Re: [PATCH] sg: atomize check and set sdp->exclude in sg_open

[vaughan]

ä 2013å06æ05æ 21:27, JÃrn Engel åé:
> On Wed, 5 June 2013 17:18:33 +0800, vaughan wrote:
> > Check and set sdp->exclude should be atomic when set in sg_open().
>
> The patch is line-wrapped.  More importantly, it doesn't seem to do
It's shorter than the original line, so I just leave it like this...

> what your description indicates it should do.  And lastly, does this
> fix a bug, possibly even one you have a testcase for, or was it found
> by code inspection?
I found it by code inspection. A race condition may happen with the old 
code if two threads are both trying to open the same sg with O_EXCL 
simultaneously. It's possible that they both find fsds list is empty and 
get_exclude(sdp) returns 0, then they both call set_exclude() and break 
out from wait_event_interruptible and resume open. So it's necessary to 
check again with sg_open_exclusive_lock held to ensure only one can set 
sdp->exclude and return >0 to break out from wait_event loop.

> > Signed-off-by: Vaughan Cao <vaughan.cao@xxxxxxxxxx>
> > ---
> >   drivers/scsi/sg.c | 17 ++++++++++++++++-
> >   1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c
> > index 25b5455..0ede08f 100644
> > --- a/drivers/scsi/sg.c
> > +++ b/drivers/scsi/sg.c
> > @@ -245,6 +245,21 @@ static int set_exclude(Sg_device *sdp, char val)
> >       return val;
> >   }
> >
> > +/* Check if we can set exclude and then set, return 1 if success */
> > +static int try_set_exclude(Sg_device *sdp)
> > +{
> > +    unsigned long flags;
> > +
> > +    spin_lock_irqsave(&sg_open_exclusive_lock, flags);
> > +    if (sdp->exclude) {
> > +        spin_unlock_irqrestore(&sg_open_exclusive_lock, flags);
> > +        return 0;
> > +    }
> > +    sdp->exclude = 1;
> > +    spin_unlock_irqrestore(&sg_open_exclusive_lock, flags);
> > +    return 1;
> > +}
> > +
> >   static int sfds_list_empty(Sg_device *sdp)
> >   {
> >       unsigned long flags;
> > @@ -303,7 +318,7 @@ sg_open(struct inode *inode, struct file *filp)
> >               goto error_out;
> >           }
> >           res = wait_event_interruptible(sdp->o_excl_wait,
> > -                       ((!sfds_list_empty(sdp) || get_exclude(sdp))
> > ? 0 : set_exclude(sdp, 1)));
> > +            ((!sfds_list_empty(sdp) || get_exclude(sdp)) ? 0 :
> > try_set_exclude(sdp)));
> >           if (res) {
> >               retval = res;    /* -ERESTARTSYS because signal hit process */
> >               goto error_out;
> > --
> > 1.7.11.7
>
> JÃrn
>
> --
> Fantasy is more important than knowledge. Knowledge is limited,
> while fantasy embraces the whole world.
> -- Albert Einstein
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