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    SubjectRe: [PATCH 2/2] writeback: don't decrement wb->refcnt if !wb->bdi
    On Wed, 17 Oct 2018 16:03:11 +0200 Anders Roxell <anders.roxell@linaro.org> wrote:

    > When enabling CONFIG_DEBUG_TEST_DRIVER_REMOVE devtmpfs gets killed
    > because we try to remove a file and decrement the wb reference count
    > before the noop_backing_device_info gets initialized.
    >
    > Since arch_initcall(pl011_init) came before
    > subsys_initcall(default_bdi_init), devtmpfs' handle_remove() crashes
    > because the reference count is a NULL pointer only because bdi->wb
    > hasn't been initialized yet.

    Is this changelog correct? What does drivers/tty/serial/amba-pl011.c
    have to do with page writeback? Confused.

    > [ 0.332075] Serial: AMBA PL011 UART driver
    > [ 0.485276] 9000000.pl011: ttyAMA0 at MMIO 0x9000000 (irq = 39, base_baud = 0) is a PL011 rev1
    > [ 0.502382] console [ttyAMA0] enabled
    > [ 0.515710] Unable to handle kernel paging request at virtual address 0000800074c12000
    > [ 0.516053] Mem abort info:
    > [ 0.516222] ESR = 0x96000004
    > [ 0.516417] Exception class = DABT (current EL), IL = 32 bits
    > [ 0.516641] SET = 0, FnV = 0
    > [ 0.516826] EA = 0, S1PTW = 0
    > [ 0.516984] Data abort info:
    > [ 0.517149] ISV = 0, ISS = 0x00000004
    > [ 0.517339] CM = 0, WnR = 0
    > [ 0.517553] [0000800074c12000] user address but active_mm is swapper
    > [ 0.517928] Internal error: Oops: 96000004 [#1] PREEMPT SMP
    > [ 0.518305] Modules linked in:
    > [ 0.518839] CPU: 0 PID: 13 Comm: kdevtmpfs Not tainted 4.19.0-rc5-next-20180928-00002-g2ba39ab0cd01-dirty #82
    > [ 0.519307] Hardware name: linux,dummy-virt (DT)
    > [ 0.519681] pstate: 80000005 (Nzcv daif -PAN -UAO)
    > [ 0.519959] pc : __destroy_inode+0x94/0x2a8
    > [ 0.520212] lr : __destroy_inode+0x78/0x2a8
    > [ 0.520401] sp : ffff0000098c3b20
    > [ 0.520590] x29: ffff0000098c3b20 x28: 00000000087a3714
    > [ 0.520904] x27: 0000000000002000 x26: 0000000000002000
    > [ 0.521179] x25: ffff000009583000 x24: 0000000000000000
    > [ 0.521467] x23: ffff80007bb52000 x22: ffff80007bbaa7c0
    > [ 0.521737] x21: ffff0000093f9338 x20: 0000000000000000
    > [ 0.522033] x19: ffff80007bbb05d8 x18: 0000000000000400
    > [ 0.522376] x17: 0000000000000000 x16: 0000000000000000
    > [ 0.522727] x15: 0000000000000400 x14: 0000000000000400
    > [ 0.523068] x13: 0000000000000001 x12: 0000000000000001
    > [ 0.523421] x11: 0000000000000000 x10: 0000000000000970
    > [ 0.523749] x9 : ffff0000098c3a60 x8 : ffff80007bbab190
    > [ 0.524017] x7 : ffff80007bbaa880 x6 : 0000000000000c88
    > [ 0.524305] x5 : ffff0000093d96c8 x4 : 61c8864680b583eb
    > [ 0.524567] x3 : ffff0000093d6180 x2 : ffffffffffffffff
    > [ 0.524872] x1 : 0000800074c12000 x0 : 0000800074c12000
    > [ 0.525207] Process kdevtmpfs (pid: 13, stack limit = 0x(____ptrval____))
    > [ 0.525529] Call trace:
    > [ 0.525806] __destroy_inode+0x94/0x2a8
    > [ 0.526108] destroy_inode+0x34/0x88
    > [ 0.526370] evict+0x144/0x1c8
    > [ 0.526636] iput+0x184/0x230
    > [ 0.526871] dentry_unlink_inode+0x118/0x130
    > [ 0.527152] d_delete+0xd8/0xe0
    > [ 0.527420] vfs_unlink+0x240/0x270
    > [ 0.527665] handle_remove+0x1d8/0x330
    > [ 0.527875] devtmpfsd+0x138/0x1c8
    > [ 0.528085] kthread+0x14c/0x158
    > [ 0.528291] ret_from_fork+0x10/0x18
    > [ 0.528720] Code: 92800002 aa1403e0 d538d081 8b010000 (c85f7c04)

    Seems that there is indeed some form of linkage. Can this be spelled
    out more in the changelog please?

    >
    > Rework so that wb_put have an extra check if wb->bdi before decrement
    > wb->refcnt and also add a WARN_ON to get a warning if it happens again
    > in other drivers.
    >
    > Fixes: 52ebea749aae ("writeback: make backing_dev_info host cgroup-specific bdi_writebacks")
    > Cc: Arnd Bergmann <arnd@arndb.de>
    > Co-developed-by: Arnd Bergmann <arnd@arndb.de>
    > Signed-off-by: Anders Roxell <anders.roxell@linaro.org>

    Signed-off-by: Arnd, please.

    > --- a/include/linux/backing-dev-defs.h
    > +++ b/include/linux/backing-dev-defs.h
    > @@ -258,7 +258,7 @@ static inline void wb_get(struct bdi_writeback *wb)
    > */
    > static inline void wb_put(struct bdi_writeback *wb)
    > {
    > - if (wb != &wb->bdi->wb)
    > + if (!WARN_ON(!wb->bdi) && wb != &wb->bdi->wb)
    > percpu_ref_put(&wb->refcnt);
    > }

    The !WARN_ON(!expr) isn't very easy to follow. This:

    {
    if (WARN_ON_ONCE(!wb->bdi)) {
    /*
    * Nice comment explaining how this situation comes about
    */
    return;
    }

    if (wb != &wb->bdi->wb)
    percpu_ref_put(&wb->refcnt);
    }

    is better, no?

    Also, please note the s/WARN_ON/WARN_ON_ONCE/. I don't think we gain
    anything from reporting the same thing many times?

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