION_FIRST_USE(s) \
((s)->hazard == NULL) /* The number of hazard pointers grows dynamically. */
#define WT_HAZARD_INCR 10
uint32_t hazard_size; /* Allocated slots in hazard array. */
uint32_t nhazard; /* Count of active hazard pointers */
WT_HAZARD *hazard; /* Hazard pointer array */
};
support/hazard.c
__wt_hazard_set()
/__wt_hazard_clear()
btree.i
在__wt_page_hazard_check前, cas保证WT_REF为 WT_REF_LOCKED(btmem.h, struct __wt_ref)
__wt_page_hazard_check(WT_SESSION_IMPL *session, WT_PAGE *page)
{
WT_CONNECTION_IMPL *conn;
WT_HAZARD *hp;
WT_SESSION_IMPL *s;
uint32_t i, hazard_size, session_cnt; conn = S2C(session); /*
* No lock is required because the session array is fixed size, but it
* may contain inactive entries. We must review any active session
* that might contain a hazard pointer, so insert a barrier before
* reading the active session count. That way, no matter what sessions
* come or go, we'll check the slots for all of the sessions that could
* have been active when we started our check.
*/
WT_ORDERED_READ(session_cnt, conn->session_cnt);
for (s = conn->sessions, i = ; i < session_cnt; ++s, ++i) {
if (!s->active)
continue;
WT_ORDERED_READ(hazard_size, s->hazard_size);
for (hp = s->hazard; hp < s->hazard + hazard_size; ++hp)
if (hp->page == page)
return (hp);
}
return (NULL);
}
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