From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tony Lindgren Subject: Re: [Letux-kernel] BUG: drivers/pinctrl/core: races in pinctrl_groups and deferred probing Date: Sun, 17 Jun 2018 23:46:51 -0700 Message-ID: <20180618064651.GL112168@atomide.com> References: <20180614120157.GE112168@atomide.com> <20180615065810.GI112168@atomide.com> <20180615111318.GJ112168@atomide.com> <029F115C-480E-485A-B547-9D5873925CEF@goldelico.com> <4DE2E482-B0D3-4799-9E2D-74E2180B305B@goldelico.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4DE2E482-B0D3-4799-9E2D-74E2180B305B@goldelico.com> Sender: linux-kernel-owner@vger.kernel.org To: "H. Nikolaus Schaller" Cc: "open list:GPIO SUBSYSTEM" , Linus Walleij , Linux Kernel Mailing List , kernel@pyra-handheld.com, Discussions about the Letux Kernel List-Id: linux-gpio@vger.kernel.org * H. Nikolaus Schaller [180616 11:10]: > There is also good news: the selectors are now assigned in strict sequence. > This is a big improvement: OK thanks for testing. > If slots 17 and 18 do not really exist (or are deleted after failed probing), there is > a NULL return. Which triggers the strcmp(NULL), because the strcmp is not guarded against > non-existing slots in the radix tree. > > So a simple workaround could be: > > if (gname && !strcmp(gname, pin_group)) { > > But I think the fundamental problem is that the same driver assigns multiple slots if > probing is deferred. > > Therefore, I tried to find out more why this happens. Here are some more observations by > studying the code and adding a dump_stack() inside pinctrl_generic_add_group(): > > 1. the records stored in the radix tree are allocated through devm_kzalloc() within > pinctrl_generic_add_group(). > 2. I have not found a mechanism that removes them from the radix tree if they are > devm freed which IMHO happens if the probe fails (and all devm objects are > rewound). > 3. I could not observe calls to pinctrl_generic_remove_group() > 4. this means a stale pointer to the group_desc is still stored in the radix tree > if driver probing fails > 5. scanning through the radix tree for a matching gname in pinctrl_get_group_selector() > accesses this stale radix tree entry. > It has either been overwritten by something else - or contains a dangling pointer for > group->name. This explains the randomness of the problem but that it is also repeatable > to some extent. For a pure race of some 100 instructions it happens IMHO too often. > 6. the pinctrl_generic_add_group() is called from create_pinctrl() through > pinctrl_dt_to_map() and pcs_dt_node_to_map(), i.e. before the pinctrl_maps_mutex > is locked in create_pinctrl(). This means that pinctrl_generic_add_group() is > not locked in this case. > > I hope this helps to pinpoint and solve the remaining bugs. Yes sounds like that's where things still go wrong. I'll take a look if it makes sense to assign the selector from the pin controller driver instead or just ignoring the holes. I'll try to debug the devm issue too. Regards, Tony