On 03/13/2013 07:22 PM, Stephen Warren wrote: > On 03/13/2013 09:44 AM, Linus Walleij wrote: >> From: Patrice Chotard <patrice.chot...@st.com> >> >> This mutex avoids deadlock in case of use of multiple pin >> controllers. Before this modification, by using a global >> mutex, deadlock appeared when, for example, a call to >> pinctrl_pins_show() locked the pinctrl_mutex, called the >> ops->pin_dbg_show of a particular pin controller. If this >> pin controller needs I2C access to retrieve configuration >> information and I2C driver is using pinctrl to drive its >> pins, a call to pinctrl_select_state() try to lock again >> pinctrl_mutex which leads to a deadlock. >> >> Notice that the mutex grab from the two direction functions >> was moved into pinctrl_gpio_direction(). >> >> For two cases, we can't replace pinctrl_mutex by >> pctldev->mutex, because at this stage, pctldev is >> not accessible : >> - pinctrl_get()/pinctrl_put() >> - pinctrl_register_maps() >> >> So add respectively pinctrl_list_mutex and >> pinctrl_maps_mutex in order to protect >> pinctrl_list and pinctrl_maps list instead. > > I can't see how this would be safe, or even how it would solve the > problem (and still be safe). > > In the scenario described above, pinctrl_pins_show() would need to lock > the list mutex since it's interacting with the list of pinctrl devices. > Then, the I2C drivers'c pinctrl_select() also needs to acquire that same > lock, since it's interacting with another entry in that same list in > order to re-program the other pinctrl device to route I2C to the correct > location. >
Hi Stephen, Thanks for your review. I don't understand why, in pinctrl_pins_show(), pinctrl_list_mutex should be locked whereas pinctrl_list is not updated or parsed ? > So, I can't see how separating out the map lock would make any difference. > > Also, why is the map lock relevant here at all anyway? The I2C mux's > probe() should have executed pinctrl_get(), and isn't the map parsed at > that time, and converted to a struct pinctrl, and hence any later call > to pinctrl_select() doesn't touch the map? Sorry, but i don't follow what you mean .... In create_pinctrl(), maps is protected by pinctrl_maps_mutex. I don't understand the link between maps and pinctrl_select(), pinctrl_select_state_locked() doesn't touch the map. > > Is there a recursive lock type that could be used instead? I'm not sure > if that'd still be safe though. > > Finally, a long while ago when I removed these separate locks and > created the single lock, I raised a slew of complex points re: why it > was extremely hard to split up the locking. I talked about a number of > AB/BA deadlock cases IIRC mostly w.r.t pinctrl device registration. Were > those considered when writing this patch? What's the solution? I suppose you make reference to the comment you put on this commit ? 57b676f9c1b7cd84397fe5a86c9bd2788ac4bd32 : pinctrl: fix and simplify locking Added in CC Seraphin Bonnafe as he has reported the deadlock issue using several pin ctrollers.-- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
