From nobody Sun Feb 13 22:00:44 2022 X-Original-To: dev-commits-src-main@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 30EF419C22E6; Sun, 13 Feb 2022 22:01:05 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4JxhBN1Lx4z3nsX; Sun, 13 Feb 2022 22:01:00 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.16.1/8.16.1) with ESMTPS id 21DM0jt3024966 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Mon, 14 Feb 2022 00:00:48 +0200 (EET) (envelope-from kostikbel@gmail.com) Received: (from kostik@localhost) by tom.home (8.16.1/8.16.1/Submit) id 21DM0iq6024965; Mon, 14 Feb 2022 00:00:44 +0200 (EET) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Mon, 14 Feb 2022 00:00:44 +0200 From: Konstantin Belousov To: Mateusz Guzik Cc: src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 32114b639fa1 - main - Add PROC_COW_CHANGECOUNT and thread_cow_synced Message-ID: References: <202202111357.21BDvxhr093033@gitrepo.freebsd.org> List-Id: Commit messages for the main branch of the src repository List-Archive: https://lists.freebsd.org/archives/dev-commits-src-main List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-dev-commits-src-main@freebsd.org X-BeenThere: dev-commits-src-main@freebsd.org MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FROM, NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.5 X-Spam-Checker-Version: SpamAssassin 3.4.5 (2021-03-20) on tom.home X-Rspamd-Queue-Id: 4JxhBN1Lx4z3nsX X-Spamd-Bar: -- Authentication-Results: mx1.freebsd.org; dkim=none; dmarc=fail reason="No valid SPF, No valid DKIM" header.from=gmail.com (policy=none); spf=softfail (mx1.freebsd.org: 2001:470:d5e7:1::1 is neither permitted nor denied by domain of kostikbel@gmail.com) smtp.mailfrom=kostikbel@gmail.com X-Spamd-Result: default: False [-2.94 / 15.00]; ARC_NA(0.00)[]; NEURAL_HAM_MEDIUM(-0.98)[-0.982]; RCVD_TLS_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_THREE(0.00)[4]; FREEMAIL_FROM(0.00)[gmail.com]; NEURAL_HAM_LONG(-1.00)[-0.995]; MIME_GOOD(-0.10)[text/plain]; HAS_XAW(0.00)[]; R_SPF_SOFTFAIL(0.00)[~all:c]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_SOME(0.00)[]; BLOCKLISTDE_FAIL(0.00)[2001:470:d5e7:1::1:server fail]; NEURAL_HAM_SHORT(-0.97)[-0.966]; MLMMJ_DEST(0.00)[dev-commits-src-all,dev-commits-src-main]; FREEMAIL_TO(0.00)[gmail.com]; FROM_EQ_ENVFROM(0.00)[]; R_DKIM_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; ASN(0.00)[asn:6939, ipnet:2001:470::/32, country:US]; RCVD_COUNT_TWO(0.00)[2]; FREEMAIL_ENVFROM(0.00)[gmail.com]; DMARC_POLICY_SOFTFAIL(0.10)[gmail.com : No valid SPF, No valid DKIM,none] X-ThisMailContainsUnwantedMimeParts: N On Sat, Feb 12, 2022 at 10:22:28PM +0100, Mateusz Guzik wrote: > On 2/12/22, Konstantin Belousov wrote: > > On Sat, Feb 12, 2022 at 07:50:21PM +0100, Mateusz Guzik wrote: > >> On 2/11/22, Konstantin Belousov wrote: > >> > On Fri, Feb 11, 2022 at 01:57:59PM +0000, Mateusz Guzik wrote: > >> >> The branch main has been updated by mjg: > >> >> > >> >> URL: > >> >> https://cgit.FreeBSD.org/src/commit/?id=32114b639fa1ad777312eebe14a9f677bd7be2ea > >> >> > >> >> commit 32114b639fa1ad777312eebe14a9f677bd7be2ea > >> >> Author: Mateusz Guzik > >> >> AuthorDate: 2022-02-01 13:13:13 +0000 > >> >> Commit: Mateusz Guzik > >> >> CommitDate: 2022-02-11 11:44:07 +0000 > >> >> > >> >> Add PROC_COW_CHANGECOUNT and thread_cow_synced > >> >> > >> >> Combined they can be used to avoid a proc lock/unlock cycle in the > >> >> syscall handler for curthread, see upcoming examples. > >> >> --- > >> >> sys/kern/kern_thread.c | 13 +++++++++++++ > >> >> sys/sys/proc.h | 9 +++++++++ > >> >> 2 files changed, 22 insertions(+) > >> >> > >> >> diff --git