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    SubjectRe: [PATCH v9 09/12] lib/vsprintf: Make use of fwnode API to obtain node names and separators
    On Fri 2020-01-03 13:21:45, Sakari Ailus wrote:
    > Hi Guenter,
    >
    > On Thu, Jan 02, 2020 at 02:20:41PM -0800, Guenter Roeck wrote:
    > > Hi,
    > >
    > > On Thu, Oct 03, 2019 at 03:32:16PM +0300, Sakari Ailus wrote:
    > > > Instead of implementing our own means of discovering parent nodes, node
    > > > names or counting how many parents a node has, use the newly added
    > > > functions in the fwnode API to obtain that information.
    > > >
    > > > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
    > > > Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
    > > > Reviewed-by: Petr Mladek <pmladek@suse.com>
    > > > ---
    > >
    > > This patch results in a lockdep splat when running one of my qemu
    > > emulations. See below for log and bisect results. A complete log
    > > is available at
    > > https://kerneltests.org/builders/qemu-arm-master/builds/1408/steps/qemubuildcommand/logs/stdio
    > >
    > > Guenter
    >
    > Thank you for reporting this.
    >
    > I looked into the issue, and indeed I can conform the patch introduces this
    > as it takes the devtree_lock for printing the name of the fwnode. There is

    I guess that you meant "is not".


    > however chance of a deadlock in practice as the code in mm/slub.c does not
    > deal with fwnodes (in which case acquiring devtree_lock could be possible),
    > maybe for other reasons as well. The patch however introduces an unpleasant
    > source of such warnings.

    I agree that it is a false positive. alloc/free is called in OF code
    under devtree_lock. But OF code is not called from alloc/free (slub.c)
    and it should not happen.

    lockdep sees the cycle only because the chains are connected via
    printk() and logbuf_lock.


    > One approach to address this could be not allocating memory while holding
    > devtree_lock spinlock. That seems entirely feasible. But could also
    > releasing memory cause something to be printed, effectively causing the
    > same problem?

    I expect that &n->list_lock)->rlock will be needed in kfree() as well.

    Anyway, IMHO, allocation outside devtree_lock spinlock would create hairy
    and tricky code.

    The number of needed "cache_entries" need to be counted under
    devtree_lock before the allocation. It means that we would need to
    take and release the lock twice. It might create a bunch of possible
    races. For example, when new entries are added in the mean time. Or
    when this function is called twice in parallel.


    > Perhaps a safest way to fix this could be returning to use dn->full_name
    > for printing node names, in which case the devtree_lock would no longer be
    > taken for printing names. The effect would be though that there would be
    > again one more user for the full_name field, information that can be
    > reconstructed from the node's parents.

    Would this avoid using devtree_lock in all %pO? modifiers?

    Removing the lock usage in vsprintf() would make sense
    from two reasons:

    + It would allow to use %pOF from inside drivers/of/
    code called under devtree_lock. The current implementation
    would cause a deadlock because of the recursion.

    + There is a huge effort to make printk() lockless. Any lock
    in vsprintf() is just a call for troubles. We probably should
    not have allowed this in the first place.


    Finally, this problem will be gone when printk uses a lockless
    ringbuffer. I hope that it will happen either for-5.6 or 5.7.
    The most tricky part, the ringbuffer itself is in good shape now, see
    https://lkml.kernel.org/r/20191128015235.12940-1-john.ogness@linutronix.de

    Temporary solution would be to disable lockdep in vsprintf() code.
    But I would really prefer to avoid the lock in vsprintf() at all.

