Sorry to scare you, I was a little unhappy about dump hanging.
Based on the cvs repository I wrote a little patch that combines two changes made by scott and jeff that works agains 7.0-RELEASE (looking at the commit logs scared me away from trying STABLE on production right now. After applying this patch I could run dumps successfully on seven machines where it hung before on every single attempt (use is at your own risk of course).

cd /usr/src/sys/kern
patch < /tmp/mysleepqueue.patch
recompile and install kernel
reboot

Since I also found a fatal bug in ipv6 (panic on ping6) it might be better for you to wait for 7.1, for us there is no way back now.

cheers
michael

Mike Tancsa wrote:
At 05:07 AM 9/1/2008, Derek Kuliński wrote:

Now I'm honestly a bit scared about it (even if it will be fixed
before 7.1, I'm not sure I'll hurry with the update).

There have been a number of commits to releng_7 that fixed dump issues for me. A box that used to regularly exhibit hung dump processes have been working fine since April. e.g. a kernel from
7.0-STABLE FreeBSD 7.0-STABLE #4: Wed Apr 30

does weekly level 0 dumps and daily differential dumps on the file systems below without issue
% df -i
Filesystem 1K-blocks Used Avail Capacity iused ifree %iused Mounted on /dev/twed0s1a 2026030 284346 1579602 15% 2937 279685 1% / devfs 1 1 0 100% 0 0 100% /dev /dev/twed0s1d 5077038 575828 4095048 12% 1197 658257 0% /tmp /dev/twed0s1e 20308398 11072840 7610888 59% 1065406 1572416 40% /usr /dev/twed0s1f 20308398 13275050 5408678 71% 13750 2624072 1% /var /dev/twed0s1g 246875258 186393906 40731332 82% 9118036 22794922 29% /zoo

However, you should test and make sure it works for you.

        ---Mike

        ---Mike

--- subr_sleepqueue.c~  2008-09-01 05:14:28.000000000 +0200
+++ subr_sleepqueue.c   2008-09-01 05:14:28.000000000 +0200
@@ -177,7 +177,7 @@
        for (i = 0; i < SC_TABLESIZE; i++) {
                LIST_INIT(&sleepq_chains[i].sc_queues);
                mtx_init(&sleepq_chains[i].sc_lock, "sleepq chain", NULL,
-                   MTX_SPIN);
+                   MTX_SPIN | MTX_RECURSE);
 #ifdef SLEEPQUEUE_PROFILING
                snprintf(chain_name, sizeof(chain_name), "%d", i);
                chain_oid = SYSCTL_ADD_NODE(NULL, 
@@ -403,12 +403,15 @@
                mtx_unlock(&ps->ps_mtx);
        }
        /*
-        * Lock sleepq chain before unlocking proc
-        * without this, we could lose a race.
-        */
+        * Lock the per-process spinlock prior to dropping the PROC_LOCK
+        * to avoid a signal delivery race.  PROC_LOCK, PROC_SLOCK, and
+        * thread_lock() are currently held in tdsignal().
+        */
+       PROC_SLOCK(p);
        mtx_lock_spin(&sc->sc_lock);
        PROC_UNLOCK(p);
        thread_lock(td);
+       PROC_SUNLOCK(p);
        if (ret == 0) {
                if (!(td->td_flags & TDF_INTERRUPT)) {
                        sleepq_switch(wchan);
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