a/sys/kern/kern_thread.c b/sys/kern/kern_thread.c > >> >> index dcb52b137b58..bb724a17803e 100644 > >> >> --- a/sys/kern/kern_thread.c > >> >> +++ b/sys/kern/kern_thread.c > >> >> @@ -868,6 +868,19 @@ thread_cow_update(struct thread *td) > >> >> lim_free(oldlimit); > >> >> } > >> >> > >> >> +void > >> >> +thread_cow_synced(struct thread *td) > >> >> +{ > >> >> + struct proc *p; > >> >> + > >> >> + p = td->td_proc; > >> >> + PROC_LOCK_ASSERT(p, MA_OWNED); > >> >> + MPASS(td->td_cowgen != p->p_cowgen); > >> >> + MPASS(td->td_ucred == p->p_ucred); > >> >> + MPASS(td->td_limit == p->p_limit); > >> >> + td->td_cowgen = p->p_cowgen; > >> > This should be store-release, I think. > >> > And corresponding loads in trap() needs to get acquire semantic. > >> > > >> > This is probably a pre-existing bug. > >> > >> I don't think adding fences would improve anything here. First note > >> fences or not, the thread can still race against cowgen changing and > >> miss it this time around. At the same time all updates to cowgen are > >> done with process lock, which will also be taken to sync. Consequently > >> the thread at hand in the worst case will miss cowgen being updated > >> and will act on it next time. If it decides to act on cowgen, it takes > >> the lock which guarantees everything is stable. > > If thread missed generation update, it is it. > > > > Fence would handle the other case, when the thread observed cowgen udate, > > but continue to use old cow values. > > > > The process lock does not help there at all. > > > > What do you mean by 'cow values'? You mean the pointers like p_ucred? > > td_ucred and td_limit are only modified by curthread, thus they don't > require synchronization. p_ucred and p_limit are only accessed with > the lock held, which provides the necessary synchronization. The only > thing inspected locklessly is the cowgen counter, which is safe to > race against. You are talking about removing the process lock, right in the commit message. The only lock case related to cow values is around reading of the p_ pointers, to stash them into td_. > > >> > >> The code definitely should use atomic_store/load_int though, but there > >> are numerous bugs of this sort all over, so I don't think this is > >> pressing. > >> > >> > > >> >> +} > >> >> + > >> >> /* > >> >> * Discard the current thread and exit from its context. > >> >> * Always called with scheduler locked. > >> >> diff --git a/sys/sys/proc.h b/sys/sys/proc.h > >> >> index ff97bfbd54a9..0e33192303f4 100644 > >> >> --- a/sys/sys/proc.h > >> >> +++ b/sys/sys/proc.h > >> >> @@ -1009,6 +1009,14 @@ extern pid_t pid_max; > >> >> (p)->p_cowgen++; \ > >> >> } while (0) > >> >> > >> >> +#define PROC_COW_CHANGECOUNT(td, p) ({ \ > >> >> + struct thread *_td = (td); \ > >> >> + struct proc *_p = (p); \ > >> >> + MPASS(_td == curthread); \ > >> >> + PROC_LOCK_ASSERT(_p, MA_OWNED); \ > >> >> + _p->p_cowgen - _td->td_cowgen; \ > >> >> +}) > >> >> + > >> >> /* Check whether a thread is safe to be swapped out. */ > >> >> #define thread_safetoswapout(td) ((td)->td_flags & TDF_CANSWAP) > >> >> > >> >> @@ -1200,6 +1208,7 @@ void thread_cow_get_proc(struct thread *newtd, > >> >> struct proc *p); > >> >> void thread_cow_get(struct thread *newtd, struct thread *td); > >> >> void thread_cow_free(struct thread *td); > >> >> void thread_cow_update(struct thread *td); > >> >> +void thread_cow_synced(struct thread *td); > >> >> int thread_create(struct thread *td, struct rtprio *rtp, > >> >> int (*initialize_thread)(struct thread *, void *), void *thunk); > >> >> void thread_exit(void) __dead2; > >> > > >> > >> > >> -- > >> Mateusz Guzik > > > > > -- > Mateusz Guzik