svn commit: r329448 - head/sys/kern
Peter Holm
peter at holm.cc
Sat Feb 17 16:38:33 UTC 2018
On Sat, Feb 17, 2018 at 06:26:32PM +0200, Konstantin Belousov wrote:
> On Sat, Feb 17, 2018 at 05:07:07PM +0100, Mateusz Guzik wrote:
> > On Sat, Feb 17, 2018 at 01:27:38PM +0200, Konstantin Belousov wrote:
> > > On Sat, Feb 17, 2018 at 08:48:46AM +0000, Mateusz Guzik wrote:
> > > > Author: mjg
> > > > Date: Sat Feb 17 08:48:45 2018
> > > > New Revision: 329448
> > > > URL: https://svnweb.freebsd.org/changeset/base/329448
> > > >
> > > > Log:
> > > > exit: get rid of PROC_SLOCK when checking a process to report
> > > Was this tested ?
> > >
> >
> > I was trussing multithreaded microbenchmarks, no issues.
> >
> > > In particular, are you aware of r309539 ?
> > >
> >
> > So it looks like I misread the code - I have grepped
> > thread_suspend_switch operating with the proc locked and misread
> > thread_suspend_one's assert as PROC_LOCK_ASSERT.
> >
> > That said, I think this is harmless. Regardless of the lock the
> > inspecting thread can race and check "too soon". Even for a case where
> > it decides to report, I don't see anything which would depend on the
> > suspending thread to finish.
> It was definitely not harmless when I tried to avoid the spin lock there,
> but I do not remember exact failure mode. Most likely, it was a missed
> report of the traced child indeed, but I am not sure that truss triggered
> it. Most likely, Peter Holm was the reporter, since he is listed in
> the commit.
>
I ran a truss(1) test on r329456 and it fails. I have not had a
chance to look closer at this, but this is what I see:
[root at mercat1 /home/pho]# pgrep truss | xargs ps -Hlp
UID PID PPID CPU PRI NI VSZ RSS MWCHAN STAT TT TIME COMMAND
0 41149 41118 0 52 0 11532 2588 wait I 0 0:01.38 truss /tmp/ttruss 10
0 41151 41149 0 52 0 13156 2300 - TX 0 0:00.98 /tmp/ttruss 10
0 41151 41149 0 52 0 13156 2300 - TX 0 0:00.00 /tmp/ttruss 10
[root at mercat1 /home/pho]# procstat -k 41151
PID TID COMM TDNAME KSTACK
41151 100211 ttruss - mi_switch thread_suspend_switch ptracestop amd64_syscall fast_syscall_common
41151 100765 ttruss - mi_switch thread_suspend_check ast doreti_ast
[root at mercat1 /home/pho]#
> >
> > However, locking can be employed in a way which is avoided in the common
> > case:
> >
> > diff --git a/sys/kern/kern_exit.c b/sys/kern/kern_exit.c
> > index b063bda5b7ff..4ae24bcd7059 100644
> > --- a/sys/kern/kern_exit.c
> > +++ b/sys/kern/kern_exit.c
> > @@ -1174,6 +1174,7 @@ kern_wait6(struct thread *td, idtype_t idtype, id_t
> > id, int *status,
> > struct proc *p, *q;
> > pid_t pid;
> > int error, nfound, ret;
> > + bool report;
> >
> > AUDIT_ARG_VALUE((int)idtype); /* XXX - This is likely wrong! */
> > AUDIT_ARG_PID((pid_t)id); /* XXX - This may be wrong! */
> > @@ -1226,27 +1227,36 @@ kern_wait6(struct thread *td, idtype_t idtype, id_t
> > id, int *status,
> > PROC_LOCK_ASSERT(p, MA_OWNED);
> >
> > if ((options & WTRAPPED) != 0 &&
> > - (p->p_flag & P_TRACED) != 0 &&
> > - (p->p_flag & (P_STOPPED_TRACE | P_STOPPED_SIG)) != 0 &&
> > - p->p_suspcount == p->p_numthreads &&
> > - (p->p_flag & P_WAITED) == 0) {
> > + (p->p_flag & P_TRACED) != 0) {
> > + PROC_SLOCK(p);
> > + report =
> > + ((p->p_flag & (P_STOPPED_TRACE |
> > P_STOPPED_SIG)) &&
> > + p->p_suspcount == p->p_numthreads &&
> > + (p->p_flag & P_WAITED) == 0);
> > + PROC_SUNLOCK(p);
> > + if (report) {
> > CTR4(KTR_PTRACE,
> > "wait: returning trapped pid %d status %#x "
> > "(xstat %d) xthread %d",
> > p->p_pid, W_STOPCODE(p->p_xsig), p->p_xsig,
> > p->p_xthread != NULL ?
> > p->p_xthread->td_tid : -1);
> > - report_alive_proc(td, p, siginfo, status, options,
> > - CLD_TRAPPED);
> > - return (0);
> > + report_alive_proc(td, p, siginfo, status,
> > + options, CLD_TRAPPED);
> > + return (0);
> > + }
> > }
> > if ((options & WUNTRACED) != 0 &&
> > - (p->p_flag & P_STOPPED_SIG) != 0 &&
> > - p->p_suspcount == p->p_numthreads &&
> > - (p->p_flag & P_WAITED) == 0) {
> > - report_alive_proc(td, p, siginfo, status, options,
> > + report_alive_proc(td, p, siginfo, status,
> > + options, CLD_TRAPPED);
> > + return (0);
> > + }
> > }
> > if ((options & WUNTRACED) != 0 &&
> > - (p->p_flag & P_STOPPED_SIG) != 0 &&
> > - p->p_suspcount == p->p_numthreads &&
> > - (p->p_flag & P_WAITED) == 0) {
> > - report_alive_proc(td, p, siginfo, status, options,
> > - CLD_STOPPED);
> > - return (0);
> > + (p->p_flag & P_STOPPED_SIG) != 0) {
> > + PROC_SLOCK(p);
> > + report = (p->p_suspcount == p->p_numthreads &&
> > + ((p->p_flag & P_WAITED) == 0));
> > + PROC_SUNLOCK(p);
> > + if (report) {
> > + report_alive_proc(td, p, siginfo, status,
> > + options, CLD_STOPPED);
> > + return (0);
> > + }
> > }
> > if ((options & WCONTINUED) != 0 &&
> > (p->p_flag & P_CONTINUED) != 0) {
> >
> If this works, I am fine with the patch. Still, I would prefer to have
> the original commit reverted for now, until the fix is tested. Then,
> commit updated patch.
--
Peter
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