Sorry about replying so late.The critical part of open is to add a new sfd to the list and its protected by the
On Mon, 17 June 2013 21:10:53 +0800, vaughan wrote:Rewrite the last patch....
Add a new field 'toopen' in sg_device to count ongoing sg_open's. By checking both 'toopen' and 'exclude' marks when do exclusive open, old race conditions can be avoided.
Replace global sg_open_exclusive_lock with a per device lock - sfd_lock. Since sfds list is now protected by the lock owned by the same sg_device, sg_index_lock becomes a real global lock to only protect sg devices lookup.
Also did some cleanup, such as remove get_exclude() and rename set_exclude() to clear_exclude().
@@ -171,10 +168,10 @@ typedef struct sg_device { /* holds the state of each scsi generic device */^
wait_queue_head_t o_excl_wait; /* queue open() when O_EXCL in use */
int sg_tablesize; /* adapter's max scatter-gather table size */
u32 index; /* device index number */
- /* sfds is protected by sg_index_lock */
+ spinlock_t sfd_lock; /* protect sfds, exclude, toopen */
struct list_head sfds;
+ int toopen; /* number of who are ready to open sg */
I think the 'toopen' is a bad choice. I'm having trouble wrapping my
head around the semantics of this variable, your description feels a
bit handwavy, the main noun is missing in the command above, I think I
found one more overflow bug,...
What you ended up doing is reimplement a rw_semaphone. Why not use
one instead? down_write() for exclusive access, down_read() for
non-exclusive, _trylock variants for nonblocking opens, etc.
Would this work?
JÃrn
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