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SubjectRe: [PATCH] sg: atomize check and set sdp->exclude in sg_open
于 2013年06月05日 21:27, Jörn Engel 写道:
> 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@oracle.com>
>> ---
>> 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
>>
>
> Jörn
>
> --
> Fantasy is more important than knowledge. Knowledge is limited,
> while fantasy embraces the whole world.
> -- Albert Einstein
>
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