On Mon, Feb 6, 2023 at 4:28 PM Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> wrote: > > Hello, > > On Wed, Jan 18, 2023 at 02:49:40PM +0100, Bartosz Golaszewski wrote: > > From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx> > > > > If we open an i2c character device and then unbind the underlying i2c > > adapter (either by unbinding it manually via sysfs or - for a real-life > > example - when unplugging a USB device with an i2c adaper), the kernel > > thread calling i2c_del_adapter() will become blocked waiting for the > > completion that only completes once all references to the character > > device get dropped. > > Is this bad? > > > In order to fix that, we introduce a couple changes. They need to be > > part of a single commit in order to preserve bisectability. First, drop > > the dev_release completion. That removes the risk of a deadlock but > > we now need to protect the character device structures against NULL > > pointer dereferences. To that end introduce an rw semaphore. It will > > protect the dummy i2c_client structure against dropping the adapter from > > under it. It will be taken for reading by all file_operations callbacks > > and for writing by the notifier's unbind handler. This way we don't > > prohibit the syscalls that don't get in each other's way from running > > concurrently but the adapter will not be unbound before all syscalls > > return. > > > > Finally: upon being notified about an unbind event for the i2c adapter, > > we take the lock for writing and set the adapter pointer in the character > > device's structure to NULL. This "numbs down" the device - it still exists > > but is no longer functional. Meanwhile every syscall callback checks that > > pointer after taking the lock but before executing any code that requires > > it. If it's NULL, we return an error to user-space. > > > > This way we can safely open an i2c device from user-space, unbind the > > device without triggering a deadlock and any subsequent system-call for > > the file descriptor associated with the removed adapter will gracefully > > fail. > > > > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx> > > @Bartosz, is that the patch you talked about on FOSDEM? I thought > Wolfram had some concerns but I thought they were unaddressed still. > What am I missing? Hi Uwe, yes, this patch was dropped, I can see the rest of the discussion on the list, have you got the rest of the email too? Bart
