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Linus Torvalds 3de34cc5ea UPSTREAM: pipe: make pipe writes always wake up readers 
Since commit 1b6b26ae70 ("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 1b6b26ae70 ('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 f467a6a664 ("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/

Bug: 193851993
Reported-by: Sandeep Patil <sspatil@android.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 3a34b13a88)
Signed-off-by: Sandeep Patil <sspatil@android.com>
Change-Id: Idcf3e8faa31bff47ada4b815237a355e0757b964
2021-08-03 21:20:32 +00:00
android ANDROID: ABI: update generic symbol list and ABI XML 2021-07-28 15:32:59 +00:00
arch ANDROID: GKI: Enable CONFIG_USB_EHCI_ROOT_HUB_TT 2021-08-02 16:28:45 +08:00
block ANDROID: Enable GKI Dr. No Enforcement 2021-07-21 20:51:47 +01:00
certs certs: Fix blacklist flag type confusion 2021-03-04 11:37:59 +01:00
crypto ANDROID: Enable GKI Dr. No Enforcement 2021-07-21 20:51:47 +01:00
Documentation ANDROID: Enable GKI Dr. No Enforcement 2021-07-21 20:51:47 +01:00
drivers FROMLIST: scsi: ufs: Allow async suspend/resume callbacks 2021-07-28 20:55:57 +00:00
fs UPSTREAM: pipe: make pipe writes always wake up readers 2021-08-03 21:20:32 +00:00
include ANDROID: scsi: ufs: add vendor hook to override key reprogramming 2021-07-28 15:32:51 +00:00
init Revert "ANDROID: make per-cgroup PSI tracking configurable" 2021-06-23 18:35:27 +00:00
ipc ipc/mqueue, msg, sem: avoid relying on a stack reference past its expiry 2021-05-26 12:06:54 +02:00
kernel UPSTREAM: kernel/irq: export irq_gc_set_wake 2021-07-30 06:41:28 +00:00
lib FROMLIST: lockdep: Remove console_verbose when disable lock debugging 2021-07-07 15:25:28 +00:00
LICENSES LICENSES/deprecated: add Zlib license text 2020-09-16 14:33:49 +02:00
mm UPSTREAM: mm/compaction: correct deferral logic for proactive compaction 2021-07-21 22:18:24 +00:00
net ANDROID: vendor_hooks: Add hooks to tcp/udp send/recv msg functions. 2021-07-28 09:53:04 +08:00
samples samples: vfio-mdev: fix error handing in mdpy_fb_probe() 2021-06-10 13:39:15 +02:00
scripts FROMLIST: kbuild: mkcompile_h: consider timestamp if KBUILD_BUILD_TIMESTAMP is set 2021-06-16 12:19:04 +00:00
security ANDROID: selinux: modify RTM_GETNEIGH{TBL} 2021-06-23 14:06:26 +00:00
sound ANDROID: add kabi padding for structures for the android12 release 2021-07-14 20:51:51 -07:00
tools ANDROID: Enable GKI Dr. No Enforcement 2021-07-21 20:51:47 +01:00
usr Merge branch 'work.fdpic' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2020-08-07 13:29:39 -07:00
virt Revert "irqbypass: do not start cons/prod when failed connect" 2021-06-03 09:00:34 +02:00
.clang-format RDMA 5.10 pull request 2020-10-17 11:18:18 -07:00
.cocciconfig
.get_maintainer.ignore
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Merge 5.10.38 into android12-5.10 2021-05-20 15:35:25 +02:00
.mailmap mailmap: add two more addresses of Uwe Kleine-König 2020-12-06 10:19:07 -08:00
