[patch] Userland DTrace
Andrey Zonov
zont at FreeBSD.org
Sat Feb 9 22:29:47 UTC 2013
On 2/8/13 8:04 PM, Matt Burke wrote:
> 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
> dtrace:target
>
Great! This fixes panics for me too, but I still cannot get something
useful tracing and after detaching by ctrl+c my programs still segfaults.
Please look at one style comment below.
>
> 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"
> is?
>
>
> Thanks
>
>
> --- 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)
> mutex_enter(&dtrace_provider_lock);
> mutex_enter(&mod_lock);
> mutex_enter(&dtrace_lock);
> +#else
> + if (sx_try_xlock(&dtrace_provider_lock) == 0)
s/sx_try_xlock/mutex_tryenter/
Look at sys/cddl/compat/opensolaris/sys/mutex.h
> + 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);
> + }
> +#endif
> }
>
> /*
> --- 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);
> + }
> +#else
> + if ((p = pfind(probe->ftp_pid)) != NULL) {
> + _PHOLD(p);
> + PROC_UNLOCK(p);
> }
> -#ifdef __FreeBSD__
> - _PHOLD(p);
> - PROC_UNLOCK(p);
> #endif
>
> + 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;
> mutex_exit(&provider->ftp_mtx);
> +
> +#if defined(sun)
> + mutex_enter(&p->p_lock);
> + sprunlock(p);
> +#else
> + PRELE(p);
> +#endif
> } 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)
> fasttrap_pid_cleanup();
>
> -#ifdef __FreeBSD__
> - PRELE(p);
> -#endif
> if (!probe->ftp_enabled)
> return;
>
>
--
Andrey Zonov
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