Messages in this thread | | | Subject | Re: [BUG] bus: mhi: possible ABBA deadlock in mhi_pm_m0_transition() and mhi_send_cmd() | From | Jia-Ju Bai <> | Date | Mon, 19 Jul 2021 10:08:55 +0800 |
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On 2021/7/16 14:45, Manivannan Sadhasivam wrote: > On Thu, Jul 15, 2021 at 09:57:22AM -0700, Bhaumik Bhatt wrote: >> On 2021-07-15 02:45 AM, Jia-Ju Bai wrote: >>> Hello, >>> >>> I find there is a possible ABBA deadlock in the MHI driver in Linux >>> 5.10: >>> >>> In mhi_pm_m0_transition(): >>> 262: read_lock_bh(&mhi_cntrl->pm_lock); >>> 281: spin_lock_irq(&mhi_cmd->lock); >>> >>> In mhi_send_cmd(): >>> 1181: spin_lock_bh(&mhi_cmd->lock); >>> 1207: read_lock_bh(&mhi_cntrl->pm_lock); >>> >>> When mhi_pm_m0_transition() and mhi_send_cmd() are concurrently >>> executed, the deadlock can occur. >>> >>> I check the code and find a possible case of such concurrent execution: >>> >>> #CPU1: >>> mhi_poll (mhi_event->process_event(...)) >>> mhi_process_ctrl_ev_ring >>> mhi_pm_m0_transition >>> >>> #CPU2: >>> mhi_prepare_for_transfer >>> mhi_prepare_channel >>> mhi_send_cmd >>> >>> Note that mhi_poll() and mhi_prepare_for_transfer() are both exported >>> by EXPORT_SYMBOL. >>> Thus, I guess these two functions could be concurrently called by a MHI >>> driver. >>> >>> I am not quite sure whether this possible deadlock is real and how to >>> fix it if it is real. >>> Any feedback would be appreciated, thanks :) >>> >>> >>> Best wishes, >>> Jia-Ju Bai >> Few pointers from your example: >> >> 1. mhi_poll() is currently not used by any client upstream yet. > Then this shouldn't be added in first place... :/ > >> 2. Polling is not to be used for single event ring (shared control + data) >> cases >> since it is meant to be for client drivers with dedicated data packets only. >> 3. mhi_send_cmd() will always be called after an mhi_pm_m0_transition() has >> completed by design since we wait for the device to be held in M0 prior to >> it. >> > But client can be unloaded during M0 event! > > Anyway, I don't think the deadlock scenario is valid because of the usage > of "read_lock_bh()". So if "mhi_send_cmd()" has acquired > "spin_lock_bh(&mhi_cmd->lock)", it can always acquire > "read_lock_bh(&mhi_cntrl->pm_lock)" as multiple readers can acquire the > read lock. > > Deadlock would only occur if one of the functions take write lock. > > Thanks for auditing.
Thanks for the feedback and explanation :)
Best wishes, Jia-Ju Bai
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