Date: Sat, 17 Feb 2018 18:26:32 +0200 From: Konstantin Belousov <kib@freebsd.org> To: Mateusz Guzik <mjguzik@gmail.com>, peter@holm.cc Cc: 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: <20180217162632.GQ94212@kib.kiev.ua> In-Reply-To: <CAGudoHG%2BXxobj9ziASdW3ugQoxd843K%2BncDSyLbEEGQ20QzAQg@mail.gmail.com> References: <201802170848.w1H8mkfb081764@repo.freebsd.org> <20180217112738.GO94212@kib.kiev.ua> <CAGudoHG%2BXxobj9ziASdW3ugQoxd843K%2BncDSyLbEEGQ20QzAQg@mail.gmail.com>
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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. > > 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.
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