    Best Regards,
    Petr

    > > ---
    > > ======================================================
    > > WARNING: possible circular locking dependency detected
    > > 5.5.0-rc4-00066-g738d2902773e #1 Not tainted
    > > ------------------------------------------------------
    > > swapper/0/1 is trying to acquire lock:
    > > c1313b00 (logbuf_lock){-.-.}, at: vprintk_emit+0x68/0x2d4
    > >
    > > but task is already holding lock:
    > > ef030b90 (&(&n->list_lock)->rlock){..-.}, at: free_debug_processing+0x38/0x418
    > >
    > > which lock already depends on the new lock.
    > >
    > >
    > > the existing dependency chain (in reverse order) is:
    > >
    > > -> #2 (&(&n->list_lock)->rlock){..-.}:
    > > ___slab_alloc.constprop.23+0x12c/0x798
    > > __slab_alloc.constprop.22+0x44/0x70
    > > __kmalloc+0x384/0x41c
    > > of_populate_phandle_cache+0xcc/0x148
    > > of_core_init+0x8/0xbc
    > > driver_init+0x1c/0x2c
    > > kernel_init_freeable+0xac/0x1b4
    > > kernel_init+0x8/0x118
    > > ret_from_fork+0x14/0x20
    > > 0x0
    > >
    > > -> #1 (devtree_lock){....}:
    > > of_get_parent+0x18/0x34
    > > of_fwnode_get_parent+0x34/0x40
    > > fwnode_count_parents+0x28/0x58
    > > fwnode_full_name_string+0x18/0xa0
    > > device_node_string+0x490/0x4f0
    > > pointer+0x440/0x4d8
    > > vsnprintf+0x1bc/0x3d8
    > > vscnprintf+0xc/0x24
    > > vprintk_store+0x34/0x204
    > > vprintk_emit+0x94/0x2d4
    > > vprintk_default+0x20/0x28
    > > printk+0x30/0x54
    > > exynos4_pm_init_power_domain+0x220/0x258
    > > do_one_initcall+0x8c/0x440
    > > kernel_init_freeable+0x150/0x1b4
    > > kernel_init+0x8/0x118
    > > ret_from_fork+0x14/0x20
    > > 0x0
    > >
    > > -> #0 (logbuf_lock){-.-.}:
    > > lock_acquire+0xec/0x290
    > > _raw_spin_lock+0x38/0x48
    > > vprintk_emit+0x68/0x2d4
    > > vprintk_default+0x20/0x28
    > > printk+0x30/0x54
    > > unwind_frame+0x6a8/0x6fc
    > > walk_stackframe+0x2c/0x38
    > > __save_stack_trace+0x84/0x8c
    > > stack_trace_save+0x3c/0x5c
    > > set_track+0x40/0x9c
    > > free_debug_processing+0x1a4/0x418
    > > __slab_free+0x2d4/0x510
    > > kmem_cache_free+0x44c/0x49c
    > > rcu_core+0x348/0x994
    > > __do_softirq+0x164/0x668
    > > irq_exit+0x16c/0x170
    > > __handle_domain_irq+0x80/0xec
    > > gic_handle_irq+0x58/0x9c
    > > __irq_svc+0x70/0xb0
    > > raid6_neon8_gen_syndrome_real+0x264/0x39c
    > > raid6_neon8_gen_syndrome_real+0x264/0x39c
    > >
    > > other info that might help us debug this:
    > >
    > > Chain exists of:
    > > logbuf_lock --> devtree_lock --> &(&n->list_lock)->rlock
    > >
    > > Possible unsafe locking scenario:
    > >
    > > CPU0 CPU1
    > > ---- ----
    > > lock(&(&n->list_lock)->rlock);
    > > lock(devtree_lock);
    > > lock(&(&n->list_lock)->rlock);
    > > lock(logbuf_lock);
    > >
    > > *** DEADLOCK ***
    > >
    > > 2 locks held by swapper/0/1:
    > > #0: c131466c (rcu_callback){....}, at: rcu_core+0x304/0x994
    > > #1: ef030b90 (&(&n->list_lock)->rlock){..-.}, at: free_debug_processing+0x38/0x418
    > >
    > > stack backtrace:
    > > CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.5.0-rc4-00066-g738d2902773e #1
    > > Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
    > > [<c0113264>] (unwind_backtrace) from [<c010e448>] (show_stack+0x10/0x14)
    > > [<c010e448>] (show_stack) from [<c0c9b754>] (dump_stack+0xa4/0xd0)
    > > [<c0c9b754>] (dump_stack) from [<c018cbd8>] (check_noncircular+0x258/0x274)
    > > [<c018cbd8>] (check_noncircular) from [<c019043c>] (__lock_acquire+0x1870/0x2860)
    > > [<c019043c>] (__lock_acquire) from [<c018e088>] (lock_acquire+0xec/0x290)
    > > [<c018e088>] (lock_acquire) from [<c0cbf278>] (_raw_spin_lock+0x38/0x48)
    > > [<c0cbf278>] (_raw_spin_lock) from [<c01a1f84>] (vprintk_emit+0x68/0x2d4)
    > > [<c01a1f84>] (vprintk_emit) from [<c01a2210>] (vprintk_default+0x20/0x28)
    > > [<c01a2210>] (vprintk_default) from [<c01a2844>] (printk+0x30/0x54)
    > > [<c01a2844>] (printk) from [<c0113210>] (unwind_frame+0x6a8/0x6fc)
    > > [<c0113210>] (unwind_frame) from [<c010ddf0>] (walk_stackframe+0x2c/0x38)
    > > [<c010ddf0>] (walk_stackframe) from [<c010df54>] (__save_stack_trace+0x84/0x8c)
    > > [<c010df54>] (__save_stack_trace) from [<c01c2d40>] (stack_trace_save+0x3c/0x5c)
