[patch] Userland DTrace

Matt Burke mattblists at icritical.com
Fri Feb 8 16:11:23 UTC 2013

I've been spending some time trying to get the fasttrap provider to work
on FreeBSD without panicing. I believe I have succeeded, at least to the
point where it's no longer panicing.

There were two panic causes. The first was
http://www.freebsd.org/cgi/query-pr.cgi?pr=165541 - the FreeBSD port of
fasttrap.c caused ftp_rcount to be left >0. To fix this I've got rid of
the early return and reverted to the opensolaris way.

A second panic then showed up intermittently when fasttrap_pid_cleanup_cb
was run while something in userland had locks. Using sx_try_xlock calls
has stopped the panics and shouldn't affect operation AFAICT.

This is against r246454.

Although this has fixed the panics for me, I'm finding a lot of stuff just
isn't actually working, with dtrace and the traced process just chewing
CPU. Truss on the dtrace shows a heck of a lot of ptrace() calls and I
have no idea what the target is doing... CPU time is split 2:1

Also noteworthy is the LOR on the first time you try to use the fasttrap
provider: http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/165479

The lock order there seems right, so I'm guessing "something else" must
have done it wrong first? How can I find out what the "something else"


--- a/sys/cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c
@@ -7536,9 +7536,23 @@ dtrace_unregister(dtrace_provider_id_t id)
                        return (EBUSY);
        } else {
+#if defined(sun)
+               if (sx_try_xlock(&dtrace_provider_lock) == 0)
+                       return (EBUSY);
+               if (sx_try_xlock(&mod_lock) == 0) {
+                       mutex_exit(&dtrace_provider_lock);
+                       return (EBUSY);
+               }
+               if (sx_try_xlock(&dtrace_lock) == 0) {
+                       mutex_exit(&mod_lock);
+                       mutex_exit(&dtrace_provider_lock);
+                       return (EBUSY);
+               }
--- a/sys/cddl/contrib/opensolaris/uts/common/dtrace/fasttrap.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/dtrace/fasttrap.c
@@ -1116,23 +1116,28 @@ fasttrap_pid_disable(void *arg, dtrace_id_t id, void *parg)
        ASSERT(id == probe->ftp_id);
-       mutex_enter(&provider->ftp_mtx);
         * We won't be able to acquire a /proc-esque lock on the process
         * iff the process is dead and gone. In this case, we rely on the
         * provider lock as a point of mutual exclusion to prevent other
         * DTrace consumers from disabling this probe.
-       if ((p = pfind(probe->ftp_pid)) == NULL) {
-               mutex_exit(&provider->ftp_mtx);
-               return;
+#if defined(sun)
+       if ((p = sprlock(probe->ftp_pid)) != NULL) {
+               ASSERT(!(p->p_flag & SVFORK));
+               mutex_exit(&p->p_lock);
+       }
+       if ((p = pfind(probe->ftp_pid)) != NULL) {
+               _PHOLD(p);
+               PROC_UNLOCK(p);
-#ifdef __FreeBSD__
-       _PHOLD(p);
-       PROC_UNLOCK(p);
+       mutex_enter(&provider->ftp_mtx);
         * Disable all the associated tracepoints (for fully enabled probes).
@@ -1154,6 +1159,13 @@ fasttrap_pid_disable(void *arg, dtrace_id_t id, void *parg)
                if (provider->ftp_retired && !provider->ftp_marked)
                        whack = provider->ftp_marked = 1;
+#if defined(sun)
+               mutex_enter(&p->p_lock);
+               sprunlock(p);
+               PRELE(p);
        } else {
                 * If the process is dead, we're just waiting for the
@@ -1167,9 +1179,6 @@ fasttrap_pid_disable(void *arg, dtrace_id_t id, void *parg)
        if (whack)
-#ifdef __FreeBSD__
-       PRELE(p);
        if (!probe->ftp_enabled)

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