Re: [PATCH] HID: i2c-hid: remove I2C_HID_READ_PENDING flag to prevent lock-up

[Jiri Kosina]

On Mon, 18 Mar 2024, Nam Cao wrote:

> The flag I2C_HID_READ_PENDING is used to serialize I2C operations.
> However, this is not necessary, because I2C core already has its own
> locking for that.
> 
> More importantly, this flag can cause a lock-up: if the flag is set in
> i2c_hid_xfer() and an interrupt happens, the interrupt handler
> (i2c_hid_irq) will check this flag and return immediately without doing
> anything, then the interrupt handler will be invoked again in an
> infinite loop.
> 
> Since interrupt handler is an RT task, it takes over the CPU and the
> flag-clearing task never gets scheduled, thus we have a lock-up.
> 
> Delete this unnecessary flag.

Hmm, right, good catch, I can't figure out what extra semantic 
I2C_HID_READ_PENDING would be adding (rather than deadlock :) ).
Why RT throttling didn't happen and let the system in a completely locked 
up state is something I don't understand, but that's separate.

I have now queued this in hid.git#for-6.9/upstream-fixes

Thanks,

-- 
Jiri Kosina
SUSE Labs