Export the newly introduced diskseq in uevents:

    $ udevadm info /sys/class/block/* |grep -e DEVNAME -e DISKSEQ
    E: DEVNAME=/dev/loop0
    E: DISKSEQ=1
    E: DEVNAME=/dev/loop1
    E: DISKSEQ=2
    E: DEVNAME=/dev/loop2
    E: DISKSEQ=3
    E: DEVNAME=/dev/loop3
    E: DISKSEQ=4
    E: DEVNAME=/dev/loop4
    E: DISKSEQ=5
    E: DEVNAME=/dev/loop5
    E: DISKSEQ=6
    E: DEVNAME=/dev/loop6
    E: DISKSEQ=7
    E: DEVNAME=/dev/loop7
    E: DISKSEQ=8
    E: DEVNAME=/dev/nvme0n1
    E: DISKSEQ=9
    E: DEVNAME=/dev/nvme0n1p1
    E: DISKSEQ=9
    E: DEVNAME=/dev/nvme0n1p2
    E: DISKSEQ=9
    E: DEVNAME=/dev/nvme0n1p3
    E: DISKSEQ=9
    E: DEVNAME=/dev/nvme0n1p4
    E: DISKSEQ=9
    E: DEVNAME=/dev/nvme0n1p5
    E: DISKSEQ=9
    E: DEVNAME=/dev/sda
    E: DISKSEQ=10
    E: DEVNAME=/dev/sda1
    E: DISKSEQ=10
    E: DEVNAME=/dev/sda2
    E: DISKSEQ=10

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Tested-by: Luca Boccassi <bluca@debian.org>
Link: https://lore.kernel.org/r/20210712230530.29323-3-mcroce@linux.microsoft.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Associating uevents with block devices in userspace is difficult and racy:
the uevent netlink socket is lossy, and on slow and overloaded systems
has a very high latency.
Block devices do not have exclusive owners in userspace, any process can
set one up (e.g. loop devices). Moreover, device names can be reused
(e.g. loop0 can be reused again and again). A userspace process setting
up a block device and watching for its events cannot thus reliably tell
whether an event relates to the device it just set up or another earlier
instance with the same name.

Being able to set a UUID on a loop device would solve the race conditions.
But it does not allow to derive orderings from uevents: if you see a
uevent with a UUID that does not match the device you are waiting for,
you cannot tell whether it's because the right uevent has not arrived yet,
or it was already sent and you missed it. So you cannot tell whether you
should wait for it or not.

Associating a unique, monotonically increasing sequential number to the
lifetime of each block device, which can be retrieved with an ioctl
immediately upon setting it up, allows to solve the race conditions with
uevents, and also allows userspace processes to know whether they should
wait for the uevent they need or if it was dropped and thus they should
move on.

Additionally, increment the disk sequence number when the media change,
i.e. on DISK_EVENT_MEDIA_CHANGE event.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Tested-by: Luca Boccassi <bluca@debian.org>
Link: https://lore.kernel.org/r/20210712230530.29323-2-mcroce@linux.microsoft.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>

cmdline-parser.c is only used by the cmdline faux partition format,
so merge the code into that and avoid an indirect call.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210728053756.409654-1-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Remove the disk_name function now that all users are gone.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727062518.122108-7-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

disk_name for partition 0 just copies out the disk_name field.  Replace
the call to disk_name with a %s format specifier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727062518.122108-6-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Printk ->disk_name directly for the disk and use the %pg format specifier
for the block device, which is equivalent to a bdevname call.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727062518.122108-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Simplify printing the partition name by using the %pg format specifier
that is equivalent to a bdevname call.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727062518.122108-4-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Simplify printing the partition name by using the %pg format specifier
that is equivalent to a bdevname call.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727062518.122108-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

I have compiled the kernel with a cross compiler "hppa-linux-gnu-" v9.3.0
on x86-64 host machine. I got the following warning:

block/genhd.c: In function ‘diskstats_show’:
block/genhd.c:1227:1: warning: the frame size of 1688 bytes is larger
than 1280 bytes [-Wframe-larger-than=]
 1227  |  }

By Reduced the stack footprint by using the %pg printk specifier instead
of disk_name to remove the need for the on-stack buffer.

