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SubjectRe: [GIT PULL] pipe: Notification queue preparation
On Mon, Nov 25, 2019 at 10:39:49PM +0000, David Howells wrote:
> David Howells (12):
> pipe: Reduce #inclusion of pipe_fs_i.h
> Remove the nr_exclusive argument from __wake_up_sync_key()
> Add wake_up_interruptible_sync_poll_locked()
> pipe: Use head and tail pointers for the ring, not cursor and length

This commit (8cefc107ca54c8b06438b7dc9) causes hangs of 'btrfs send'
commands, that's eg. inside fstests or in btrfs-progs testsuite. The
'send' uses pipe/splice to get data from kernel to userspace.

The test that reliably worked for me leaves the process hanging with
this stacktrace (no CPU or IO activity):

[<0>] pipe_write+0x1be/0x4b0
[<0>] new_sync_write+0x11e/0x1c0
[<0>] vfs_write+0xc1/0x1d0
[<0>] kernel_write+0x2c/0x40
[<0>] send_cmd+0x78/0xf0 [btrfs]
[<0>] send_extent_data+0x4b1/0x52c [btrfs]
[<0>] process_extent+0xe46/0xe9d [btrfs]
[<0>] changed_cb+0xcf5/0xd2f [btrfs]
[<0>] send_subvol+0x8ad/0xc0b [btrfs]
[<0>] btrfs_ioctl_send+0xe50/0xf30 [btrfs]
[<0>] _btrfs_ioctl_send+0x80/0x110 [btrfs]
[<0>] btrfs_ioctl+0x18e1/0x3450 [btrfs]
[<0>] do_vfs_ioctl+0xa5/0x710
[<0>] ksys_ioctl+0x70/0x80
[<0>] __x64_sys_ioctl+0x16/0x20
[<0>] do_syscall_64+0x56/0x220
[<0>] entry_SYSCALL_64_after_hwframe+0x49/0xbe

I've bisected that to the mentioned commit, using test in btrfs-progs
testsuite with command "make TEST=016\* test-misc". Normally the test
can run up to 10 seconds, in the buggy case it stays there.

I can help testing fixes or gathering more information, please let me
know.

Full bisect log:

# bad: [3c0edea9b29f9be6c093f236f762202b30ac9431] pipe: Remove sync on wake_ups
# good: [da0c9ea146cbe92b832f1b0f694840ea8eb33cce] Linux 5.4-rc2
git bisect start '3c0edea9b29f9be6c093f236f762202b30ac9431' 'd055b4fb4d165b06d912e7f846610d120c3bb9fb^'
# bad: [b667b867344301e24f21d4a4c844675ff61d89e1] pipe: Advance tail pointer inside of wait spinlock in pipe_read()
git bisect bad b667b867344301e24f21d4a4c844675ff61d89e1
# good: [f94df9890e98f2090c6a8d70c795134863b70201] Add wake_up_interruptible_sync_poll_locked()
git bisect good f94df9890e98f2090c6a8d70c795134863b70201
# bad: [6718b6f855a0b4962d54bd625be2718cb820cec6] pipe: Allow pipes to have kernel-reserved slots
git bisect bad 6718b6f855a0b4962d54bd625be2718cb820cec6
# bad: [8cefc107ca54c8b06438b7dc9cc08bc0a11d5b98] pipe: Use head and tail pointers for the ring, not cursor and length
git bisect bad 8cefc107ca54c8b06438b7dc9cc08bc0a11d5b98
# first bad commit: [8cefc107ca54c8b06438b7dc9cc08bc0a11d5b98] pipe: Use head and tail pointers for the ring, not cursor and length

> pipe: Allow pipes to have kernel-reserved slots
> pipe: Advance tail pointer inside of wait spinlock in pipe_read()
> pipe: Conditionalise wakeup in pipe_read()
> pipe: Rearrange sequence in pipe_write() to preallocate slot
> pipe: Remove redundant wakeup from pipe_write()
> pipe: Check for ring full inside of the spinlock in pipe_write()
> pipe: Increase the writer-wakeup threshold to reduce context-switch count
> pipe: Remove sync on wake_ups

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