alistair23-linux/drivers/staging
David Howells cc53ce53c8 Add a dentry op to allow processes to be held during pathwalk transit
Add a dentry op (d_manage) to permit a filesystem to hold a process and make it
sleep when it tries to transit away from one of that filesystem's directories
during a pathwalk.  The operation is keyed off a new dentry flag
(DCACHE_MANAGE_TRANSIT).

The filesystem is allowed to be selective about which processes it holds and
which it permits to continue on or prohibits from transiting from each flagged
directory.  This will allow autofs to hold up client processes whilst letting
its userspace daemon through to maintain the directory or the stuff behind it
or mounted upon it.

The ->d_manage() dentry operation:

	int (*d_manage)(struct path *path, bool mounting_here);

takes a pointer to the directory about to be transited away from and a flag
indicating whether the transit is undertaken by do_add_mount() or
do_move_mount() skipping through a pile of filesystems mounted on a mountpoint.

It should return 0 if successful and to let the process continue on its way;
-EISDIR to prohibit the caller from skipping to overmounted filesystems or
automounting, and to use this directory; or some other error code to return to
the user.

->d_manage() is called with namespace_sem writelocked if mounting_here is true
and no other locks held, so it may sleep.  However, if mounting_here is true,
it may not initiate or wait for a mount or unmount upon the parameter
directory, even if the act is actually performed by userspace.

Within fs/namei.c, follow_managed() is extended to check with d_manage() first
on each managed directory, before transiting away from it or attempting to
automount upon it.

follow_down() is renamed follow_down_one() and should only be used where the
filesystem deliberately intends to avoid management steps (e.g. autofs).

A new follow_down() is added that incorporates the loop done by all other
callers of follow_down() (do_add/move_mount(), autofs and NFSD; whilst AFS, NFS
and CIFS do use it, their use is removed by converting them to use
d_automount()).  The new follow_down() calls d_manage() as appropriate.  It
also takes an extra parameter to indicate if it is being called from mount code
(with namespace_sem writelocked) which it passes to d_manage().  follow_down()
ignores automount points so that it can be used to mount on them.

__follow_mount_rcu() is made to abort rcu-walk mode if it hits a directory with
DCACHE_MANAGE_TRANSIT set on the basis that we're probably going to have to
sleep.  It would be possible to enter d_manage() in rcu-walk mode too, and have
that determine whether to abort or not itself.  That would allow the autofs
daemon to continue on in rcu-walk mode.

Note that DCACHE_MANAGE_TRANSIT on a directory should be cleared when it isn't
required as every tranist from that directory will cause d_manage() to be
invoked.  It can always be set again when necessary.

==========================
WHAT THIS MEANS FOR AUTOFS
==========================

Autofs currently uses the lookup() inode op and the d_revalidate() dentry op to
trigger the automounting of indirect mounts, and both of these can be called
with i_mutex held.

autofs knows that the i_mutex will be held by the caller in lookup(), and so
can drop it before invoking the daemon - but this isn't so for d_revalidate(),
since the lock is only held on _some_ of the code paths that call it.  This
means that autofs can't risk dropping i_mutex from its d_revalidate() function
before it calls the daemon.

The bug could manifest itself as, for example, a process that's trying to
validate an automount dentry that gets made to wait because that dentry is
expired and needs cleaning up:

	mkdir         S ffffffff8014e05a     0 32580  24956
	Call Trace:
	 [<ffffffff885371fd>] :autofs4:autofs4_wait+0x674/0x897
	 [<ffffffff80127f7d>] avc_has_perm+0x46/0x58
	 [<ffffffff8009fdcf>] autoremove_wake_function+0x0/0x2e
	 [<ffffffff88537be6>] :autofs4:autofs4_expire_wait+0x41/0x6b
	 [<ffffffff88535cfc>] :autofs4:autofs4_revalidate+0x91/0x149
	 [<ffffffff80036d96>] __lookup_hash+0xa0/0x12f
	 [<ffffffff80057a2f>] lookup_create+0x46/0x80
	 [<ffffffff800e6e31>] sys_mkdirat+0x56/0xe4

versus the automount daemon which wants to remove that dentry, but can't
because the normal process is holding the i_mutex lock:

	automount     D ffffffff8014e05a     0 32581      1              32561
	Call Trace:
	 [<ffffffff80063c3f>] __mutex_lock_slowpath+0x60/0x9b
	 [<ffffffff8000ccf1>] do_path_lookup+0x2ca/0x2f1
	 [<ffffffff80063c89>] .text.lock.mutex+0xf/0x14
	 [<ffffffff800e6d55>] do_rmdir+0x77/0xde
	 [<ffffffff8005d229>] tracesys+0x71/0xe0
	 [<ffffffff8005d28d>] tracesys+0xd5/0xe0

which means that the system is deadlocked.

This patch allows autofs to hold up normal processes whilst the daemon goes
ahead and does things to the dentry tree behind the automouter point without
risking a deadlock as almost no locks are held in d_manage() and none in
d_automount().

Signed-off-by: David Howells <dhowells@redhat.com>
Was-Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2011-01-15 20:07:31 -05:00
..
asus_oled Staging: Merge 2.6.37-rc5 into staging-next 2010-12-07 10:47:56 -08:00
ath6kl drivers/staging: Remove unnecessary semicolons 2010-11-16 12:06:47 -08:00
autofs Add a dentry op to allow processes to be held during pathwalk transit 2011-01-15 20:07:31 -05:00
bcm Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging-2.6 2011-01-10 16:04:53 -08:00
brcm80211 staging: brcm80211: Fix WL_<type> logging macros 2010-12-16 12:25:37 -08:00
comedi Staging: comedi: Fix checkpatch.pl issues in file s526.c 2010-12-16 12:20:06 -08:00
cptm1217 staging: clearpad_tm1217 depends on INPUT 2010-11-19 17:17:45 -08:00
crystalhd drivers/staging: Remove unnecessary casts of pci_get_drvdata 2010-11-16 12:06:47 -08:00
cs5535_gpio cs5535: deprecate older cs5535_gpio driver 2011-01-13 08:03:13 -08:00
cx25821 Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging-2.6 2011-01-10 16:04:53 -08:00
cxt1e1 Staging: cxt1e1: rename global log_level variable 2010-12-10 10:46:30 -08:00
dabusb [media] dabusb: Move it to staging to be deprecated 2010-12-29 08:17:15 -02:00
dt3155v4l [media] V4L: remove V4L1 compatibility mode 2010-12-29 08:17:07 -02:00
easycap Staging: Merge 2.6.37-rc5 into staging-next 2010-12-07 10:47:56 -08:00
echo
et131x drivers/staging: Remove unnecessary casts of pci_get_drvdata 2010-11-16 12:06:47 -08:00
frontier Staging: Merge 2.6.37-rc5 into staging-next 2010-12-07 10:47:56 -08:00
ft1000 staging: ft1000: Fix kernel panic when use ioctl. 2010-12-16 12:20:05 -08:00
go7007 Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging-2.6 2011-01-10 16:04:53 -08:00
hv Staging: hv: Use only one txf buffer per channel and kmalloc/GFP_KERNEL on initialize 2010-12-16 12:37:51 -08:00
iio Staging: IIO: DDS: AD9833 / AD9834 driver 2010-12-16 12:23:03 -08:00
intel_sst Staging: Merge 2.6.37-rc5 into staging-next 2010-12-07 10:47:56 -08:00
keucr Staging: Merge 2.6.37-rc5 into staging-next 2010-12-07 10:47:56 -08:00
line6 staging/line6: Fix sparse warning 'Using plain integer as NULL pointer' 2010-12-08 14:31:01 -08:00
lirc Merge branch 'for-2.6.38' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu 2011-01-07 17:02:58 -08:00
memrar Staging: sep: handle the memrar stuff in the headers 2010-11-29 11:17:27 -08:00
msm Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial 2011-01-13 10:05:56 -08:00
octeon Merge 'staging-next' to Linus's tree 2010-10-28 09:44:56 -07:00
olpc_dcon Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/trivial 2011-01-13 10:05:56 -08:00
