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include/waiting_threads.h:
  comment with a deadlock example
This commit is contained in:
Sergei Golubchik
2008-11-04 14:09:32 +01:00
parent f91219ed47
commit 8a655c6042
1 changed files with 25 additions and 1 deletions
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26
include/waiting_threads.h
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@@ -80,7 +80,31 @@ typedef struct st_wt_resource {
#ifdef WT_RWLOCKS_USE_MUTEXES
/*
we need a special rwlock-like 'lock' to allow readers bypass
waiting writers, otherwise readers can deadlock.
waiting writers, otherwise readers can deadlock. For example:
A waits on resource x, owned by B, B waits on resource y, owned
by A, we have a cycle (A->x->B->y->A)
Both A and B start deadlock detection:
A locks x B locks y
A goes deeper B goes deeper
A locks y B locks x
with mutexes it would deadlock. With rwlocks it won't, as long
as both A and B are taking read locks (and they do).
But other threads may take write locks. Assume there's
C who wants to start waiting on x, and D who wants to start
waiting on y.
A read-locks x B read-locks y
A goes deeper B goes deeper
=> C write-locks x (to add a new edge) D write-locks y
.. C is blocked D is blocked
A read-locks y B read-locks x
Now, if a read lock can bypass a pending wrote lock request, we're fine.
If it can not, we have a deadlock.
writer starvation is technically possible, but unlikely, because
the contention is expected to be low.
*/