Date: Wed, 6 Apr 2022 10:24:18 -0400 From: Mark Johnston <markj@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> 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: <Yk2ikhXUhi4tzzhf@nuc> In-Reply-To: <Ykt5MtaYq976GFPg@kib.kiev.ua> References: <202203011741.221Hftwu093303@gitrepo.freebsd.org> <Ykt4qtwH9qRg9kkV@kib.kiev.ua> <Ykt5MtaYq976GFPg@kib.kiev.ua>
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On Tue, Apr 05, 2022 at 02:03:14AM +0300, Konstantin Belousov wrote: > On Tue, Apr 05, 2022 at 02:01:04AM +0300, Konstantin Belousov wrote: > > 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(). > > What probably should be done is referencing the target process vmspace, > instead. You mean, callers should use something like this, with the proc locked: bool vmspace_reference_live(struct vmspace *vm) { return (refcount_acquire_if_not_zero(&vm->vm_refcnt)); } ? Yes, I think that'd be sufficient. Though, in practice most callers of proc_rwmem() really do need the proc to be held for other purposes, so the existing assertion serves as a close enough approximation. We could maybe just relax the assertion in proc_rwmem() to MPASS(refcount_load(&p->p_vmspace->vm_refcnt) > 0); since this is implied by p_hold > 0. > > > 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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