panel llseek: automatically add .llseek fop 2010-10-15 15:53:27 +02:00
phison Staging: phison: use DEFINE_PCI_DEVICE_TABLE 2010-12-09 16:29:48 -08:00
pohmelfs pohmelfs: double-free and leak 2011-01-12 20:02:44 -05:00
quatech_usb2 staging/trivial: fix typos concerning "initiali[zs]e" 2010-09-04 21:23:26 -07:00
quickstart Staging: quickstart: free after input_unregister_device() 2010-11-16 12:25:52 -08:00
rt2860 Staging: rt2860: Sanitize DBGPRINT_ERR macro 2010-12-16 12:17:29 -08:00
rt2870 Staging: rt2870: Makefile: replace the use of <module>-objs with <module>-y 2010-10-08 07:23:46 -07:00
rtl8187se staging: rtl8187se: Change panic to warn when RF switch turned off 2010-11-16 12:25:53 -08:00
rtl8192e Staging: rtl8192e: use DEFINE_PCI_DEVICE_TABLE 2010-12-09 16:29:48 -08:00
rtl8192u Staging: rtl8192u: add missing curly braces 2010-12-10 10:39:55 -08:00
rtl8712 Staging: Merge 2.6.37-rc5 into staging-next 2010-12-07 10:47:56 -08:00
samsung-laptop Merge branch 'master' into for-next 2010-12-22 18:57:02 +01:00
sbe-2t3e3 Staging: sbe-2t3e3: Makefile: replace the use of <module>-objs with <module>-y 2010-10-08 07:23:45 -07:00
se401 [media] staging: se401 depends on USB 2010-12-30 22:29:41 -02:00
sep staging: sep: fix comments in sep_get_static_pool_addr_handler 2010-12-10 10:39:55 -08:00
serqt_usb2
slicoss staging: slicoss: use free_netdev(netdev) instead of kfree() 2010-09-30 04:14:21 -07:00
sm7xx Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jbarnes/pci-2.6 2011-01-14 09:29:05 -08:00
smbfs convert smbfs 2011-01-12 20:02:47 -05:00
solo6x10 staging: solo6010 depends on I2C 2010-12-09 16:29:49 -08:00
speakup Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging-2.6 2011-01-10 16:04:53 -08:00
spectra Staging: Merge 2.6.37-rc5 into staging-next 2010-12-07 10:47:56 -08:00
ste_rmi4 staging: fix ste_rmi4 build, depends on INPUT 2010-11-16 12:31:48 -08:00
ti-st staging: ti-st: move TI_ST from staging to misc/ 2010-10-06 08:25:06 -07:00
tidspbridge Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging-2.6 2011-01-10 16:04:53 -08:00
tm6000 [media] tm6000: Fix mutex unbalance 2010-12-29 08:17:21 -02:00
usbip staging: usbip: remove double giveback of URB 2011-01-10 12:26:02 -08:00
usbvideo [media] staging: usbvideo/vicam depends on USB 2010-12-30 22:29:39 -02:00
vme Staging: vme_tsi148: use DEFINE_PCI_DEVICE_TABLE 2010-12-09 16:29:47 -08:00
vt6655 drivers/staging: Remove unnecessary semicolons 2010-11-16 12:06:47 -08:00
vt6656 drivers/staging: Remove unnecessary semicolons 2010-11-16 12:06:47 -08:00
westbridge Staging: Merge 2.6.37-rc2 into staging-next 2010-11-16 10:44:50 -08:00
winbond Staging: w35und: Kill struct wb_usb 2010-12-01 14:45:47 -08:00
wlags49_h2 drivers/staging: Remove unnecessary casts of pci_get_drvdata 2010-11-16 12:06:47 -08:00
wlags49_h25 Staging: wlags49_h25: Makefile: cleaned up Makefile cflag lines 2010-10-05 11:56:38 -07:00
wlan-ng Staging: Merge 2.6.37-rc2 into staging-next 2010-11-16 10:44:50 -08:00
xgifb staging: Use vzalloc 2010-11-09 16:34:01 -08:00
zram Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging-2.6 2011-01-10 16:04:53 -08:00
Kconfig cs5535: deprecate older cs5535_gpio driver 2011-01-13 08:03:13 -08:00
Makefile cs5535: deprecate older cs5535_gpio driver 2011-01-13 08:03:13 -08:00
staging.c