On Sun, Sep 22, 2013 at 9:34 PM, Konstantin Belousov <kostik...@gmail.com> wrote: > On Sun, Sep 22, 2013 at 11:19:16PM +0300, Konstantin Belousov wrote: >> On Sun, Sep 22, 2013 at 01:14:21PM -0700, Matthew Fleming wrote: >> > On Sun, Sep 22, 2013 at 12:23 PM, Konstantin Belousov >> > <k...@freebsd.org>wrote: >> >> > > Author: kib >> > > Date: Sun Sep 22 19:23:48 2013 >> > > New Revision: 255797 >> > > URL: http://svnweb.freebsd.org/changeset/base/255797 >> > > >> > > Log: >> > > Increase the chance of the buffer write from the bufdaemon helper >> > > context to succeed. If the locked vnode which owns the buffer to be >> > > written is shared locked, try the non-blocking upgrade of the lock to >> > > exclusive. >> > > >> > > PR: kern/178997 >> > > Reported and tested by: Klaus Weber < >> > > fbsd-bugs-201...@unix-admin.de> >> > > Sponsored by: The FreeBSD Foundation >> > > MFC after: 1 week >> > > Approved by: re (marius) >> > > >> > > Modified: >> > > head/sys/kern/vfs_bio.c >> > > >> > > Modified: head/sys/kern/vfs_bio.c >> > > >> > > ============================================================================== >> > > --- head/sys/kern/vfs_bio.c Sun Sep 22 19:15:24 2013 (r255796) >> > > +++ head/sys/kern/vfs_bio.c Sun Sep 22 19:23:48 2013 (r255797) >> > > @@ -2624,6 +2624,8 @@ flushbufqueues(struct vnode *lvp, int ta >> > > int hasdeps; >> > > int flushed; >> > > int queue; >> > > + int error; >> > > + bool unlock; >> > > >> > > flushed = 0; >> > > queue = QUEUE_DIRTY; >> > > @@ -2699,7 +2701,16 @@ flushbufqueues(struct vnode *lvp, int ta >> > > BUF_UNLOCK(bp); >> > > continue; >> > > } >> > > - if (vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT | LK_CANRECURSE) >> > > == 0) { >> > > + if (lvp == NULL) { >> > > + unlock = true; >> > > + error = vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT); >> > > + } else { >> > > + ASSERT_VOP_LOCKED(vp, "getbuf"); >> > > + unlock = false; >> > > + error = VOP_ISLOCKED(vp) == LK_EXCLUSIVE ? 0 : >> > > + vn_lock(vp, LK_UPGRADE | LK_NOWAIT); >> > > >> > >> > I don't think this is quite right. >> > >> > When the lock is held shared, and VOP_LOCK is implemented by lockmgr(9), >> > (i.e. all in-tree filesystems?), LK_UPGRADE may drop the lock, and not >> > reacquire it. This would happen when the vnode is locked shared, the >> > upgrade fails (2 shared owners), then lockmgr(9) will try to lock EX, which >> > will also fail (still one shared owner). The caller's lock is no longer >> > held. >> > >> > Doesn't that scenario (LK_UPGRADE failing) cause problems both for the >> > caller (unexpected unlock) and for flushbufqueues(), which expects the >> > vnode lock to be held since lvp is non-NULL? >> >> Does it ? If the lock is dropped, the code is indeed in trouble. >> Please note that LK_NOWAIT is specified for upgrade, and I believe >> that this causes lockmgr to return with EBUSY without dropping >> the lock. > > Yes, you are right, I reverted the patch. Thank you for noting this. > > I am bitten by unreasonable behaviour of non-blocking upgrade once more. > It has a history. > > Some time ago I proposed the following patch, which was turned down. > That time, I was able to work-around the case. For the bufdaemon helper, > I do not see any way to avoid this, except of sometimes locking the > reader vnode exclusive in anticipation of the too high dirty buffer > mark.
If you are speaking about me, you are mistaken, I never turned out this patch. What I said is completely different: I said that LK_UPGRADE is a completely wrong semantic because it can hide wrong things like the one you hit today. I wanted to see it removed and replaced by explicit LK_RELEASE + LK_EXCLUSIVE operations. Note that this would have avoided this patch. I'm completely in favour of LK_TRYUPGRADE. Attilio _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
