phy: improve safety of fixed-phy MII register reading

There is no prevention of a concurrent call to both fixed_mdio_read()
and fixed_phy_update_state(), which can result in the state being
modified while it's being inspected.  Fix this by using a seqcount
to detect modifications, and memcpy()ing the state.

We remain slightly naughty here, calling link_update() and updating
the link status within the read-side loop - which would need rework
of the design to change.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/phy/fixed_phy.c b/drivers/net/phy/fixed_phy.c
index 0dfed86..b376ada 100644 (file)
--- a/drivers/net/phy/fixed_phy.c
+++ b/drivers/net/phy/fixed_phy.c
@@ -23,6 +23,7 @@
 #include <linux/slab.h>
 #include <linux/of.h>
 #include <linux/gpio.h>
+#include <linux/seqlock.h>
 
 #include "swphy.h"
 
@@ -34,6 +35,7 @@ struct fixed_mdio_bus {
 struct fixed_phy {
        int addr;
        struct phy_device *phydev;
+       seqcount_t seqcount;
        struct fixed_phy_status status;
        int (*link_update)(struct net_device *, struct fixed_phy_status *);
        struct list_head node;
@@ -58,13 +60,21 @@ static int fixed_mdio_read(struct mii_bus *bus, int phy_addr, int reg_num)
 
        list_for_each_entry(fp, &fmb->phys, node) {
                if (fp->addr == phy_addr) {
-                       /* Issue callback if user registered it. */
-                       if (fp->link_update) {
-                               fp->link_update(fp->phydev->attached_dev,
-                                               &fp->status);
-                               fixed_phy_update(fp);
-                       }
-                       return swphy_read_reg(reg_num, &fp->status);
+                       struct fixed_phy_status state;
+                       int s;
+
+                       do {
+                               s = read_seqcount_begin(&fp->seqcount);
+                               /* Issue callback if user registered it. */
+                               if (fp->link_update) {
+                                       fp->link_update(fp->phydev->attached_dev,
+                                                       &fp->status);
+                                       fixed_phy_update(fp);
+                               }
+                               state = fp->status;
+                       } while (read_seqcount_retry(&fp->seqcount, s));
+
+                       return swphy_read_reg(reg_num, &state);
                }
        }
 
@@ -116,6 +126,7 @@ int fixed_phy_update_state(struct phy_device *phydev,
 
        list_for_each_entry(fp, &fmb->phys, node) {
                if (fp->addr == phydev->mdio.addr) {
+                       write_seqcount_begin(&fp->seqcount);
 #define _UPD(x) if (changed->x) \
        fp->status.x = status->x
                        _UPD(link);
@@ -125,6 +136,7 @@ int fixed_phy_update_state(struct phy_device *phydev,
                        _UPD(asym_pause);
 #undef _UPD
                        fixed_phy_update(fp);
+                       write_seqcount_end(&fp->seqcount);
                        return 0;
                }
        }
@@ -149,6 +161,8 @@ int fixed_phy_add(unsigned int irq, int phy_addr,
        if (!fp)
                return -ENOMEM;
 
+       seqcount_init(&fp->seqcount);
+
        if (irq != PHY_POLL)
                fmb->mii_bus->irq[phy_addr] = irq;