MonetDB: Dec2023 - Fixed a potential deadlock.

[Sjoerd Mullender via checkin-list]

Changeset: 630d6c4d0e55 for MonetDB
URL: https://dev.monetdb.org/hg/MonetDB/rev/630d6c4d0e55
Modified Files:
        gdk/gdk_select.c
Branch: Dec2023
Log Message:

Fixed a potential deadlock.
Thread A has heaplock for bat 24688 and tries to unfix its parent, bat
112.
Thread B is trying to incref bat 24688 and already has the swaplock and
now tries to get the heaplock.

Note that the swaplock for bats 24688 and 112 are the same (modulo
8192); so thread A wants swaplock(112) while it has heaplock(24688), and
thread B has swaplock(112) and wants heaplock(24688).
Fixed by delaying the unfix in thread A until after the heaplock was
released.  This happened in function BATrange.


diffs (40 lines):

diff --git a/gdk/gdk_select.c b/gdk/gdk_select.c
--- a/gdk/gdk_select.c
+++ b/gdk/gdk_select.c
@@ -1235,10 +1235,10 @@ BATrange(BATiter *bi, const void *tl, co
                maxval = VALptr(maxprop);
                maxincl = false;
        }
+       bool keep = false;      /* keep lock on parent bat? */
        if (minprop == NULL || maxprop == NULL) {
                if (VIEWtparent(bi->b) &&
                    (pb = BATdescriptor(VIEWtparent(bi->b))) != NULL) {
-                       bool keep = false;
                        MT_lock_set(&pb->theaplock);
                        if (minprop == NULL && (minprop = BATgetprop_nolock(pb, 
GDK_MIN_BOUND)) != NULL) {
                                keep = true;
@@ -1251,8 +1251,6 @@ BATrange(BATiter *bi, const void *tl, co
                        }
                        if (!keep) {
                                MT_lock_unset(&pb->theaplock);
-                               BBPreclaim(pb);
-                               pb = NULL;
                        }
                }
        }
@@ -1340,12 +1338,13 @@ BATrange(BATiter *bi, const void *tl, co
                }
        }
 
+       MT_lock_unset(&bi->b->theaplock);
        if (pb) {
-               MT_lock_unset(&pb->theaplock);
+               if (keep)
+                       MT_lock_unset(&pb->theaplock);
                BBPreclaim(pb);
        }
 
-       MT_lock_unset(&bi->b->theaplock);
        return range;
 }
 
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