From owner-dev-commits-src-main@freebsd.org Sat May 29 22:04:17 2021 Return-Path: Delivered-To: dev-commits-src-main@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 67FED648AC6; Sat, 29 May 2021 22:04:17 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from mxrelay.nyi.freebsd.org (mxrelay.nyi.freebsd.org [IPv6:2610:1c1:1:606c::19:3]) (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-signature RSA-PSS (4096 bits) client-digest SHA256) (Client CN "mxrelay.nyi.freebsd.org", Issuer "R3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4FswZ926Czz4hMc; Sat, 29 May 2021 22:04:17 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from gitrepo.freebsd.org (gitrepo.freebsd.org [IPv6:2610:1c1:1:6068::e6a:5]) (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 mxrelay.nyi.freebsd.org (Postfix) with ESMTPS id 30221120E9; Sat, 29 May 2021 22:04:17 +0000 (UTC) (envelope-from git@FreeBSD.org) Received: from gitrepo.freebsd.org ([127.0.1.44]) by gitrepo.freebsd.org (8.16.1/8.16.1) with ESMTP id 14TM4HHr026166; Sat, 29 May 2021 22:04:17 GMT (envelope-from git@gitrepo.freebsd.org) Received: (from git@localhost) by gitrepo.freebsd.org (8.16.1/8.16.1/Submit) id 14TM4HSc026165; Sat, 29 May 2021 22:04:17 GMT (envelope-from git) Date: Sat, 29 May 2021 22:04:17 GMT Message-Id: <202105292204.14TM4HSc026165@gitrepo.freebsd.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org From: Mateusz Guzik Subject: git: 9bfddb3ac4ce - main - fd: use PROC_WAIT_UNLOCKED when clearing p_fd/p_pd MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Git-Committer: mjg X-Git-Repository: src X-Git-Refname: refs/heads/main X-Git-Reftype: branch X-Git-Commit: 9bfddb3ac4ce8a2fbd5bb212a263747343a931e7 Auto-Submitted: auto-generated X-BeenThere: dev-commits-src-main@freebsd.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: Commit messages for the main branch of the src repository List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 29 May 2021 22:04:17 -0000 The branch main has been updated by mjg: URL: https://cgit.FreeBSD.org/src/commit/?id=9bfddb3ac4ce8a2fbd5bb212a263747343a931e7 commit 9bfddb3ac4ce8a2fbd5bb212a263747343a931e7 Author: Mateusz Guzik AuthorDate: 2021-05-27 14:29:26 +0000 Commit: Mateusz Guzik CommitDate: 2021-05-29 22:04:09 +0000 fd: use PROC_WAIT_UNLOCKED when clearing p_fd/p_pd --- sys/kern/kern_descrip.c | 37 +++++++++++++++++++++++++++++-------- 1 file changed, 29 insertions(+), 8 deletions(-) diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c index 09eb0770bcda..36092c9acd42 100644 --- a/sys/kern/kern_descrip.c +++ b/sys/kern/kern_descrip.c @@ -2198,13 +2198,28 @@ pdinit(struct pwddesc *pdp, bool keeplock) return (newpdp); } +/* + * Hold either filedesc or pwddesc of the passed process. + * + * The process lock is used to synchronize against the target exiting and + * freeing the data. + * + * Clearing can be ilustrated in 3 steps: + * 1. set the pointer to NULL. Either routine can race against it, hence + * atomic_load_ptr. + * 2. observe the process lock as not taken. Until then fdhold/pdhold can + * race to either still see the pointer or find NULL. It is still safe to + * grab a reference as clearing is stalled. + * 3. after the lock is observed as not taken, any fdhold/pdhold calls are + * guaranteed to see NULL, making it safe to finish clearing + */ static struct filedesc * fdhold(struct proc *p) { struct filedesc *fdp; PROC_LOCK_ASSERT(p, MA_OWNED); - fdp = p->p_fd; + fdp = atomic_load_ptr(&p->p_fd); if (fdp != NULL) refcount_acquire(&fdp->fd_holdcnt); return (fdp); @@ -2216,7 +2231,7 @@ pdhold(struct proc *p) struct pwddesc *pdp; PROC_LOCK_ASSERT(p, MA_OWNED); - pdp = p->p_pd; + pdp = atomic_load_ptr(&p->p_pd); if (pdp != NULL) refcount_acquire(&pdp->pd_refcount); return (pdp); @@ -2582,9 +2597,12 @@ fdescfree(struct thread *td) if (p->p_fdtol != NULL) fdclearlocks(td); - PROC_LOCK(p); - p->p_fd = NULL; - PROC_UNLOCK(p); + /* + * Check fdhold for an explanation. + */ + atomic_store_ptr(&p->p_fd, NULL); + atomic_thread_fence_seq_cst(); + PROC_WAIT_UNLOCKED(p); if (refcount_release(&fdp->fd_refcnt) == 0) return; @@ -2602,9 +2620,12 @@ pdescfree(struct thread *td) pdp = p->p_pd; MPASS(pdp != NULL); - PROC_LOCK(p); - p->p_pd = NULL; - PROC_UNLOCK(p); + /* + * Check pdhold for an explanation. + */ + atomic_store_ptr(&p->p_pd, NULL); + atomic_thread_fence_seq_cst(); + PROC_WAIT_UNLOCKED(p); pddrop(pdp); }