[Bug 35935] since 2.6.38.1, screen become garbled after some times (RV770)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=35935

--- Comment #5 from Mjules <mjulien.m at gmail.com> 2011-04-26 14:20:30 PDT ---
The result of the bisection is :
ba3ffdc68b4c80fe4cc42bdae6040eca1067ebb2 is the first bad commit
commit ba3ffdc68b4c80fe4cc42bdae6040eca1067ebb2
Author: Stanislav Kinsbursky <skinsbursky at parallels.com>
Date:   Thu Mar 17 18:54:23 2011 +0300

    RPC: killing RPC tasks races fixed

    commit 8e26de238fd794c8ea56a5c98bf67c40cfeb051d upstream.

    RPC task RPC_TASK_QUEUED bit is set must be checked before trying to wake
up
    task rpc_killall_tasks() because task->tk_waitqueue can not be set (equal
to
    NULL).
    Also, as Trond Myklebust mentioned, such approach (instead of checking
    tk_waitqueue to NULL) allows us to "optimise away the call to
    rpc_wake_up_queued_task() altogether for those
    tasks that aren't queued".
    Here is an example of dereferencing of tk_waitqueue equal to NULL:

    CPU 0                   CPU 1                CPU 2
    --------------------    ---------------------    --------------------------
    nfs4_run_open_task
    rpc_run_task
    rpc_execute
    rpc_set_active
    rpc_make_runnable
    (waiting)
                rpc_async_schedule
                nfs4_open_prepare
                nfs_wait_on_sequence
                            nfs_umount_begin
                            rpc_killall_tasks
                            rpc_wake_up_task
                            rpc_wake_up_queued_task
                            spin_lock(tk_waitqueue == NULL)
                            BUG()
                rpc_sleep_on
                spin_lock(&q->lock)
                __rpc_sleep_on
                task->tk_waitqueue = q

    Signed-off-by: Stanislav Kinsbursky <skinsbursky at openvz.org>
    Signed-off-by: Trond Myklebust <Trond.Myklebust at netapp.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh at suse.de>

:040000 040000 91b88f0611c9ff1cf2691d9d65ec13c652a554ac
e23b6d459a8bef93de6ea4821754e2f51e3ad32f M    net



which seems completely bogus to me (I don't use nfs).

BTW, the bug is still present with 2.6.38.3 and seems more frequent with it. 

Another detail is it occurs only one time during a session.

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