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    Subject[PATCH 3.14 126/158] hvc: ensure hvc_init is only ever called once in hvc_console.c
    Date
    3.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Paul Gortmaker <paul.gortmaker@windriver.com>

    commit f76a1cbed18c86e2d192455f0daebb48458965f3 upstream.

    Commit 3e6c6f630a5282df8f3393a59f10eb9c56536d23 ("Delay creation of
    khcvd thread") moved the call of hvc_init from being a device_initcall
    into hvc_alloc, and used a non-null hvc_driver as indication of whether
    hvc_init had already been called.

    The problem with this is that hvc_driver is only assigned a value
    at the bottom of hvc_init, and so there is a window where multiple
    hvc_alloc calls can be in progress at the same time and hence try
    and call hvc_init multiple times. Previously the use of device_init
    guaranteed that hvc_init was only called once.

    This manifests itself as sporadic instances of two hvc_init calls
    racing each other, and with the loser of the race getting -EBUSY
    from tty_register_driver() and hence that virtual console fails:

    Couldn't register hvc console driver
    virtio-ports vport0p1: error -16 allocating hvc for port

    Here we add an atomic_t to guarantee we'll never run hvc_init twice.

    Cc: Rusty Russell <rusty@rustcorp.com.au>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Fixes: 3e6c6f630a52 ("Delay creation of khcvd thread")
    Reported-by: Jim Somerville <Jim.Somerville@windriver.com>
    Tested-by: Jim Somerville <Jim.Somerville@windriver.com>
    Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/tty/hvc/hvc_console.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    --- a/drivers/tty/hvc/hvc_console.c
    +++ b/drivers/tty/hvc/hvc_console.c
    @@ -31,6 +31,7 @@
    #include <linux/list.h>
    #include <linux/module.h>
    #include <linux/major.h>
    +#include <linux/atomic.h>
    #include <linux/sysrq.h>
    #include <linux/tty.h>
    #include <linux/tty_flip.h>
    @@ -70,6 +71,9 @@ static struct task_struct *hvc_task;
    /* Picks up late kicks after list walk but before schedule() */
    static int hvc_kicked;

    +/* hvc_init is triggered from hvc_alloc, i.e. only when actually used */
    +static atomic_t hvc_needs_init __read_mostly = ATOMIC_INIT(-1);
    +
    static int hvc_init(void);

    #ifdef CONFIG_MAGIC_SYSRQ
    @@ -851,7 +855,7 @@ struct hvc_struct *hvc_alloc(uint32_t vt
    int i;

    /* We wait until a driver actually comes along */
    - if (!hvc_driver) {
    + if (atomic_inc_not_zero(&hvc_needs_init)) {
    int err = hvc_init();
    if (err)
    return ERR_PTR(err);



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