Date: Mon, 23 Jun 2003 23:27:45 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: current@FreeBSD.org Subject: patch to let witness monitor the mtx pool Message-ID: <200306240627.h5O6RjM7070708@gw.catspoiler.org>
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I've been running with the patch below for a little while now. It
helped me find a situation where a thread attemped to grab a "pool
mutex" while it already held one, which I suspect could have caused a
deadlock in certain circumstances. In any case, this was illegal
because these mutexes are only supposed be used as leaf mutexes.
I had to patch kern_sx.c to quiet warnings about lock order reversal
that look bogus to me. I believe the way that kern_sx.c uses the pool
mutexes is safe.
Index: sys/kern/kern_mtxpool.c
===================================================================
RCS file: /home/ncvs/src/sys/kern/kern_mtxpool.c,v
retrieving revision 1.7
diff -u -r1.7 kern_mtxpool.c
--- sys/kern/kern_mtxpool.c 13 Jun 2003 19:39:21 -0000 1.7
+++ sys/kern/kern_mtxpool.c 20 Jun 2003 06:36:55 -0000
@@ -33,6 +33,7 @@
#include <sys/lock.h>
#include <sys/malloc.h>
#include <sys/mutex.h>
+#include <sys/sysctl.h>
#include <sys/systm.h>
#ifndef MTX_POOL_SIZE
@@ -44,6 +45,12 @@
int mtx_pool_valid = 0;
+int witness_skip_mtx_pool = 0;
+
+TUNABLE_INT("debug.witness_skip_mtx_pool", &witness_skip_mtx_pool);
+SYSCTL_INT(_debug, OID_AUTO, witness_skip_mtx_pool, CTLFLAG_RD,
+ &witness_skip_mtx_pool, 0, "");
+
/*
* Inline version of mtx_pool_find(), used to streamline our main API
* function calls.
@@ -60,11 +67,13 @@
static void
mtx_pool_setup(void *dummy __unused)
{
- int i;
+ int i, mtx_opts;
+ mtx_opts = MTX_DEF | MTX_QUIET;
+ if (witness_skip_mtx_pool)
+ mtx_opts |= MTX_NOWITNESS;
for (i = 0; i < MTX_POOL_SIZE; ++i)
- mtx_init(&mtx_pool_ary[i], "pool mutex", NULL,
- MTX_DEF | MTX_NOWITNESS | MTX_QUIET);
+ mtx_init(&mtx_pool_ary[i], "pool mutex", NULL, mtx_opts);
mtx_pool_valid = 1;
}
Index: sys/kern/kern_sx.c
===================================================================
RCS file: /home/ncvs/src/sys/kern/kern_sx.c,v
retrieving revision 1.19
diff -u -r1.19 kern_sx.c
--- sys/kern/kern_sx.c 11 Jun 2003 00:56:56 -0000 1.19
+++ sys/kern/kern_sx.c 19 Jun 2003 20:28:19 -0000
@@ -126,9 +126,8 @@
sx->sx_cnt++;
LOCK_LOG_LOCK("SLOCK", &sx->sx_object, 0, 0, file, line);
- WITNESS_LOCK(&sx->sx_object, 0, file, line);
-
mtx_unlock(sx->sx_lock);
+ WITNESS_LOCK(&sx->sx_object, 0, file, line);
}
int
@@ -139,8 +138,8 @@
if (sx->sx_cnt >= 0) {
sx->sx_cnt++;
LOCK_LOG_TRY("SLOCK", &sx->sx_object, 0, 1, file, line);
- WITNESS_LOCK(&sx->sx_object, LOP_TRYLOCK, file, line);
mtx_unlock(sx->sx_lock);
+ WITNESS_LOCK(&sx->sx_object, LOP_TRYLOCK, file, line);
return (1);
} else {
LOCK_LOG_TRY("SLOCK", &sx->sx_object, 0, 0, file, line);
@@ -180,9 +179,8 @@
sx->sx_xholder = curthread;
LOCK_LOG_LOCK("XLOCK", &sx->sx_object, 0, 0, file, line);
- WITNESS_LOCK(&sx->sx_object, LOP_EXCLUSIVE, file, line);
-
mtx_unlock(sx->sx_lock);
+ WITNESS_LOCK(&sx->sx_object, LOP_EXCLUSIVE, file, line);
}
int
@@ -194,9 +192,9 @@
sx->sx_cnt--;
sx->sx_xholder = curthread;
LOCK_LOG_TRY("XLOCK", &sx->sx_object, 0, 1, file, line);
+ mtx_unlock(sx->sx_lock);
WITNESS_LOCK(&sx->sx_object, LOP_EXCLUSIVE | LOP_TRYLOCK, file,
line);
- mtx_unlock(sx->sx_lock);
return (1);
} else {
LOCK_LOG_TRY("XLOCK", &sx->sx_object, 0, 0, file, line);
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