On 26/03/2019 23:20, Kirill Smelkov wrote: > Commit 9c225f2655 (vfs: atomic f_pos accesses as per POSIX) added locking for > file.f_pos access and in particular made concurrent read and write not possible > - now both those functions take f_pos lock for the whole run, and so if e.g. a > read is blocked waiting for data, write will deadlock waiting for that read to > complete. This caused regression for stream-like files where previously read > and write could run simultaneously, but after that patch could not do so > anymore. See e.g. 581d21a2d0 (xenbus: fix deadlock on writes to /proc/xen/xenbus) > which fixes such regression for particular case of /proc/xen/xenbus. > > The patch that added f_pos lock in 2014 (see https://lkml.org/lkml/2014/2/17/324 > for background discussion) did so to guarantee POSIX thread safety for > read/write/lseek and added the locking to file descriptors of all regular > files. In 2014 that thread-safety problem was not new as it was already discussed > earlier in 2006: https://lwn.net/Articles/180387. However even though 2006'th > version of Linus's patch (https://lwn.net/Articles/180396) was adding f_pos > locking "only for files that are marked seekable with FMODE_LSEEK (thus avoiding > the stream-like objects like pipes and sockets)", 2014'th version - the one that > actually made it into the tree as 9c225f2655 - is doing so irregardless of whether > a file is seekable or not. The reason that it did so is, probably, that there are > many files that are marked non-seekable, but e.g. their read implementation > actually depends on knowing current position to correctly handle the read. Some > examples: > > kernel/power/user.c snapshot_read > fs/debugfs/file.c u32_array_read > fs/fuse/control.c fuse_conn_waiting_read + ... > drivers/hwmon/asus_atk0110.c atk_debugfs_ggrp_read > arch/s390/hypfs/inode.c hypfs_read_iter > ... > > In despite that, many nonseekable_open users implement read and write with pure > stream semantics - they don't depend on passed ppos at all. And for those cases > where read could wait for something inside, it creates a situation similar to > xenbus - the write could be never made to go until read is done, and read is > waiting for some, potentially external, event, for potentially unbounded time > -> deadlock. Besides xenbus, there are 14 such places in the kernel that I've > found with semantic patch (see below): > > drivers/xen/evtchn.c:667:8-24: ERROR: evtchn_fops: .read() can deadlock .write() > drivers/isdn/capi/capi.c:963:8-24: ERROR: capi_fops: .read() can deadlock .write() > drivers/input/evdev.c:527:1-17: ERROR: evdev_fops: .read() can deadlock .write() > drivers/char/pcmcia/cm4000_cs.c:1685:7-23: ERROR: cm4000_fops: .read() can deadlock .write() > net/rfkill/core.c:1146:8-24: ERROR: rfkill_fops: .read() can deadlock .write() > drivers/s390/char/fs3270.c:488:1-17: ERROR: fs3270_fops: .read() can deadlock .write() > drivers/usb/misc/ldusb.c:310:1-17: ERROR: ld_usb_fops: .read() can deadlock .write() > drivers/hid/uhid.c:635:1-17: ERROR: uhid_fops: .read() can deadlock .write() > net/batman-adv/icmp_socket.c:80:1-17: ERROR: batadv_fops: .read() can deadlock .write() > drivers/media/rc/lirc_dev.c:198:1-17: ERROR: lirc_fops: .read() can deadlock .write() > drivers/leds/uleds.c:77:1-17: ERROR: uleds_fops: .read() can deadlock .write() > drivers/input/misc/uinput.c:400:1-17: ERROR: uinput_fops: .read() can deadlock .write() > drivers/infiniband/core/user_mad.c:985:7-23: ERROR: umad_fops: .read() can deadlock .write() > drivers/gnss/core.c:45:1-17: ERROR: gnss_fops: .read() can deadlock .write() > > In addition to the cases above another regression caused by f_pos locking is > that now FUSE filesystems that implement open with FOPEN_NONSEEKABLE flag, can > no longer implement bidirectional stream-like files - for the same reason > as above e.g. read can deadlock write locking on file.f_pos in the kernel. > FUSE's FOPEN_NONSEEKABLE was added in 2008 in a7c1b990f7 (fuse: implement > nonseekable open) to support OSSPD (https://github.com/libfuse/osspd; > https://lwn.net/Articles/308445). OSSPD implements /dev/dsp in userspace with > FOPEN_NONSEEKABLE flag, with corresponding read and write routines not > depending on current position at all, and with both read and write being > potentially blocking operations: > > https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1406 > https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1438-L1477 > https://github.com/libfuse/osspd/blob/14a9cff0/osspd.c#L1479-L1510 > > Corresponding libfuse example/test also describes FOPEN_NONSEEKABLE as > "somewhat pipe-like files ..." with read handler not using offset. However > that test implements