On Wed, Jan 18, 2023 at 2:47 PM Bartosz Golaszewski <brgl@xxxxxxxx> 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. > > 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> > --- Sorry for the noise, it was supposed to be v3. Bart
