develoment workflow: how to avoid duplicate work ?

[Hugo Lefeuvre]

> 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)

Interesting. I don't know what's happening here, but the idea that
release() would be delayed until all operations returned (as nice
as it sounds) seems pretty odd to me.

I'll not have time to experiment on this today, but this is definitely
worth taking a look.

Cheers,
 Hugo

-- 
             Hugo Lefeuvre (hle)    |    www.owl.eu.com
4096/ 9C4F C8BF A4B0 8FC5 48EB 56B8 1962 765B B9A8 BACA
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