only read without write and cannot exercise the deadlock > scenario: > > https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L124-L131 > https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L146-L163 > https://github.com/libfuse/libfuse/blob/fuse-3.4.2-3-ga1bff7d/example/poll.c#L209-L216 > > I've actually hit the read vs write deadlock for real while implementing my > FUSE filesystem where there is /head/watch file, for which open creates > separate bidirectional socket-like stream in between filesystem and its user > with both read and write being later performed simultaneously. And there it is > semantically not easy to split the stream into two separate read-only and > write-only channels: > > https://lab.nexedi.com/kirr/wendelin.core/blob/f13aa600/wcfs/wcfs.go#L88-169 > > Let's fix this regression. The plan is: > > 1. We can't change nonseekable_open to include &~FMODE_ATOMIC_POS - doing so would > break many in-kernel nonseekable_open users which actually use ppos in > read/write handlers. > > 2. Add stream_open() to kernel to open stream-like non-seekable file descriptors. > Read and write on such file descriptors would never use nor change ppos. And > with that property on stream-like files read and write will be running without > taking f_pos lock - i.e. read and write could be running simultaneously. > > 3. With semantic patch search and convert to stream_open all in-kernel > nonseekable_open users for which read and write actually do not depend on ppos and > where there is no other methods in file_operations which assume @offset access. > > 4. Add FOPEN_STREAM to fs/fuse/ and open in-kernel file-descriptors via steam_open > if that bit is present in filesystem open reply. > > It was tempting to change fs/fuse/ open handler to use stream_open instead of > nonseekable_open on just FOPEN_NONSEEKABLE flags, but grepping through Debian > codesearch shows users of FOPEN_NONSEEKABLE, and in particular GVFS which actually > uses offset in its read and write handlers > > https://codesearch.debian.net/search?q=-%3Enonseekable+%3D > https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1080 > https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1247-1346 > https://gitlab.gnome.org/GNOME/gvfs/blob/1.40.0-6-gcbc54396/client/gvfsfusedaemon.c#L1399-1481 > > so if we would do such a change it will break a real user. > > 5. Add stream_open and FOPEN_STREAM handling to stable kernels starting from > v3.14+ (the kernel where 9c225f2655 first appeared). This will allow to patch > OSSPD and other FUSE filesystems that provide stream-like files to return > FOPEN_STREAM | FOPEN_NONSEEKABLE in open handler and this way avoid the deadlock on > all kernel versions. This should work because fs/fuse/ ignores unknown open > flags returned from a filesystem and so passing FOPEN_STREAM to a kernel that > is not aware of this flag cannot hurt. In turn the kernel that is not aware of > FOPEN_STREAM will be < v3.14 where just FOPEN_NONSEEKABLE is sufficient to > implement streams without read vs write deadlock. > > This patch: adds stream_open, converts /proc/xen/xenbus to it and adds semantic > patch to automatically locate in-kernel places that are either required to be > converted due to read vs write deadlock, or that are just safe to be converted > because read and write do not use ppos and there are no other funky methods in > file_operations. > > Followup patches are: > > - apply the result of semantic patch; > - add FOPEN_STREAM to fs/fuse. > > Regarding semantic patch I've verified each generated change manually - that it is > correct to convert - and each other nonseekable_open instance left - that it is > either not correct to convert there, or that it is not converted due to current > stream_open.cocci limitations. The script also does not convert files that should > be valid to convert, but that currently have .llseek = noop_llseek or > generic_file_llseek for unknown reason despite file being opened with > nonseekable_open (e.g. drivers/input/mousedev.c) > > Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx> > Cc: Yongzhi Pan <panyongzhi@xxxxxxxxx> > Cc: Jonathan Corbet <corbet@xxxxxxx> > Cc: David Vrabel <david.vrabel@xxxxxxxxxx> > Cc: Juergen Gross <jgross@xxxxxxxx> > Cc: Miklos Szeredi <miklos@xxxxxxxxxx> > Cc: Tejun Heo <tj@xxxxxxxxxx> > Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: Christoph Hellwig <hch@xxxxxx> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > Cc: Julia Lawall <Julia.Lawall@xxxxxxx> > Cc: Nikolaus Rath <Nikolaus@xxxxxxxx> > Cc: Han-Wen Nienhuys <hanwen@xxxxxxxxxx> > Signed-off-by: Kirill Smelkov <kirr@xxxxxxxxxx> For the Xen changes: Reviewed-by: Juergen Gross <jgross@xxxxxxxx> Juergen
