[PATCH] hysdn: fix to a race condition in put_log_buffer

[Anton Volkov]

The synchronization type that was used earlier to guard the loop that
deletes unused log buffers may have lead to a situation that prevents 
any thread from going through the loop.

The patch deletes previously used synchronization mechanism and moves
the loop under the spin_lock so the similar cases won't be feasible in
the future.

Found by by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Anton Volkov <avol...@ispras.ru>
---
 drivers/isdn/hysdn/hysdn_proclog.c | 27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/drivers/isdn/hysdn/hysdn_proclog.c 
b/drivers/isdn/hysdn/hysdn_proclog.c
index 7b5fd8fb1761..b152c6ca444b 100644
--- a/drivers/isdn/hysdn/hysdn_proclog.c
+++ b/drivers/isdn/hysdn/hysdn_proclog.c
@@ -44,7 +44,6 @@ struct procdata {
        char log_name[15];      /* log filename */
        struct log_data *log_head, *log_tail;   /* head and tail for queue */
        int if_used;            /* open count for interface */
-       int volatile del_lock;  /* lock for delete operations */
        unsigned char logtmp[LOG_MAX_LINELEN];
        wait_queue_head_t rd_queue;
 };
@@ -102,7 +101,6 @@ put_log_buffer(hysdn_card *card, char *cp)
 {
        struct log_data *ib;
        struct procdata *pd = card->proclog;
-       int i;
        unsigned long flags;

        if (!pd)
@@ -126,21 +124,20 @@ put_log_buffer(hysdn_card *card, char *cp)
        else
                pd->log_tail->next = ib;  /* follows existing messages */
        pd->log_tail = ib;   /* new tail */
-       i = pd->del_lock++;  /* get lock state */
-       spin_unlock_irqrestore(&card->hysdn_lock, flags);

        /* delete old entrys */
-       if (!i)
-               while (pd->log_head->next) {
-                       if ((pd->log_head->usage_cnt <= 0) &&
-                           (pd->log_head->next->usage_cnt <= 0)) {
-                               ib = pd->log_head;
-                               pd->log_head = pd->log_head->next;
-                               kfree(ib);
-                       } else
-                               break;
-               }               /* pd->log_head->next */
-       pd->del_lock--;              /* release lock level */
+       while (pd->log_head->next) {
+               if ((pd->log_head->usage_cnt <= 0) &&
+                   (pd->log_head->next->usage_cnt <= 0)) {
+                       ib = pd->log_head;
+                       pd->log_head = pd->log_head->next;
+                       kfree(ib);
+               } else
+                       break;
+       }               /* pd->log_head->next */
+
+       spin_unlock_irqrestore(&card->hysdn_lock, flags);
+
        wake_up_interruptible(&(pd->rd_queue));          /* announce new entry 
*/
 }                              /* put_log_buffer */

--
2.11.0