Re: nwbutton: lost wakeup in button_read() -- worth fixing, or is this driver done?

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Aug 22, 2026 at 11:01:38AM -0500, old king wrote:
> Hi,
> 
> button_read() in drivers/char/nwbutton.c sleeps unconditionally: nothing is
> tested before schedule(), not even whether the buffer is empty. If
> button_sequence_finished() wakes button_wait_queue in the window between
> read() being entered and prepare_to_wait() queuing the reader, that wakeup
> is lost and the reader sleeps until some later, unrelated button sequence
> completes. The race predates commit eb831743f2b4, whose changelog already called
> the code "inherently racy". I have reproduced it under QEMU and prepared a fix:
> a sequence counter captured before queuing and re-checked after.
> 
> Before I send that: is this driver still worth fixing? footbridge is
> deprecated and its Kconfig says support goes away in 2027 unless remaining
> users speak up. Every change to nwbutton.c in git history has been a
> treewide cleanup rather than anything a user asked for, and
> button_add_callback()/button_del_callback() have no callers in the tree. I
> have no NetWinder hardware and no way to tell whether anyone else does.

If you don't have the hardware to test it, don't worry about the driver
at all.

thanks,

greg k-h