On 07/06/2013 01:39 AM, Jörn Engel wrote:
Sorry about replying so late.
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.
The critical part of open is to add a new sfd to the list and its
protected by the
spin_lock(sg_index_lock previously) well. So I added an counter as a
sign rather than
introducing another spinlock or mutex which means I should deal with
potential deadlock.
The code may be simpler with a rwsem implementation as you suggest, I'll
modify it in
this way.
There is no overflow bug, I eliminated it with the following line :)
if (!sdp->exclude && sdp->toopen != INT_MAX) { ...
Do you agree that I use a per device spin_lock 'sfd_lock' to protect
sfds list and leave sg_index_lock
only protect the global sg device lookup? I think it's reasonable for
concurrency.
Thanks,
Vaughan
Would this work?
Jörn
--
I've never met a human being who would want to read 17,000 pages of
documentation, and if there was, I'd kill him to get him out of the
gene pool.
-- Joseph Costello
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