develoment workflow: how to avoid duplicate work ?

[Valentin Vidic]

On Mon, Jun 04, 2018 at 06:27:43AM +0000, Nicholas Mc Guire wrote:
> are you using kprobes to get this information or how did you trace
> these calls ? could you explain how you did that ? I assume that
> would be of interest to others as well not only me.

Nothing fancy, just added debug print statements to the release
and ioctl methods of the pi433 driver.  The ioctl method also has
a 5s sleep so it takes some time to execute while the other thread
calls close on the same file descriptor.

The timing should be as follows:

0s thread1 and thread2 start
0s thread1 calls ioctl
1s thread2 calls close
1s thread2 close returns
5s thread1 ioctl returns

Adding a dump_stack() call to both driver methods shows that ioctl is
called directly:

[236449.534358] [<8010ffd8>] (unwind_backtrace) from [<8010c240>] (show_stack+0x20/0x24)
[236449.534378] [<8010c240>] (show_stack) from [<807840a4>] (dump_stack+0xd4/0x118)
[236449.534413] [<807840a4>] (dump_stack) from [<7f5e5348>] (pi433_ioctl+0x64/0x324 [pi433])
[236449.534457] [<7f5e5348>] (pi433_ioctl [pi433]) from [<8029dbe0>] (do_vfs_ioctl+0xac/0x7c4)
[236449.534472] [<8029dbe0>] (do_vfs_ioctl) from [<8029e33c>] (SyS_ioctl+0x44/0x6c)
[236449.534488] [<8029e33c>] (SyS_ioctl) from [<80108060>] (ret_fast_syscall+0x0/0x28)

while the release gets called from task_work_run:

[236454.624185] [<8010ffd8>] (unwind_backtrace) from [<8010c240>] (show_stack+0x20/0x24)
[236454.624204] [<8010c240>] (show_stack) from [<807840a4>] (dump_stack+0xd4/0x118)
[236454.624240] [<807840a4>] (dump_stack) from [<7f5e508c>] (pi433_release+0x48/0xc0 [pi433])
[236454.624270] [<7f5e508c>] (pi433_release [pi433]) from [<8028bab0>] (__fput+0x9c/0x1e8)
[236454.624288] [<8028bab0>] (__fput) from [<8028bc6c>] (____fput+0x18/0x1c)
[236454.624304] [<8028bc6c>] (____fput) from [<8013bbf8>] (task_work_run+0xbc/0xe0)
[236454.624322] [<8013bbf8>] (task_work_run) from [<8010b810>] (do_work_pending+0xcc/0xd0)
[236454.624340] [<8010b810>] (do_work_pending) from [<80108094>] (slow_work_pending+0xc/0x20)

-- 
Valentin