From: Prasanna S. Panchamukhi <prasannax.s.panchamukhi at intel.com>
This patch fixes an infinite loop caused by i2400m_tx_fifo_push() due
to a corner case where there is no tail space in the TX FIFO.
Please refer the documentation in the code for details.
Signed-off-by: Prasanna S. Panchamukhi <prasannax.s.panchamukhi at intel.com>
---
drivers/net/wimax/i2400m/tx.c | 65 +++++++++++++++++++++++++++++++++++++---
1 files changed, 60 insertions(+), 5 deletions(-)
diff --git a/drivers/net/wimax/i2400m/tx.c b/drivers/net/wimax/i2400m/tx.c
index 101550a..609f1ca 100644
--- a/drivers/net/wimax/i2400m/tx.c
+++ b/drivers/net/wimax/i2400m/tx.c
@@ -341,6 +341,14 @@ size_t __i2400m_tx_tail_room(struct i2400m *i2400m)
* @padding: ensure that there is at least this many bytes of free
* contiguous space in the fifo. This is needed because later on
* we might need to add padding.
+ * @try_head: specify either to allocate head room or tail room space
+ * in the TX FIFO. This boolean is required to avoids a system hang
+ * due to an infinite loop caused by i2400m_tx_fifo_push().
+ * The caller must always try to allocate tail room space first by
+ * calling this routine with try_head = 0. In case if there
+ * is not enough tail room space but there is enough head room space,
+ * (i2400m_tx_fifo_push() returns TAIL_FULL) try to allocate head
+ * room space, by calling this routine again with try_head = 1.
*
* Returns:
*
@@ -372,6 +380,48 @@ size_t __i2400m_tx_tail_room(struct i2400m *i2400m)
* fail and return TAIL_FULL and let the caller figure out if we wants to
* skip the tail room and try to allocate from the head.
*
+ * There is a corner case, wherein i2400m_tx_new() can get into
+ * an infinite loop calling i2400m_tx_fifo_push().
+ * In certain situations, tx_in would have reached on the top of TX FIFO
+ * and i2400m_tx_tail_room() returns 0, as described below:
+ *
+ * N ___________ tail room is zero
+ * |<- IN ->|
+ * | |
+ * | |
+ * | |
+ * | data |
+ * |<- OUT ->|
+ * | |
+ * | |
+ * | head room |
+ * 0 -----------
+ * During such a time, where tail room is zero in the TX FIFO and if there
+ * is a request to add a payload to TX FIFO, which calls:
+ * i2400m_tx()
+ * ->calls i2400m_tx_close()
+ * ->calls i2400m_tx_skip_tail()
+ * goto try_new;
+ * ->calls i2400m_tx_new()
+ * |----> [try_head:]
+ * infinite loop | ->calls i2400m_tx_fifo_push()
+ * | if (tail_room < needed)
+ * | if (head_room => needed)
+ * | return TAIL_FULL;
+ * |<---- goto try_head;
+ *
+ * i2400m_tx() calls i2400m_tx_close() to close the message, since there
+ * is no tail room to accommodate the payload and calls
+ * i2400m_tx_skip_tail() to skip the tail space. Now i2400m_tx() calls
+ * i2400m_tx_new() to allocate space for new message header calling
+ * i2400m_tx_fifo_push() that returns TAIL_FULL, since there is no tail space
+ * to accommodate the message header, but there is enough head space.
+ * The i2400m_tx_new() keeps re-retrying by calling i2400m_tx_fifo_push()
+ * ending up in a loop causing system freeze.
+ *
+ * This corner case is avoided by using a try_head boolean,
+ * as an argument to i2400m_tx_fifo_push().
+ *
* Note:
*
* Assumes i2400m->tx_lock is taken, and we use that as a barrier
@@ -380,7 +430,8 @@ size_t __i2400m_tx_tail_room(struct i2400m *i2400m)
* pop data off the queue
*/
static
-void *i2400m_tx_fifo_push(struct i2400m *i2400m, size_t size, size_t padding)
+void *i2400m_tx_fifo_push(struct i2400m *i2400m, size_t size,
+ size_t padding, bool try_head)
{
struct device *dev = i2400m_dev(i2400m);
size_t room, tail_room, needed_size;
@@ -395,7 +446,7 @@ void *i2400m_tx_fifo_push(struct i2400m *i2400m, size_t size, size_t padding)
}
/* Is there space at the tail? */
tail_room = __i2400m_tx_tail_room(i2400m);
- if (tail_room < needed_size) {
+ if (!try_head && tail_room < needed_size) {
/*
* If the tail room space is not enough to push the message
* in the TX FIFO, then there are two possibilities:
@@ -510,14 +561,16 @@ void i2400m_tx_new(struct i2400m *i2400m)
{
struct device *dev = i2400m_dev(i2400m);
struct i2400m_msg_hdr *tx_msg;
+ bool try_head = 0;
BUG_ON(i2400m->tx_msg != NULL);
try_head:
- tx_msg = i2400m_tx_fifo_push(i2400m, I2400M_TX_PLD_SIZE, 0);
+ tx_msg = i2400m_tx_fifo_push(i2400m, I2400M_TX_PLD_SIZE, 0, try_head);
if (tx_msg == NULL)
goto out;
else if (tx_msg == TAIL_FULL) {
i2400m_tx_skip_tail(i2400m);
d_printf(2, dev, "new TX message: tail full, trying head\n");
+ try_head = 1;
goto try_head;
}
memset(tx_msg, 0, I2400M_TX_PLD_SIZE);
@@ -591,7 +644,7 @@ void i2400m_tx_close(struct i2400m *i2400m)
aligned_size = ALIGN(tx_msg_moved->size, i2400m->bus_tx_block_size);
padding = aligned_size - tx_msg_moved->size;
if (padding > 0) {
- pad_buf = i2400m_tx_fifo_push(i2400m, padding, 0);
+ pad_buf = i2400m_tx_fifo_push(i2400m, padding, 0, 0);
if (unlikely(WARN_ON(pad_buf == NULL
|| pad_buf == TAIL_FULL))) {
/* This should not happen -- append should verify
@@ -657,6 +710,7 @@ int i2400m_tx(struct i2400m *i2400m, const void *buf, size_t buf_len,
unsigned long flags;
size_t padded_len;
void *ptr;
+ bool try_head = 0;
unsigned is_singleton = pl_type == I2400M_PT_RESET_WARM
|| pl_type == I2400M_PT_RESET_COLD;
@@ -702,11 +756,12 @@ try_new:
/* So we have a current message header; now append space for
* the message -- if there is not enough, try the head */
ptr = i2400m_tx_fifo_push(i2400m, padded_len,
- i2400m->bus_tx_block_size);
+ i2400m->bus_tx_block_size, try_head);
if (ptr == TAIL_FULL) { /* Tail is full, try head */
d_printf(2, dev, "pl append: tail full\n");
i2400m_tx_close(i2400m);
i2400m_tx_skip_tail(i2400m);
+ try_head = 1;
goto try_new;
} else if (ptr == NULL) { /* All full */
result = -ENOSPC;
--
1.6.6.1
