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

From: vaughan
Date: Wed Jun 05 2013 - 12:14:28 EST

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 :
if (res) {
retval = res; /* -ERESTARTSYS because signal hit process */
goto error_out;


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while fantasy embraces the whole world.
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