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SubjectRe: [PATCH 4/7] hvc_console: Fix race between hvc_close and hvc_remove
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On Fri, 2010-03-19 at 08:18 -0700, Greg Kroah-Hartman wrote:
> From: Amit Shah <amit.shah@redhat.com>
>
> Alan pointed out a race in the code where hvc_remove is invoked. The
> recent virtio_console work is the first user of hvc_remove().

This causes hangs during boot on pseries machines. Haven't had a chance
to track that down yet, but please revert
e74d098c66543d0731de62eb747ccd5b636a6f4c for now.

Cheers,
Ben.


> Alan describes it thus:
>
> The hvc_console assumes that a close and remove call can't occur at the
> same time.
>
> In addition tty_hangup(tty) is problematic as tty_hangup is asynchronous
> itself....
>
> So this can happen
>
> hvc_close hvc_remove
> hung up ? - no
> lock
> tty = hp->tty
> unlock
> lock
> hp->tty = NULL
> unlock
> notify del
> kref_put the hvc struct
> close completes
> tty is destroyed
> tty_hangup dead tty
> tty->ops will be NULL
> NULL->...
>
> This patch adds some tty krefs and also converts to using tty_vhangup().
>
> Reported-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
> Signed-off-by: Amit Shah <amit.shah@redhat.com>
> CC: Alan Cox <alan@lxorguk.ukuu.org.uk>
> CC: linuxppc-dev@ozlabs.org
> CC: Rusty Russell <rusty@rustcorp.com.au>
> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
> ---
> drivers/char/hvc_console.c | 31 +++++++++++++++++++++----------
> 1 files changed, 21 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/char/hvc_console.c b/drivers/char/hvc_console.c
> index 465185f..ba55bba 100644
> --- a/drivers/char/hvc_console.c
> +++ b/drivers/char/hvc_console.c
> @@ -312,6 +312,7 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
> spin_lock_irqsave(&hp->lock, flags);
> /* Check and then increment for fast path open. */
> if (hp->count++ > 0) {
> + tty_kref_get(tty);
> spin_unlock_irqrestore(&hp->lock, flags);
> hvc_kick();
> return 0;
> @@ -319,7 +320,7 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
>
> tty->driver_data = hp;
>
> - hp->tty = tty;
> + hp->tty = tty_kref_get(tty);
>
> spin_unlock_irqrestore(&hp->lock, flags);
>
> @@ -336,6 +337,7 @@ static int hvc_open(struct tty_struct *tty, struct file * filp)
> spin_lock_irqsave(&hp->lock, flags);
> hp->tty = NULL;
> spin_unlock_irqrestore(&hp->lock, flags);
> + tty_kref_put(tty);
> tty->driver_data = NULL;
> kref_put(&hp->kref, destroy_hvc_struct);
> printk(KERN_ERR "hvc_open: request_irq failed with rc %d.\n", rc);
> @@ -363,13 +365,18 @@ static void hvc_close(struct tty_struct *tty, struct file * filp)
> return;
>
> hp = tty->driver_data;
> +
> spin_lock_irqsave(&hp->lock, flags);
> + tty_kref_get(tty);
>
> if (--hp->count == 0) {
> /* We are done with the tty pointer now. */
> hp->tty = NULL;
> spin_unlock_irqrestore(&hp->lock, flags);
>
> + /* Put the ref obtained in hvc_open() */
> + tty_kref_put(tty);
> +
> if (hp->ops->notifier_del)
> hp->ops->notifier_del(hp, hp->data);
>
> @@ -389,6 +396,7 @@ static void hvc_close(struct tty_struct *tty, struct file * filp)
> spin_unlock_irqrestore(&hp->lock, flags);
> }
>
> + tty_kref_put(tty);
> kref_put(&hp->kref, destroy_hvc_struct);
> }
>
> @@ -424,10 +432,11 @@ static void hvc_hangup(struct tty_struct *tty)
> spin_unlock_irqrestore(&hp->lock, flags);
>
> if (hp->ops->notifier_hangup)
> - hp->ops->notifier_hangup(hp, hp->data);
> + hp->ops->notifier_hangup(hp, hp->data);
>
> while(temp_open_count) {
> --temp_open_count;
> + tty_kref_put(tty);
> kref_put(&hp->kref, destroy_hvc_struct);
> }
> }
> @@ -592,7 +601,7 @@ int hvc_poll(struct hvc_struct *hp)
> }
>
> /* No tty attached, just skip */
> - tty = hp->tty;
> + tty = tty_kref_get(hp->tty);
> if (tty == NULL)
> goto bail;
>
> @@ -672,6 +681,8 @@ int hvc_poll(struct hvc_struct *hp)
>
> tty_flip_buffer_push(tty);
> }
> + if (tty)
> + tty_kref_put(tty);
>
> return poll_mask;
> }
> @@ -807,7 +818,7 @@ int hvc_remove(struct hvc_struct *hp)
> struct tty_struct *tty;
>
> spin_lock_irqsave(&hp->lock, flags);
> - tty = hp->tty;
> + tty = tty_kref_get(hp->tty);
>
> if (hp->index < MAX_NR_HVC_CONSOLES)
> vtermnos[hp->index] = -1;
> @@ -819,18 +830,18 @@ int hvc_remove(struct hvc_struct *hp)
> /*
> * We 'put' the instance that was grabbed when the kref instance
> * was initialized using kref_init(). Let the last holder of this
> - * kref cause it to be removed, which will probably be the tty_hangup
> + * kref cause it to be removed, which will probably be the tty_vhangup
> * below.
> */
> kref_put(&hp->kref, destroy_hvc_struct);
>
> /*
> - * This function call will auto chain call hvc_hangup. The tty should
> - * always be valid at this time unless a simultaneous tty close already
> - * cleaned up the hvc_struct.
> + * This function call will auto chain call hvc_hangup.
> */
> - if (tty)
> - tty_hangup(tty);
> + if (tty) {
> + tty_vhangup(tty);
> + tty_kref_put(tty);
> + }
> return 0;
> }
> EXPORT_SYMBOL_GPL(hvc_remove);




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