USB-related problem

[Adrian Chadd]

hiya,

any updates? I caan't trigger this one myself :(


-a


On 12 November 2015 at 14:28, Eugene Grosbein <eugen at grosbein.net> wrote:
> On 11.11.2015 01:57, Adrian Chadd wrote:
>> + benno, jeff, kib
>>
>> cool, so now we know where syncer is hanging. nice work!
>>
>> That syncer loop that calls it waiting for the list to be empty is
>> also a bit suss; it looks like it could also get stuck in a loop and
>> never yield.
>
> I've used new state variables spread all over the kernel
> (plus KTR facility) to track this down and came to a result I cannot explain.
>
> First, here is call sequence leading to kernel hang, always the same:
>
> sys/kern/vfs_subr.c, sync_vnode() -> VOP_FSYNC(vp, MNT_LAZY, td) -> VOP_FSYNC_APV()
> in generated vnode_if.c in the kernel compile directory.
>
> Here we have KTR_START3(KTR_VOP, ...) that is latest record that DDB shows for
> "show ktr /v". There should be KTR_STOP3(KTR_VOP, "VOP", ...) recorded after, but it is not.
> In fact, execution proceeds to vop_fsync(a) and never returns, precisely:
>
> vop_fsync(a) -> sys/fs/devfs/devfs_vnops.c, devfs_fsync() ->
> "return (vop_stdfsync(ap))" - never returns.
>
> It reaches sys/kern/vfs_default.c, vop_stdfsync() and hangs exactly at this KASSERT:
> https://svnweb.freebsd.org/base/head/sys/kern/vfs_default.c?annotate=288451#l683
>
> I've defined another global variable "volatile unsigned vopsfsyncstate",
> set it to 15 just before this KASSERT and to 126 just after the KASSERT
> and it always equals to 15 when kernel hangs. I tested kernel compiled
> with and without WITNESS (and WITHNESS_SKIPSPIN), no changes.
>
> I do not understand how and why does it hang here but that's a fact.
> Here I'm stuck.
>
> Eugene Grosbein
>