Signed-off-by: Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727062518.122108-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Now that we've stopped using inode references for anything meaninful
in the block layer get rid of the helper to put it and just open code
the call to iput on the block_device inode.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Link: https://lore.kernel.org/r/20210722075402.983367-10-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

All callers are gone, and no one should grab a pure inode reference to
a block device anymore.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20210722075402.983367-9-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

The whole device block device won't be removed while the disk is still
alive, so don't bother to grab a reference to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Ming Lei <ming.lei@rehat.com>
Reviewed-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Link: https://lore.kernel.org/r/20210722075402.983367-8-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Instead of acquiring an inode reference on open make sure partitions
always hold device model references to the disk while alive, and switch
open to grab only a device model reference to the opened block device.
If that is a partition the disk reference is transitively held by the
partition already.

Link: https://lore.kernel.org/r/20210722075402.983367-6-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Move the allocation of bd_meta_info after initializing the struct device
to avoid the special bdput error handling path.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210722075402.983367-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Unhash the whole device inode early in del_gendisk.  This allows to
remove the first GENHD_FL_UP check in the open path as we simply
won't find a just removed inode.  The second non-racy check after
taking open_mutex is still kept.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210722075402.983367-4-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Add a lockdep assert instead of the outdated locking comment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Link: https://lore.kernel.org/r/20210722075402.983367-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Using local kmaps slightly reduces the chances to stray writes, and
the bvec interface cleans up the code a little bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-16-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Using local kmaps slightly reduces the chances to stray writes, and
the bvec interface cleans up the code a little bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-15-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Rewrite the actual bounce buffering loop in __blk_queue_bounce to that
the memcpy_to_bvec helper can be used to perform the data copies.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-14-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Use memcpy_from_bvec instead of open coding the logic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-13-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Use memcpy_to_bvec instead of opencoding the logic.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-12-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Use the proper helpers instead of open coding the copy.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-11-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

These two helpers are entirely unused now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20210727055646.118787-10-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Use the bvec helpers instead of open coding the copy.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Tested-by: Geoff Levand <geoff@infradead.org>
Link: https://lore.kernel.org/r/20210727055646.118787-9-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

There is no need to disable interrupts in bio_copy_block, and the local
only mappings helps to avoid any sort of problems with stray writes
into the bio data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-8-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Use memzero_bvec instead of reimplementing it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-7-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Use memzero_bvec to zero each segment in the bio instead of manually
mapping and zeroing the data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-6-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Add helpers to perform common memory operation on a bvec.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Add a helper to call kmap_local_page on a bvec.  There is no need for
an unmap helper given that kunmap_local accept any address in the mapped
page.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-4-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Fix the include guards to match the file naming.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-3-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

There is no need to include genhd.h from a random arch header, and not
doing so prevents the possibility for nasty include loops.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210727055646.118787-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>

systemd added a modified copy of include/linux/ioprio.h into its
code to get the relevant content definitions for the exposed
ioprio_[get|set] system calls.

Move the user space relevant ioprio bits to the UAPI includes to be
able to use the ioprio_[get|set] syscalls as intended.

Cc: Kay Sievers <kay@vrfy.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/r/20210714195655.181943-1-socketcan@hartkopp.net


Signed-off-by: Jens Axboe <axboe@kernel.dk>

Merge tag 'perf-tools-fixes-for-v5.14-2021-08-01' of git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux

Pull perf tools fixes from Arnaldo Carvalho de Melo:

 - Revert "perf map: Fix dso->nsinfo refcounting", this makes 'perf top'
   abort, uncovering a design flaw on how namespace information is kept.
   The fix for that is more than we can do right now, leave it for the
   next merge window.

 - Split --dump-raw-trace by AUX records for ARM's CoreSight, fixing up
   the decoding of some records.

 - Fix PMU alias matching.

Thanks to James Clark and John Garry for these fixes.

* tag 'perf-tools-fixes-for-v5.14-2021-08-01' of git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux:
  Revert "perf map: Fix dso->nsinfo refcounting"
  perf pmu: Fix alias matching
  perf cs-etm: Split --dump-raw-trace by AUX records

Pull powerpc fixes from Michael Ellerman:

 - Don't use r30 in VDSO code, to avoid breaking existing Go lang
   programs.

 - Change an export symbol to allow non-GPL modules to use spinlocks
   again.

Thanks to Paul Menzel, and Srikar Dronamraju.

