Date: Sat, 29 May 2021 22:04:17 GMT From: Mateusz Guzik <mjg@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: 9bfddb3ac4ce - main - fd: use PROC_WAIT_UNLOCKED when clearing p_fd/p_pd Message-ID: <202105292204.14TM4HSc026165@gitrepo.freebsd.org>
next in thread | raw e-mail | index | archive | help
The branch main has been updated by mjg: URL: https://cgit.FreeBSD.org/src/commit/?id=9bfddb3ac4ce8a2fbd5bb212a263747343a931e7 commit 9bfddb3ac4ce8a2fbd5bb212a263747343a931e7 Author: Mateusz Guzik <mjg@FreeBSD.org> AuthorDate: 2021-05-27 14:29:26 +0000 Commit: Mateusz Guzik <mjg@FreeBSD.org> 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); }
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?202105292204.14TM4HSc026165>