On Thu, 2016-08-11 at 15:55 +0000, Trond Myklebust wrote: > > > > On Aug 11, 2016, at 11:23, Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > I was playing around with the in-kernel flexfiles server today, and > > I > > seem to be hitting a deadlock when using it on an XFS-exported > > filesystem. Here's the stack trace of how the CB_LAYOUTRECALL > > occurs: > > > > [ 928.736139] CPU: 0 PID: 846 Comm: nfsd Tainted: > > G OE 4.8.0-rc1+ #3 > > [ 928.737040] Hardware name: QEMU Standard PC (i440FX + PIIX, > > 1996), BIOS 1.9.1-1.fc24 04/01/2014 > > [ 928.738009] 0000000000000286 000000006125f50e ffff91153845b878 > > ffffffff8f463853 > > [ 928.738906] ffff91152ec194d0 ffff91152d31d9c0 ffff91153845b8a8 > > ffffffffc045936f > > [ 928.739788] ffff91152c051980 ffff91152d31d9c0 ffff91152c051540 > > ffff9115361b8a58 > > [ 928.740697] Call Trace: > > [ 928.740998] [<ffffffff8f463853>] dump_stack+0x86/0xc3 > > [ 928.741570] [<ffffffffc045936f>] > > nfsd4_recall_file_layout+0x17f/0x190 [nfsd] > > [ 928.742380] [<ffffffffc045939d>] > > nfsd4_layout_lm_break+0x1d/0x30 [nfsd] > > [ 928.743115] [<ffffffff8f3056d8>] __break_lease+0x118/0x6a0 > > [ 928.743759] [<ffffffffc02dea69>] xfs_break_layouts+0x79/0x120 > > [xfs] > > [ 928.744462] [<ffffffffc029ea04>] > > xfs_file_aio_write_checks+0x94/0x1f0 [xfs] > > [ 928.745251] [<ffffffffc029f36b>] > > xfs_file_buffered_aio_write+0x7b/0x330 [xfs] > > [ 928.746063] [<ffffffffc029f70c>] xfs_file_write_iter+0xec/0x140 > > [xfs] > > [ 928.746803] [<ffffffff8f2a0599>] > > do_iter_readv_writev+0xb9/0x140 > > [ 928.747478] [<ffffffff8f2a126b>] do_readv_writev+0x19b/0x240 > > [ 928.748146] [<ffffffffc029f620>] ? > > xfs_file_buffered_aio_write+0x330/0x330 [xfs] > > [ 928.748956] [<ffffffff8f29e02b>] ? do_dentry_open+0x28b/0x310 > > [ 928.749614] [<ffffffffc029c800>] ? > > xfs_extent_busy_ag_cmp+0x20/0x20 [xfs] > > [ 928.750367] [<ffffffff8f2a156f>] vfs_writev+0x3f/0x50 > > [ 928.750934] [<ffffffffc04276ca>] nfsd_vfs_write+0xca/0x3a0 > > [nfsd] > > [ 928.751608] [<ffffffffc0429ec5>] nfsd_write+0x485/0x780 [nfsd] > > [ 928.752263] [<ffffffffc043144c>] nfsd3_proc_write+0xbc/0x150 > > [nfsd] > > [ 928.752973] [<ffffffffc0421388>] nfsd_dispatch+0xb8/0x1f0 > > [nfsd] > > [ 928.753642] [<ffffffffc036d78f>] svc_process_common+0x42f/0x690 > > [sunrpc] > > [ 928.754395] [<ffffffffc036e8e8>] svc_process+0x118/0x330 > > [sunrpc] > > [ 928.755080] [<ffffffffc04208ac>] nfsd+0x19c/0x2b0 [nfsd] > > [ 928.755681] [<ffffffffc0420715>] ? nfsd+0x5/0x2b0 [nfsd] > > [ 928.756274] [<ffffffffc0420710>] ? nfsd_destroy+0x190/0x190 > > [nfsd] > > [ 928.756991] [<ffffffff8f0d5891>] kthread+0x101/0x120 > > [ 928.757563] [<ffffffff8f10dcc5>] ? > > trace_hardirqs_on_caller+0xf5/0x1b0 > > [ 928.758282] [<ffffffff8f8f2fef>] ret_from_fork+0x1f/0x40 > > [ 928.758875] [<ffffffff8f0d5790>] ? > > kthread_create_on_node+0x250/0x250 > > > > > > So the client gets a flexfiles layout, and then tries to issue a v3 > > WRITE against the file. XFS then recalls the layout, but the client > > can't return the layout until the v3 WRITE completes. Eventually > > this > > should resolve itself after 2 lease periods, but that's quite a > > long > > time. > > What’s the sequence of operations here? If the client has outstanding > I/O, I should now be returning NFS_OK, and then completing the recall > with a LAYOUTRETURN as soon as the outstanding I/O (and layoutcommit, > if one is due) is done. > > The server is expected to return NFS4ERR_RECALLCONFLICT to any > LAYOUTGET attempts that occur before the LAYOUTRETURN. > Basically, I'm just doing this on the client: $ echo "foo" > /mnt/knfsdsrv/testfile The client does: OPEN LAYOUTGET (for RW) GETDEVICEINFO ...and then a v3 WRITE under the aegis of the layout it got. The server then issues a CB_LAYOUTRECALL (because XFS wants to do that whenever there is a local write, apparently). The client returns NFS_OK, but it can't return the layout until the v3 WRITE completes. The v3 write is hung though because it's waiting for the layout to be returned. > > > > > > I guess XFS requires recalling block and SCSI layouts when the > > server > > wants to issue a write (or someone writes to it locally), but that > > seems like it shouldn't be happening when the layout is a flexfiles > > layout. > > > > Any thoughts on what the right fix is here? > > > > On a related note, knfsd will spam the heck out of the client with > > CB_LAYOUTRECALLs during this time. I think we ought to consider > > fixing > > the server not to treat an NFS_OK return from the client like > > NFS4ERR_DELAY there, but that would mean a different mechanism for > > timing out a CB_LAYOUTRECALL. > > There is a big difference between NFS_OK and NFS4ERR_DELAY as far as > the server is concerned: > > - NFS_OK means that the client has now seen the stateid with the > updated sequence id that was sent in CB_LAYOUTRECALL, and is > processing it. No resend of the CB_LAYOUTRECALL is required. > - OTOH, NFS4ERR_DELAY means the same thing in the back channel as it > does in the forward channel: I’m busy and cannot process your > request, please resend it later. Right. The current code basically just treats them the same as a mechanism to handle eventually timing out the layoutrecall. The extra CB_LAYOUTRECALLs are entirely superfluous. It's probably not too hard to fix, but we'd need to come up with some other mechanism for timing out the layoutrecall. -- Jeff Layton <jlayton@xxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
