Re: git: 32114b639fa1 - main - Add PROC_COW_CHANGECOUNT and thread_cow_synced
Date: Sun, 13 Feb 2022 22:00:44 UTC
On Sat, Feb 12, 2022 at 10:22:28PM +0100, Mateusz Guzik wrote: > On 2/12/22, Konstantin Belousov <kostikbel@gmail.com> wrote: > > On Sat, Feb 12, 2022 at 07:50:21PM +0100, Mateusz Guzik wrote: > >> On 2/11/22, Konstantin Belousov <kostikbel@gmail.com> 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 <mjg@FreeBSD.org> > >> >> AuthorDate: 2022-02-01 13:13:13 +0000 > >> >> Commit: Mateusz Guzik <mjg@FreeBSD.org> > >> >> 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 <mjguzik gmail.com> > > > > > -- > Mateusz Guzik <mjguzik gmail.com>