* tag 'powerpc-5.14-4' of git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux:
  powerpc/vdso: Don't use r30 to avoid breaking Go lang
  powerpc/pseries: Fix regression while building external modules

Pull xfs fixes from Darrick Wong:
 "This contains a bunch of bug fixes in XFS.

  Dave and I have been busy the last couple of weeks to find and fix as
  many log recovery bugs as we can find; here are the results so far. Go
  fstests -g recoveryloop! ;)

   - Fix a number of coordination bugs relating to cache flushes for
     metadata writeback, cache flushes for multi-buffer log writes, and
     FUA writes for single-buffer log writes

   - Fix a bug with incorrect replay of attr3 blocks

   - Fix unnecessary stalls when flushing logs to disk

   - Fix spoofing problems when recovering realtime bitmap blocks"

* tag 'xfs-5.14-fixes-2' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux:
  xfs: prevent spoofing of rtbitmap blocks when recovering buffers
  xfs: limit iclog tail updates
  xfs: need to see iclog flags in tracing
  xfs: Enforce attr3 buffer recovery order
  xfs: logging the on disk inode LSN can make it go backwards
  xfs: avoid unnecessary waits in xfs_log_force_lsn()
  xfs: log forces imply data device cache flushes
  xfs: factor out forced iclog flushes
  xfs: fix ordering violation between cache flushes and tail updates
  xfs: fold __xlog_state_release_iclog into xlog_state_release_iclog
  xfs: external logs need to flush data device
  xfs: flush data dev on external log write

Pull cifs fixes from Steve French:
 "Three cifs/smb3 fixes, including two for stable, and a fix for an
  fallocate problem noticed by Clang"

* tag '5.14-rc3-smb3-fixes' of git://git.samba.org/sfrench/cifs-2.6:
  cifs: add missing parsing of backupuid
  smb3: rc uninitialized in one fallocate path
  SMB3: fix readpage for large swap cache

Pull networking fixes from Jakub Kicinski:
 "Networking fixes for 5.14-rc4, including fixes from bpf, can, WiFi
  (mac80211) and netfilter trees.

  Current release - regressions:

   - mac80211: fix starting aggregation sessions on mesh interfaces

  Current release - new code bugs:

   - sctp: send pmtu probe only if packet loss in Search Complete state

   - bnxt_en: add missing periodic PHC overflow check

   - devlink: fix phys_port_name of virtual port and merge error

   - hns3: change the method of obtaining default ptp cycle

   - can: mcba_usb_start(): add missing urb->transfer_dma initialization

  Previous releases - regressions:

   - set true network header for ECN decapsulation

   - mlx5e: RX, avoid possible data corruption w/ relaxed ordering and
     LRO

   - phy: re-add check for PHY_BRCM_DIS_TXCRXC_NOENRGY on the BCM54811
     PHY

   - sctp: fix return value check in __sctp_rcv_asconf_lookup

  Previous releases - always broken:

   - bpf:
       - more spectre corner case fixes, introduce a BPF nospec
         instruction for mitigating Spectre v4
       - fix OOB read when printing XDP link fdinfo
       - sockmap: fix cleanup related races

   - mac80211: fix enabling 4-address mode on a sta vif after assoc

   - can:
       - raw: raw_setsockopt(): fix raw_rcv panic for sock UAF
       - j1939: j1939_session_deactivate(): clarify lifetime of session
         object, avoid UAF
       - fix number of identical memory leaks in USB drivers

   - tipc:
       - do not blindly write skb_shinfo frags when doing decryption
       - fix sleeping in tipc accept routine"

* tag 'net-5.14-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net: (91 commits)
  gve: Update MAINTAINERS list
  can: esd_usb2: fix memory leak
  can: ems_usb: fix memory leak
  can: usb_8dev: fix memory leak
  can: mcba_usb_start(): add missing urb->transfer_dma initialization
  can: hi311x: fix a signedness bug in hi3110_cmd()
  MAINTAINERS: add Yasushi SHOJI as reviewer for the Microchip CAN BUS Analyzer Tool driver
  bpf: Fix leakage due to insufficient speculative store bypass mitigation
  bpf: Introduce BPF nospec instruction for mitigating Spectre v4
  sis900: Fix missing pci_disable_device() in probe and remove
  net: let flow have same hash in two directions
  nfc: nfcsim: fix use after free during module unload
  tulip: windbond-840: Fix missing pci_disable_device() in probe and remove
  sctp: fix return value check in __sctp_rcv_asconf_lookup
  nfc: s3fwrn5: fix undefined parameter values in dev_err()
  net/mlx5: Fix mlx5_vport_tbl_attr chain from u16 to u32
  net/mlx5e: Fix nullptr in mlx5e_hairpin_get_mdev()
  net/mlx5: Unload device upon firmware fatal error
  net/mlx5e: Fix page allocation failure for ptp-RQ over SF
  net/mlx5e: Fix page allocation failure for trap-RQ over SF
  ...

