Date: Wed, 27 Jul 2011 08:40:56 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-fs@freebsd.org Cc: Attilio Rao <attilio@freebsd.org> Subject: Re: Does msodsfs_readdir() require a exclusively locked vnode Message-ID: <201107270840.57104.jhb@freebsd.org> In-Reply-To: <CAJ-FndCWcxRGAuNN=OtKZnWr3JQvcWr969pDqm7KN%2Big5xSFdQ@mail.gmail.com> References: <20110726090441.GD17489@deviant.kiev.zoral.com.ua> <20110726142156.GJ17489@deviant.kiev.zoral.com.ua> <CAJ-FndCWcxRGAuNN=OtKZnWr3JQvcWr969pDqm7KN%2Big5xSFdQ@mail.gmail.com>
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On Tuesday, July 26, 2011 9:56:37 pm Attilio Rao wrote: > 2011/7/26 Kostik Belousov <kostikbel@gmail.com>: > > On Tue, Jul 26, 2011 at 10:07:28AM -0400, Rick Macklem wrote: > >> Kostik Belousov wrote: > >> > On Mon, Jul 25, 2011 at 07:22:40PM -0400, Rick Macklem wrote: > >> > > Hi, > >> > > > >> > > Currently both NFS servers set the vnode lock LK_SHARED > >> > > and so do the local syscalls (at least that's how it looks > >> > > by inspection?). > >> > > > >> > > Peter Holm just posted me this panic, where a test for an > >> > > exclusive vnode lock fails in msdosfs_readdir(). > >> > > KDB: stack backtrace: > >> > > db_trace_self_wrapper(c0efa6f6,c71627f8,c79230b0,c0f2ef29,f19154b8,...) > >> > > at db_trace_self_wrapper+0x26 > >> > > kdb_backtrace(c7f20b38,f19154fc,c0d586d5,f191550c,c7f20ae0,...) at > >> > > kdb_backtrace+0x2a > >> > > vfs_badlock(c101b180,f191550c,c1055580,c7f20ae0) at vfs_badlock+0x23 > >> > > assert_vop_elocked(c7f20ae0,c0ee5f4f,c09f3213,8,0,...) at > >> > > assert_vop_elocked+0x55 > >> > > pcbmap(c7966e00,0,f191560c,f1915618,f191561c,...) at pcbmap+0x45 > >> > > msdosfs_readdir(f1915960,c0f4b343,c7f20ae0,f1915940,0,...) at > >> > > msdosfs_readdir+0x528 > >> > > VOP_READDIR_APV(c101b180,f1915960,2,f1915a68,c7923000,...) at > >> > > VOP_READDIR_APV+0xc5 > >> > > nfsrvd_readdir(f1915b64,0,c7f20ae0,c7923000,f1915a68,...) at > >> > > nfsrvd_readdir+0x38e > >> > > nfsrvd_dorpc(f1915b64,0,c7923000,c842a200,4,...) at > >> > > nfsrvd_dorpc+0x1f79 > >> > > nfssvc_program(c7793800,c842a200,c0f24d67,492,0,...) at > >> > > nfssvc_program+0x40f > >> > > svc_run_internal(f1915d14,c09d9a98,c73dfa80,f1915d28,c0ef1130,...) > >> > > at svc_run_internal+0x952 > >> > > svc_thread_start(c73dfa80,f1915d28,c0ef1130,3a5,c7e4b2c0,...) at > >> > > svc_thread_start+0x10 > >> > > fork_exit(c0bed7d0,c73dfa80,f1915d28) at fork_exit+0xb8 > >> > > fork_trampoline() at fork_trampoline+0x8 > >> > > --- trap 0x804c12e, eip = 0xc, esp = 0x33, ebp = 0x1 --- > >> > > pcbmap: 0xc7f20ae0 is not exclusive locked but should be > >> > > KDB: enter: lock violation > >> > > > >> > > So, does anyone know if the msdosfs_readdir() really requires a > >> > > LK_EXCLUSIVE > >> > > locked vnode or is the ASSERT_VOP_ELOCKED() too strong in pcbmap()? > >> > > >> > Yes, msdosfs currently requires all vnode locks to be exclusive. One > >> > of > >> > the reasons is that each denode (the msdosfs-private vnode data) > >> > carries > >> > the fat entries cache, and this cache is updated even by the > >> > operations > >> > that do not modify vnode from the VFS POV. > >> > > >> > The locking regime is enforced by the getnewvnode() initializing the > >> > vnode > >> > lock with LK_NOSHARE flag, and msdosfs code not calling > >> > VN_LOCK_ASHARE() > >> > on the newly instantiated vnode. > >> > > >> > My question is, was the vnode in question locked at all ? > >> I think the problem is that I do a LK_DOWNGRADE. From a quick > >> look at __lockmgr_args(), it doesn't check LK_NOSHARE for a > >> LK_DOWNGRADE. > >> > >> Maybe __lockmgr_args() should have something like: > >> if (op == LK_DOWNGRADE && (lk->lock_object.lo_flags & LK_NOSHARE)) > >> return (0); /* noop */ > >> after the > >> if (op == LK_SHARED && (lk->lock_object.lo_flags & LK_NOSHARE)) > >> op = LK_EXCLUSIVE; > >> lines? > > The RELENG_7 lockmgr does not check the NOSHARE flag on downgrade, > > but I agree with the essence of your proposal. > > As long as the difference in semantic with the old lockmgr is > correctly stressed out in the doc (and eventually comments) I'm fine > with this change. I think it is a bug in the LK_NOSHARE implementation if the old lockmgr() didn't silently nop downgrade requests when LK_NOSHARE was set. :) We should definitely fix it to ignore downgrades for LK_NOSHARE. -- John Baldwin
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