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Date:      Sat, 17 Feb 2018 17:38:22 +0100
From:      Peter Holm <peter@holm.cc>
To:        Konstantin Belousov <kib@freebsd.org>
Cc:        Mateusz Guzik <mjguzik@gmail.com>, Mateusz Guzik <mjg@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   Re: svn commit: r329448 - head/sys/kern
Message-ID:  <20180217163822.GA81555@x2.osted.lan>
In-Reply-To: <20180217162632.GQ94212@kib.kiev.ua>
References:  <201802170848.w1H8mkfb081764@repo.freebsd.org> <20180217112738.GO94212@kib.kiev.ua> <CAGudoHG%2BXxobj9ziASdW3ugQoxd843K%2BncDSyLbEEGQ20QzAQg@mail.gmail.com> <20180217162632.GQ94212@kib.kiev.ua>

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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@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@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@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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