Date: Fri, 29 May 2020 07:44:02 +0000 (UTC) From: Andriy Gapon <avg@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r361620 - head/sys/kern Message-ID: <202005290744.04T7i2ZD064586@repo.freebsd.org>
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Author: avg Date: Fri May 29 07:44:02 2020 New Revision: 361620 URL: https://svnweb.freebsd.org/changeset/base/361620 Log: corefile_open_last: don't keep a locked vnode while locking other ones Consider this scenario: - kern.corefile=/var/coredumps/%N.%U.%I.core - multiple processes with the same name crash at the same time It's possible that one process selects existing file N as oldvp while it keeps looking for an unused file number. Another process scans through files and stumbles upon N. That process would be blocked on the vnode lock while holding the directory vnode exclusively locked. The first process would, thus, get blocked on the directory's vnode lock. More generally, holding a file's vnode lock (oldvp) while trying to lock its directory (for the next lookup) is a violation of the vnode locking order. I have observed this deadlock in the wild. So, the change to keep oldvp "opened" but unlocked and to lock it again only if it's to be returned as the result. As kib noted, an alternative would be to keep the directory locked and to use VOP_LOOKUP directly for scanning through existing core files. Reviewed by: kib MFC after: 2 weeks Differential Revision: https://reviews.freebsd.org/D25027 Modified: head/sys/kern/kern_sig.c Modified: head/sys/kern/kern_sig.c ============================================================================== --- head/sys/kern/kern_sig.c Fri May 29 07:38:57 2020 (r361619) +++ head/sys/kern/kern_sig.c Fri May 29 07:44:02 2020 (r361620) @@ -3454,8 +3454,9 @@ corefile_open_last(struct thread *td, char *name, int (lasttime.tv_sec == vattr.va_mtime.tv_sec && lasttime.tv_nsec >= vattr.va_mtime.tv_nsec)) { if (oldvp != NULL) - vnode_close_locked(td, oldvp); + vn_close(oldvp, FWRITE, td->td_ucred, td); oldvp = vp; + VOP_UNLOCK(oldvp); lasttime = vattr.va_mtime; } else { vnode_close_locked(td, vp); @@ -3466,12 +3467,18 @@ corefile_open_last(struct thread *td, char *name, int if (nextvp == NULL) { if ((td->td_proc->p_flag & P_SUGID) != 0) { error = EFAULT; - vnode_close_locked(td, oldvp); + vn_close(oldvp, FWRITE, td->td_ucred, td); } else { nextvp = oldvp; + error = vn_lock(nextvp, LK_EXCLUSIVE); + if (error != 0) { + vn_close(nextvp, FWRITE, td->td_ucred, + td); + nextvp = NULL; + } } } else { - vnode_close_locked(td, oldvp); + vn_close(oldvp, FWRITE, td->td_ucred, td); } } if (error != 0) {
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