#At file:///home/dlenev/src/bzr/mysql-next-4284-bg51093/ based on revid:dlenev@stripped
3096 Dmitry Lenev 2010-02-15
Fix for bug #51093 "Crash (possibly stack overflow) in
On some platform deadlock detector in metadata locking
subsystem under certain conditions might have exhausted
stack space causing server crashes.
Particularly this caused failures of rqg_mdl_stability
test on Solaris in PushBuild.
During search for deadlock MDL deadlock detector could
sometimes encounter loop in the waiters graph in which
MDL_context which has started search for a deadlock
does not participate. In such case our algorithm will
continue looping assuming that either this deadlock will
be resolved by MDL_context which has created it (i.e.
by one of loop participants) or maximum search depth
will be reached.
Since max search depth was set to 1000 in the latter case
on platforms where each iteration of deadlock search
algorithm needs more than DEFAULT_STACK_SIZE/1000 bytes
of stack (around 192 bytes for 32-bit and around 256 bytes
for 64-bit platforms) we might have exhausted stack space.
This patch solves this problem by reducing maximum search
depth for MDL deadlock detector to 100. This should be safe
at the moment as it is unlikely that each iteration of the
current deadlock detector algorithm will consume more than
512 bytes of stack (thus total amount of stack required
can't be more than 512*100 bytes) and we require at least
80K of stack in order to open any table.
Additional reasearch should be conducted in future in order
to determine the more optimal value of maximum search depth.
This patch does not include test case as existing
rqg_mdl_stability test can serve as one.
=== modified file 'sql/mdl.cc'
--- a/sql/mdl.cc 2010-02-11 10:23:39 +0000
+++ b/sql/mdl.cc 2010-02-15 08:51:07 +0000
@@ -74,7 +74,20 @@ public:
- static const uint MAX_SEARCH_DEPTH= 1000;
+ Maximum depth for deadlock searches. After this depth is
+ achieved we will unconditionally declare that there is a
+ @note This depth should be small enough to avoid stack
+ being exhausted by recursive search algorithm.
+ TODO: Find out what is the optimal value for this parameter.
+ Current value is safe, but probably sub-optimal,
+ as there is an anecdotal evidence that real-life
+ deadlocks are fairly short.
+ static const uint MAX_SEARCH_DEPTH= 100;
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