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[87.3.180.232]) by smtp.gmail.com with ESMTPSA id n14sm750005wrt.8.2020.11.05.23.40.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Nov 2020 23:40:31 -0800 (PST) Date: Fri, 6 Nov 2020 08:40:30 +0100 From: Andrea Righi To: Pavel Machek Cc: Boqun Feng , Peter Zijlstra , Ingo Molnar , Will Deacon , linux-kernel@vger.kernel.org Subject: Re: lockdep: possible irq lock inversion dependency detected (trig->leddev_list_lock) Message-ID: <20201106074030.GG1041241@xps-13-7390> References: <20201101092614.GB3989@xps-13-7390> <20201031101740.GA1875@boqun-laptop.fareast.corp.microsoft.com> <20201102073328.GA9930@xps-13-7390> <20201102085658.GA5506@amd> <20201102090927.GC9930@xps-13-7390> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201102090927.GC9930@xps-13-7390> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 02, 2020 at 10:09:28AM +0100, Andrea Righi wrote: > On Mon, Nov 02, 2020 at 09:56:58AM +0100, Pavel Machek wrote: > > 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); > > > > > > > > > > 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); > > > > > > > > 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 > > > > > > > > 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? > > Using read_lock_irqsave/irqrestore() in led_trigger_event() would be > enough to prevent the deadlock. If it's an acceptable solution I can > send a patch (already tested it and lockdep doesn't complain :)). Any comment on https://lore.kernel.org/lkml/20201102104152.GG9930@xps-13-7390/? Thanks, -Andrea