Date: Tue, 5 Apr 2022 02:00:58 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Mark Johnston <markj@freebsd.org> Cc: src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 12fb39ec3e6b - main - proc: Relax proc_rwmem()'s assertion on the process hold count Message-ID: <Ykt4qtwH9qRg9kkV@kib.kiev.ua> In-Reply-To: <202203011741.221Hftwu093303@gitrepo.freebsd.org> References: <202203011741.221Hftwu093303@gitrepo.freebsd.org>
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On Tue, Mar 01, 2022 at 05:41:55PM +0000, Mark Johnston wrote: > The branch main has been updated by markj: > > URL: https://cgit.FreeBSD.org/src/commit/?id=12fb39ec3e6bc529feff3ba2862c6a4a30bd54eb > > commit 12fb39ec3e6bc529feff3ba2862c6a4a30bd54eb > Author: Mark Johnston <markj@FreeBSD.org> > AuthorDate: 2022-03-01 16:48:39 +0000 > Commit: Mark Johnston <markj@FreeBSD.org> > CommitDate: 2022-03-01 17:40:35 +0000 > > proc: Relax proc_rwmem()'s assertion on the process hold count > > This reference ensures that the process and its associated vmspace will > not be destroyed while proc_rwmem() is executing. If, however, the > calling thread belongs to the target process, then it is unnecessary to > hold the process. In particular, fasttrap - a module which enables > userspace dtrace - may frequently call proc_rwmem(), and we'd prefer to > avoid the overhead of locking and bumping the hold count when possible. In fact I am not sure it makes much sense to disable swap out for remote process as well. With the current definition of p_hold, it only prevents kstack pages reuse, which should be irrelevant for proc_rwmem(). > > Thus, make the assertion conditional on "p != curproc". Also assert > that the process is not already exiting. No functional change intended. > > MFC after: 2 weeks > Sponsored by: The FreeBSD Foundation > --- > sys/kern/sys_process.c | 9 +++++---- > 1 file changed, 5 insertions(+), 4 deletions(-) > > diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c > index 582bff962f1a..8d8c5a1d34ff 100644 > --- a/sys/kern/sys_process.c > +++ b/sys/kern/sys_process.c > @@ -336,11 +336,12 @@ proc_rwmem(struct proc *p, struct uio *uio) > int error, fault_flags, page_offset, writing; > > /* > - * Assert that someone has locked this vmspace. (Should be > - * curthread but we can't assert that.) This keeps the process > - * from exiting out from under us until this operation completes. > + * Make sure that the process' vmspace remains live. > */ > - PROC_ASSERT_HELD(p); > + if (p != curproc) > + PROC_ASSERT_HELD(p); > + KASSERT((p->p_flag & P_WEXIT) == 0, > + ("%s: process %p is exiting", __func__, p)); > PROC_LOCK_ASSERT(p, MA_NOTOWNED); > > /*
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