From: Pavel Machek <firstname.lastname@example.org> To: Andrea Righi <email@example.com> Cc: Boqun Feng <firstname.lastname@example.org>, Peter Zijlstra <email@example.com>, Ingo Molnar <firstname.lastname@example.org>, Will Deacon <email@example.com>, firstname.lastname@example.org Subject: Re: lockdep: possible irq lock inversion dependency detected (trig->leddev_list_lock) Date: Mon, 2 Nov 2020 09:56:58 +0100 [thread overview] Message-ID: <20201102085658.GA5506@amd> (raw) In-Reply-To: <20201102073328.GA9930@xps-13-7390> [-- Attachment #1: Type: text/plain, Size: 3492 bytes --] Hi! > > > I'm getting the following lockdep splat (see below). > > > > > > Apparently this warning starts to be reported after applying: > > > > > > e918188611f0 ("locking: More accurate annotations for read_lock()") > > > > > > It looks like a false positive to me, but it made me think a bit and > > > IIUC there can be still a potential deadlock, even if the deadlock > > > scenario is a bit different than what lockdep is showing. > > > > > > In the assumption that read-locks are recursive only in_interrupt() > > > context (as stated in e918188611f0), the following scenario can still > > > happen: > > > > > > CPU0 CPU1 > > > ---- ---- > > > read_lock(&trig->leddev_list_lock); > > > write_lock(&trig->leddev_list_lock); > > > <soft-irq> > > > kbd_bh() > > > -> read_lock(&trig->leddev_list_lock); > > > > > > *** DEADLOCK *** > > > > > > The write-lock is waiting on CPU1 and the second read_lock() on CPU0 > > > would be blocked by the write-lock *waiter* on CPU1 => deadlock. > > > > > > > No, this is not a deadlock, as a write-lock waiter only blocks > > *non-recursive* readers, so since the read_lock() in kbd_bh() is called > > in soft-irq (which in_interrupt() returns true), so it's a recursive > > reader and won't get blocked by the write-lock waiter. > > That's right, I was missing that in_interrupt() returns true also from > soft-irq context. > > > > In that case we could prevent this deadlock condition using a workqueue > > > to call kbd_propagate_led_state() instead of calling it directly from > > > kbd_bh() (even if lockdep would still report the false positive). > > > > > > > The deadlock senario reported by the following splat is: > > > > > > CPU 0: CPU 1: CPU 2: > > ----- ----- ----- > > led_trigger_event(): > > read_lock(&trig->leddev_list_lock); > > <work queue processing> > > ata_hsm_qs_complete(): > > spin_lock_irqsave(&host->lock); > > write_lock(&trig->leddev_list_lock); > > ata_port_freeze(): > > ata_do_link_abort(): > > ata_qc_complete(): > > ledtrig_disk_activity(): > > led_trigger_blink_oneshot(): > > read_lock(&trig->leddev_list_lock); > > // ^ not in in_interrupt() context, so could get blocked by CPU 2 > > <interrupt> > > ata_bmdma_interrupt(): > > spin_lock_irqsave(&host->lock); > > > > , where CPU 0 is blocked by CPU 1 because of the spin_lock_irqsave() in > > ata_bmdma_interrupt() and CPU 1 is blocked by CPU 2 because of the > > read_lock() in led_trigger_blink_oneshot() and CPU 2 is blocked by CPU 0 > > because of an arbitrary writer on &trig->leddev_list_lock. > > > > So I don't think it's false positive, but I might miss something > > obvious, because I don't know what the code here actually does ;-) > > With the CPU2 part it all makes sense now and lockdep was right. :) > > At this point I think we could just schedule a separate work to do the > led trigger and avoid calling it with host->lock held and that should > prevent the deadlock. I'll send a patch to do that. Let's... not do that, unless we have no choice. Would it help if leddev_list_lock used _irqsave() locking? Best regards, Pavel -- http://www.livejournal.com/~pavelmachek [-- Attachment #2: Digital signature --] [-- Type: application/pgp-signature, Size: 181 bytes --]
next prev parent reply other threads:[~2020-11-02 8:57 UTC|newest] Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-11-01 9:26 Andrea Righi 2020-10-31 10:17 ` Boqun Feng 2020-11-02 7:33 ` Andrea Righi 2020-11-02 8:56 ` Pavel Machek [this message] 2020-11-02 9:09 ` Andrea Righi 2020-11-06 7:40 ` Andrea Righi 2020-11-01 16:28 ` Pavel Machek 2020-11-02 7:39 ` Andrea Righi
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