From owner-svn-src-head@freebsd.org Sat Feb 17 16:26:44 2018 Return-Path: Delivered-To: svn-src-head@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id C7360F1D77F; Sat, 17 Feb 2018 16:26:44 +0000 (UTC) (envelope-from kib@freebsd.org) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 51AAA87CE3; Sat, 17 Feb 2018 16:26:44 +0000 (UTC) (envelope-from kib@freebsd.org) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id w1HGQWZx063679 (version=TLSv1.2 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO); Sat, 17 Feb 2018 18:26:35 +0200 (EET) (envelope-from kib@freebsd.org) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua w1HGQWZx063679 Received: (from kostik@localhost) by tom.home (8.15.2/8.15.2/Submit) id w1HGQW2f063678; Sat, 17 Feb 2018 18:26:32 +0200 (EET) (envelope-from kib@freebsd.org) X-Authentication-Warning: tom.home: kostik set sender to kib@freebsd.org using -f Date: Sat, 17 Feb 2018 18:26:32 +0200 From: Konstantin Belousov To: Mateusz Guzik , peter@holm.cc Cc: Mateusz Guzik , src-committers , 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> References: <201802170848.w1H8mkfb081764@repo.freebsd.org> <20180217112738.GO94212@kib.kiev.ua> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.3 (2018-01-21) X-Spam-Status: No, score=-2.9 required=5.0 tests=ALL_TRUSTED,BAYES_00 autolearn=ham autolearn_force=no version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on tom.home X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.25 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 17 Feb 2018 16:26:45 -0000 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.