On Tue, Nov 09, 2021 at 10:38:47PM +0100, Peter Zijlstra wrote: > On Tue, Nov 09, 2021 at 12:06:48PM -0800, Sultan Alsawaf wrote: > > Hi, > > > > I encountered a printk deadlock on 5.13 which appears to still affect the latest > > kernel. The deadlock occurs due to printk being used while having the current > > CPU's runqueue locked, and the underlying framebuffer console attempting to lock > > the same runqueue when printk tries to flush the log buffer. > > Yes, that's a known 'feature' of some consoles. printk() is in the > process of being reworked to not call con->write() from the printk() > calling context, which would go a long way towards fixing this. I'm a bit out of the loop but from lwn articles my understanding is that part of upstreaming from -rt we no longer have the explicit "I'm a safe console for direct printing" opt-in. Which I get from a backwards compat pov, but I still think for at least fbcon we really should never attempt a direct printk con->write, it's just all around terrible. And it's getting worse by the year: - direct scanout displays (i.e. just a few mmio writes and it will show up) are on the way out at least in laptops, everyone gets self-refresh (dp psr) under software control, so without being able to kick a kthread off nothing shows up except more oopses - because of the impendence mismatch between fbdev and drm-kms we even go ever more this direction for dumb framebuffers, including the firmware boot-up framebuffer simpledrm. This could perhaps be fixed with a new dedicate console driver directly on top of drm-kms, but that's on the wishlist for years and I don't see anyone typing that. So yeah for fbcon at least I think we really should throw out direct con->write from printk completely. Also adding John Ogness. -Daniel > > > #27 [ffffc900005b8e28] enqueue_task_fair at ffffffff8129774a <-- SCHED_WARN_ON(rq->tmp_alone_branch != &rq->leaf_cfs_rq_list); > > #28 [ffffc900005b8ec0] activate_task at ffffffff8125625d > > #29 [ffffc900005b8ef0] ttwu_do_activate at ffffffff81257943 > > #30 [ffffc900005b8f28] sched_ttwu_pending at ffffffff8125c71f <-- locks this CPU's runqueue > > #31 [ffffc900005b8fa0] flush_smp_call_function_queue at ffffffff813c6833 > > #32 [ffffc900005b8fd8] generic_smp_call_function_single_interrupt at ffffffff813c7f58 > > #33 [ffffc900005b8fe0] __sysvec_call_function_single at ffffffff810f1456 > > #34 [ffffc900005b8ff0] sysvec_call_function_single at ffffffff831ec1bc > > --- <IRQ stack> --- > > #35 [ffffc9000019fda8] sysvec_call_function_single at ffffffff831ec1bc > > RIP: ffffffff831ed06e RSP: ffffed10438a6a49 RFLAGS: 00000001 > > RAX: ffff888100d832c0 RBX: 0000000000000000 RCX: 1ffff92000033fd7 > > RDX: 0000000000000000 RSI: ffff888100d832c0 RDI: ffffed10438a6a49 > > RBP: ffffffff831ec166 R8: dffffc0000000000 R9: 0000000000000000 > > R10: ffffffff83400e22 R11: 0000000000000000 R12: ffffffff831ed83e > > R13: 0000000000000000 R14: ffffc9000019fde8 R15: ffffffff814d4d9d > > ORIG_RAX: ffff88821c53524b CS: 0001 SS: ef073a2 > > WARNING: possibly bogus exception frame > > ----------------------->8----------------------- > > > > The catalyst is that CONFIG_SCHED_DEBUG is enabled and the tmp_alone_branch > > assertion fails (Peter, is this bad?). > > Yes, that's not good. IIRC Vincent and Michal were looking at that code > recently. > > > I'm not sure what the *correct* solution is here (don't use printk while having > > a runqueue locked? don't use schedule_work() from the fbcon path? tell printk > > to use one of its lock-less backends?), so I've cc'd all the relevant folks. > > I'm a firm believer in early_printk serial consoles. -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
