# git rev-parse -q --verify 472e5b056f000a778abb41f1e443de58eb259783^{commit}
472e5b056f000a778abb41f1e443de58eb259783
already have revision, skipping fetch
# git checkout -q -f -B kisskb 472e5b056f000a778abb41f1e443de58eb259783
# git clean -qxdf
# < git log -1
# commit 472e5b056f000a778abb41f1e443de58eb259783
# Author: Linus Torvalds <torvalds@linux-foundation.org>
# Date:   Thu Oct 1 19:14:36 2020 -0700
# 
#     pipe: remove pipe_wait() and fix wakeup race with splice
#     
#     The pipe splice code still used the old model of waiting for pipe IO by
#     using a non-specific "pipe_wait()" that waited for any pipe event to
#     happen, which depended on all pipe IO being entirely serialized by the
#     pipe lock.  So by checking the state you were waiting for, and then
#     adding yourself to the wait queue before dropping the lock, you were
#     guaranteed to see all the wakeups.
#     
#     Strictly speaking, the actual wakeups were not done under the lock, but
#     the pipe_wait() model still worked, because since the waiter held the
#     lock when checking whether it should sleep, it would always see the
#     current state, and the wakeup was always done after updating the state.
#     
#     However, commit 0ddad21d3e99 ("pipe: use exclusive waits when reading or
#     writing") split the single wait-queue into two, and in the process also
#     made the "wait for event" code wait for _two_ wait queues, and that then
#     showed a race with the wakers that were not serialized by the pipe lock.
#     
#     It's only splice that used that "pipe_wait()" model, so the problem
#     wasn't obvious, but Josef Bacik reports:
#     
#      "I hit a hang with fstest btrfs/187, which does a btrfs send into
#       /dev/null. This works by creating a pipe, the write side is given to
#       the kernel to write into, and the read side is handed to a thread that
#       splices into a file, in this case /dev/null.
#     
#       The box that was hung had the write side stuck here [pipe_write] and
#       the read side stuck here [splice_from_pipe_next -> pipe_wait].
#     
#       [ more details about pipe_wait() scenario ]
#     
#       The problem is we're doing the prepare_to_wait, which sets our state
#       each time, however we can be woken up either with reads or writes. In
#       the case above we race with the WRITER waking us up, and re-set our
#       state to INTERRUPTIBLE, and thus never break out of schedule"
#     
#     Josef had a patch that avoided the issue in pipe_wait() by just making
#     it set the state only once, but the deeper problem is that pipe_wait()
#     depends on a level of synchonization by the pipe mutex that it really
#     shouldn't.  And the whole "wait for any pipe state change" model really
#     isn't very good to begin with.
#     
#     So rather than trying to work around things in pipe_wait(), remove that
#     legacy model of "wait for arbitrary pipe event" entirely, and actually
#     create functions that wait for the pipe actually being readable or
#     writable, and can do so without depending on the pipe lock serializing
#     everything.
#     
#     Fixes: 0ddad21d3e99 ("pipe: use exclusive waits when reading or writing")
#     Link: https://lore.kernel.org/linux-fsdevel/bfa88b5ad6f069b2b679316b9e495a970130416c.1601567868.git.josef@toxicpanda.com/
#     Reported-by: Josef Bacik <josef@toxicpanda.com>
#     Reviewed-and-tested-by: Josef Bacik <josef@toxicpanda.com>
#     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/kisskb/korg/gcc-10.1.0-nolibc/arc-linux/bin/arc-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-10.1.0-nolibc/arc-linux/bin/arc-linux-ld --version
# < git log --format=%s --max-count=1 472e5b056f000a778abb41f1e443de58eb259783
# < make -s -j 8 ARCH=arc O=/kisskb/build/linus_axs103_smp_defconfig_arc-gcc10 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-10.1.0-nolibc/arc-linux/bin/arc-linux-  axs103_smp_defconfig
# Added to kconfig CONFIG_INITRAMFS_SOURCE=""
# < make -s -j 8 ARCH=arc O=/kisskb/build/linus_axs103_smp_defconfig_arc-gcc10 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-10.1.0-nolibc/arc-linux/bin/arc-linux-  help
# make -s -j 8 ARCH=arc O=/kisskb/build/linus_axs103_smp_defconfig_arc-gcc10 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-10.1.0-nolibc/arc-linux/bin/arc-linux-  olddefconfig
# make -s -j 8 ARCH=arc O=/kisskb/build/linus_axs103_smp_defconfig_arc-gcc10 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-10.1.0-nolibc/arc-linux/bin/arc-linux-  
Completed OK
# rm -rf /kisskb/build/linus_axs103_smp_defconfig_arc-gcc10
# Build took: 0:01:50.502132