    > > [<c01c2d40>] (stack_trace_save) from [<c02ae258>] (set_track+0x40/0x9c)
    > > [<c02ae258>] (set_track) from [<c02b06a8>] (free_debug_processing+0x1a4/0x418)
    > > [<c02b06a8>] (free_debug_processing) from [<c02b0bf0>] (__slab_free+0x2d4/0x510)
    > > [<c02b0bf0>] (__slab_free) from [<c02b17ac>] (kmem_cache_free+0x44c/0x49c)
    > > [<c02b17ac>] (kmem_cache_free) from [<c01bd608>] (rcu_core+0x348/0x994)
    > > [<c01bd608>] (rcu_core) from [<c010230c>] (__do_softirq+0x164/0x668)
    > > [<c010230c>] (__do_softirq) from [<c0131310>] (irq_exit+0x16c/0x170)
    > > [<c0131310>] (irq_exit) from [<c01a3740>] (__handle_domain_irq+0x80/0xec)
    > > [<c01a3740>] (__handle_domain_irq) from [<c0630828>] (gic_handle_irq+0x58/0x9c)
    > > [<c0630828>] (gic_handle_irq) from [<c0101a70>] (__irq_svc+0x70/0xb0)
    > > Exception stack(0xef19bd30 to 0xef19bd78)
    > > bd20: c0d43e50 c0d43e60 ef19bebc c0d43e70
    > > bd40: c0d43e20 c0d43e10 00000400 00000430 00000440 00000450 00000460 00000470
    > > bd60: c0d43e40 ef19bd80 c0d43e30 c0625c00 20000013 ffffffff
    > > [<c0101a70>] (__irq_svc) from [<c0625c00>] (raid6_neon8_gen_syndrome_real+0x264/0x39c)
    > >
    > > ---
    > > Bisect:
    > >
    > > # bad: [738d2902773e30939a982c8df7a7f94293659810] Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net
    > > # good: [219d54332a09e8d8741c1e1982f5eae56099de85] Linux 5.4
    > > git bisect start 'HEAD' 'v5.4'
    > > # bad: [8c39f71ee2019e77ee14f88b1321b2348db51820] Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net
    > > git bisect bad 8c39f71ee2019e77ee14f88b1321b2348db51820
    > > # good: [3b397c7ccafe0624018cb09fc96729f8f6165573] Merge tag 'regmap-v5.5' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap
    > > git bisect good 3b397c7ccafe0624018cb09fc96729f8f6165573
    > > # bad: [89d57dddd7d319ded00415790a0bb3c954b7e386] Merge tag 'media/v5.5-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-media
    > > git bisect bad 89d57dddd7d319ded00415790a0bb3c954b7e386
    > > # good: [9e7a03233e02afd3ee061e373355f34d7254f1e6] Merge tag 'pm-5.5-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm
    > > git bisect good 9e7a03233e02afd3ee061e373355f34d7254f1e6
    > > # good: [09578eacaaa44149738267083ccc050990409f86] Merge tag 'asoc-v5.5-2' of https://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound into for-linus
    > > git bisect good 09578eacaaa44149738267083ccc050990409f86
    > > # good: [1c7ae4a51298d52a21f63b2214657982036c7498] media: ad5820: Add support for of-autoload
    > > git bisect good 1c7ae4a51298d52a21f63b2214657982036c7498
    > > # good: [99cf8a7074c4ce3ff3685cd389f54e7bd4bbf510] media: dt-bindings: Fix building error for dt_binding_check
    > > git bisect good 99cf8a7074c4ce3ff3685cd389f54e7bd4bbf510
    > > # good: [782b59711e1561ee0da06bc478ca5e8249aa8d09] Merge branch 'acpi-mm'
    > > git bisect good 782b59711e1561ee0da06bc478ca5e8249aa8d09
    > > # good: [0ca40f41d795fd91811e44506bb73d0b9ca33bdd] Merge branch 'patchwork' into v4l_for_linus
    > > git bisect good 0ca40f41d795fd91811e44506bb73d0b9ca33bdd
    > > # bad: [a00351687f8a05773c1c57be80a5bbca68fa9ae8] software node: remove DEV_PROP_MAX
    > > git bisect bad a00351687f8a05773c1c57be80a5bbca68fa9ae8
    > > # good: [9af7706492f985867d070861fe39fee0fe41326f] lib/vsprintf: Remove support for %pF and %pf in favour of %pS and %ps
    > > git bisect good 9af7706492f985867d070861fe39fee0fe41326f
    > > # bad: [83abc5a77f3b028b8c845c39ce4053119e1de35b] lib/vsprintf: OF nodes are first and foremost, struct device_nodes
    > > git bisect bad 83abc5a77f3b028b8c845c39ce4053119e1de35b
    > > # bad: [a92eb7621b9fb2c28a588ce333d226f56fab6a85] lib/vsprintf: Make use of fwnode API to obtain node names and separators
    > > git bisect bad a92eb7621b9fb2c28a588ce333d226f56fab6a85
    > > # good: [1586c5ae2f9310235b5e70abe712c73fc32eb98f] lib/vsprintf: Add a note on re-using %pf or %pF
    > > git bisect good 1586c5ae2f9310235b5e70abe712c73fc32eb98f
    > > # first bad commit: [a92eb7621b9fb2c28a588ce333d226f56fab6a85] lib/vsprintf: Make use of fwnode API to obtain node names and separators
    >
    > --
    > Kind regards,
    >
    > Sakari Ailus

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