build.config.aarch64 ANDROID: arm64: enable compat vdso 2021-05-13 09:14:25 -07:00
build.config.allmodconfig ANDROID: allmodconfig: drop openssl from ADDITIONAL_HOST_TOOLS 2021-03-04 20:48:17 +00:00
build.config.allmodconfig.aarch64 ANDROID: drop KERNEL_DIR setting in build.config.common 2020-08-31 15:20:37 +00:00
build.config.allmodconfig.arm ANDROID: drop KERNEL_DIR setting in build.config.common 2020-08-31 15:20:37 +00:00
build.config.allmodconfig.x86_64 ANDROID: drop KERNEL_DIR setting in build.config.common 2020-08-31 15:20:37 +00:00
build.config.amlogic ANDROID: GKI: amlogic: add DTB overlays 2021-02-12 10:08:04 +01:00
build.config.arm ANDROID: enable LLVM_IAS=1 for clang's integrated assembler for arm 2021-03-24 09:21:58 -07:00
build.config.common ANDROID: GKI: 7/14/2021 KMI update 2021-07-14 21:10:40 -07:00
build.config.db845c ANDROID: db845c: Use FRAGMENT_CONFIG 2021-02-10 06:08:17 +00:00
build.config.gki ANDROID: gki: Removed cf modules from gki_defconfig 2020-02-01 00:27:50 +00:00
build.config.gki_kasan ANDROID: build.config: Disable LTO for KASAN and Kprobes builds 2021-03-08 00:31:49 +00:00
build.config.gki_kasan.aarch64 ANDROID: drop KERNEL_DIR setting in build.config.common 2020-08-31 15:20:37 +00:00
build.config.gki_kasan.x86_64 ANDROID: drop KERNEL_DIR setting in build.config.common 2020-08-31 15:20:37 +00:00
build.config.gki_kprobes ANDROID: build.config: Disable LTO for KASAN and Kprobes builds 2021-03-08 00:31:49 +00:00
build.config.gki_kprobes.aarch64 ANDROID: Adding kprobes build configs for Cuttlefish 2021-03-03 02:03:02 +00:00
build.config.gki_kprobes.x86_64 ANDROID: Adding kprobes build configs for Cuttlefish 2021-03-03 02:03:02 +00:00
build.config.gki-debug.aarch64 ANDROID: drop KERNEL_DIR setting in build.config.common 2020-08-31 15:20:37 +00:00
build.config.gki-debug.x86_64 ANDROID: drop KERNEL_DIR setting in build.config.common 2020-08-31 15:20:37 +00:00
build.config.gki.aarch64 ANDROID: GKI: Add rockchip symbol list 2021-07-28 12:51:43 +00:00
build.config.gki.aarch64.fips140 ANDROID: Fips 140: move fips symbols entirely in own list 2021-06-11 15:04:07 +00:00
build.config.gki.x86_64 ANDROID: drop KERNEL_DIR setting in build.config.common 2020-08-31 15:20:37 +00:00
build.config.hikey960 ANDROID: Add hikey960 build infrastructure file 2021-02-18 23:59:34 +00:00
build.config.khwasan ANDROID: Add a build config fragment for KHWASan. 2021-04-06 10:01:56 +00:00
build.config.x86_64 ANDROID: build.config: Add vmlinux.symvers and modules.builtin to DIST_DIR 2021-04-15 10:24:17 +01:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Jason Cooper to CREDITS 2020-11-30 10:20:34 +01:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Merge 5.10.38 into android12-5.10 2021-05-20 15:35:25 +02:00
Makefile Merge 5.10.43 into android12-5.10 2021-06-12 14:48:14 +02:00
OWNERS ANDROID: Use authoritative Dr. No list from android-mainline 2021-07-22 15:25:25 +00:00
README
README.md ANDROID: README: Add requirement for "Bug:" tag for FROMLIST 2020-05-01 15:36:12 -07:00

