Re: "upping" a semaphore from interrupt context?

[Arnd Bergmann <arnd@arndb.de>]

On Friday 22 June 2007, Florin Iucha wrote:
> I am writing a USB driver for some custom hardware, and I need to
> synchronize between the user-space and the USB subsystem.  Can I
> create a semaphore and "down" it in the reader then "up" it in the
> completion handler?

yes, but you should not. The use of semaphores is not recommended
for new code, it should be replaced with either a mutex or a
completion.

> I know the completion handler runs in interrupt context so you are not
> allowed to acquire any semaphores: but can you release them?  Will the
> waiting tasks wake up after the handler and its caller returned - IOW
> will the waking up task run in interrupt context as well?

What you should use is a 'struct completion', which you can wait for
in process context and complete from interrupt context.

	Arnd <><