Pull ACPI fixes from Rafael Wysocki:
 "These revert a recent IRQ resources handling modification that turned
  out to be problematic, fix suspend-to-idle handling on AMD platforms
  to take upcoming systems into account properly and fix the retrieval
  of the DPTF attributes of the PCH FIVR.

  Specifics:

   - Revert recent change of the ACPI IRQ resources handling that
     attempted to improve the ACPI IRQ override selection logic, but
     introduced serious regressions on some systems (Hui Wang).

   - Fix up quirks for AMD platforms in the suspend-to-idle support code
     so as to take upcoming systems using uPEP HID AMDI007 into account
     as appropriate (Mario Limonciello).

   - Fix the code retrieving DPTF attributes of the PCH FIVR so that it
     agrees on the return data type with the ACPI control method
     evaluated for this purpose (Srinivas Pandruvada)"

* tag 'acpi-5.14-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm:
  ACPI: DPTF: Fix reading of attributes
  Revert "ACPI: resources: Add checks for ACPI IRQ override"
  ACPI: PM: Add support for upcoming AMD uPEP HID AMDI007

Since commit 1b6b26ae ("pipe: fix and clarify pipe write wakeup
logic") we have sanitized the pipe write logic, and would only try to
wake up readers if they needed it.

In particular, if the pipe already had data in it before the write,
there was no point in trying to wake up a reader, since any existing
readers must have been aware of the pre-existing data already.  Doing
extraneous wakeups will only cause potential thundering herd problems.

However, it turns out that some Android libraries have misused the EPOLL
interface, and expected "edge triggered" be to "any new write will
trigger it".  Even if there was no edge in sight.

Quoting Sandeep Patil:
 "The commit 1b6b26ae ('pipe: fix and clarify pipe write wakeup
  logic') changed pipe write logic to wakeup readers only if the pipe
  was empty at the time of write. However, there are libraries that
  relied upon the older behavior for notification scheme similar to
  what's described in [1]

  One such library 'realm-core'[2] is used by numerous Android
  applications. The library uses a similar notification mechanism as GNU
  Make but it never drains the pipe until it is full. When Android moved
  to v5.10 kernel, all applications using this library stopped working.

  The library has since been fixed[3] but it will be a while before all
  applications incorporate the updated library"

Our regression rule for the kernel is that if applications break from
new behavior, it's a regression, even if it was because the application
did something patently wrong.  Also note the original report [4] by
Michal Kerrisk about a test for this epoll behavior - but at that point
we didn't know of any actual broken use case.

So add the extraneous wakeup, to approximate the old behavior.

[ I say "approximate", because the exact old behavior was to do a wakeup
  not for each write(), but for each pipe buffer chunk that was filled
  in. The behavior introduced by this change is not that - this is just
  "every write will cause a wakeup, whether necessary or not", which
  seems to be sufficient for the broken library use. ]

It's worth noting that this adds the extraneous wakeup only for the
write side, while the read side still considers the "edge" to be purely
about reading enough from the pipe to allow further writes.

See commit f467a6a6 ("pipe: fix and clarify pipe read wakeup logic")
for the pipe read case, which remains that "only wake up if the pipe was
full, and we read something from it".

Link: https://lore.kernel.org/lkml/CAHk-=wjeG0q1vgzu4iJhW5juPkTsjTYmiqiMUYAebWW+0bam6w@mail.gmail.com/ [1]
Link: https://github.com/realm/realm-core [2]
Link: https://github.com/realm/realm-core/issues/4666 [3]
Link: https://lore.kernel.org/lkml/CAKgNAkjMBGeAwF=2MKK758BhxvW58wYTgYKB2V-gY1PwXxrH+Q@mail.gmail.com/ [4]
Link: https://lore.kernel.org/lkml/20210729222635.2937453-1-sspatil@android.com/


Reported-by: Sandeep Patil <sspatil@android.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>