README.md

How do I submit patches to Android Common Kernels

  1. BEST: Make all of your changes to upstream Linux. If appropriate, backport to the stable releases. These patches will be merged automatically in the corresponding common kernels. If the patch is already in upstream Linux, post a backport of the patch that conforms to the patch requirements below.

    • Do not send patches upstream that contain only symbol exports. To be considered for upstream Linux, additions of EXPORT_SYMBOL_GPL() require an in-tree modular driver that uses the symbol -- so include the new driver or changes to an existing driver in the same patchset as the export.
    • When sending patches upstream, the commit message must contain a clear case for why the patch is needed and beneficial to the community. Enabling out-of-tree drivers or functionality is not not a persuasive case.

  2. LESS GOOD: Develop your patches out-of-tree (from an upstream Linux point-of-view). Unless these are fixing an Android-specific bug, these are very unlikely to be accepted unless they have been coordinated with kernel-team@android.com. If you want to proceed, post a patch that conforms to the patch requirements below.

Common Kernel patch requirements

  • All patches must conform to the Linux kernel coding standards and pass script/checkpatch.pl
  • Patches shall not break gki_defconfig or allmodconfig builds for arm, arm64, x86, x86_64 architectures (see https://source.android.com/setup/build/building-kernels)
  • If the patch is not merged from an upstream branch, the subject must be tagged with the type of patch: UPSTREAM:, BACKPORT:, FROMGIT:, FROMLIST:, or ANDROID:.
  • All patches must have a Change-Id: tag (see https://gerrit-review.googlesource.com/Documentation/user-changeid.html)
  • If an Android bug has been assigned, there must be a Bug: tag.
  • All patches must have a Signed-off-by: tag by the author and the submitter

Additional requirements are listed below based on patch type

Requirements for backports from mainline Linux: UPSTREAM:, BACKPORT:

  • If the patch is a cherry-pick from Linux mainline with no changes at all
    • tag the patch subject with UPSTREAM:.
    • add upstream commit information with a (cherry picked from commit ...) line
    • Example:
      • if the upstream commit message is
        important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>
  • then Joe Smith would upload the patch for the common kernel as
        UPSTREAM: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        (cherry picked from commit c31e73121f4c1ec41143423ac6ce3ce6dafdcec1)
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch requires any changes from the upstream version, tag the patch with BACKPORT: instead of UPSTREAM:.
    • use the same tags as UPSTREAM:
    • add comments about the changes under the (cherry picked from commit ...) line
    • Example:
        BACKPORT: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        (cherry picked from commit c31e73121f4c1ec41143423ac6ce3ce6dafdcec1)
        [joe: Resolved minor conflict in drivers/foo/bar.c ]
        Signed-off-by: Joe Smith <joe.smith@foo.org>

Requirements for other backports: FROMGIT:, FROMLIST:,

  • If the patch has been merged into an upstream maintainer tree, but has not yet been merged into Linux mainline
    • tag the patch subject with FROMGIT:
    • add info on where the patch came from as (cherry picked from commit <sha1> <repo> <branch>). This must be a stable maintainer branch (not rebased, so don't use linux-next for example).
    • if changes were required, use BACKPORT: FROMGIT:
    • Example:
      • if the commit message in the maintainer tree is
        important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>
  • then Joe Smith would upload the patch for the common kernel as
        FROMGIT: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        (cherry picked from commit 878a2fd9de10b03d11d2f622250285c7e63deace
         https://git.kernel.org/pub/scm/linux/kernel/git/foo/bar.git test-branch)
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch has been submitted to LKML, but not accepted into any maintainer tree
    • tag the patch subject with FROMLIST:
    • add a Link: tag with a link to the submittal on lore.kernel.org
    • add a Bug: tag with the Android bug (required for patches not accepted into a maintainer tree)
    • if changes were required, use BACKPORT: FROMLIST:
    • Example:
        FROMLIST: important patch from upstream

        This is the detailed description of the important patch

        Signed-off-by: Fred Jones <fred.jones@foo.org>

        Bug: 135791357
        Link: https://lore.kernel.org/lkml/20190619171517.GA17557@someone.com/
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>

Requirements for Android-specific patches: ANDROID:

  • If the patch is fixing a bug to Android-specific code
    • tag the patch subject with ANDROID:
    • add a Fixes: tag that cites the patch with the bug
    • Example:
        ANDROID: fix android-specific bug in foobar.c

        This is the detailed description of the important fix

        Fixes: 1234abcd2468 ("foobar: add cool feature")
        Change-Id: I4caaaa566ea080fa148c5e768bb1a0b6f7201c01
        Signed-off-by: Joe Smith <joe.smith@foo.org>
  • If the patch is a new feature
    • tag the patch subject with ANDROID:
    • add a Bug: tag with the Android bug (required for android-specific features)