net: core: device_rename: Use rwsem instead of a seqcount

Sequence counters write paths are critical sections that must never be
preempted, and blocking, even for CONFIG_PREEMPTION=n, is not allowed.

Commit 5dbe7c178d3f ("net: fix kernel deadlock with interface rename and
netdev name retrieval.") handled a deadlock, observed with
CONFIG_PREEMPTION=n, where the devnet_rename seqcount read side was
infinitely spinning: it got scheduled after the seqcount write side
blocked inside its own critical section.

To fix that deadlock, among other issues, the commit added a
cond_resched() inside the read side section. While this will get the
non-preemptible kernel eventually unstuck, the seqcount reader is fully
exhausting its slice just spinning -- until TIF_NEED_RESCHED is set.

The fix is also still broken: if the seqcount reader belongs to a
real-time scheduling policy, it can spin forever and the kernel will
livelock.

Disabling preemption over the seqcount write side critical section will
not work: inside it are a number of GFP_KERNEL allocations and mutex
locking through the drivers/base/ :: device_rename() call chain.

>From all the above, replace the seqcount with a rwsem.

Fixes: 5dbe7c178d3f (net: fix kernel deadlock with interface rename and netdev name retrieval.)
Fixes: 30e6c9fa93cf (net: devnet_rename_seq should be a seqcount)
Fixes: c91f6df2db49 (sockopt: Change getsockopt() of SO_BINDTODEVICE to return an interface name)
Cc: <stable@vger.kernel.org>
Reported-by: kbuild test robot <lkp@intel.com> [ v1 missing up_read() on error exit ]
Reported-by: Dan Carpenter <dan.carpenter@oracle.com> [ v1 missing up_read() on error exit ]
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/dev.c b/net/core/dev.c
index 10684833f86411b1f275742364bf9fdfbf14f96f..061496a1f640fe7d0d97373cf0a11b352c3bebb9 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -79,6 +79,7 @@
 #include <linux/sched.h>
 #include <linux/sched/mm.h>
 #include <linux/mutex.h>
+#include <linux/rwsem.h>
 #include <linux/string.h>
 #include <linux/mm.h>
 #include <linux/socket.h>
@@ -194,7 +195,7 @@ static DEFINE_SPINLOCK(napi_hash_lock);
 static unsigned int napi_gen_id = NR_CPUS;
 static DEFINE_READ_MOSTLY_HASHTABLE(napi_hash, 8);
 
-static seqcount_t devnet_rename_seq;
+static DECLARE_RWSEM(devnet_rename_sem);
 
 static inline void dev_base_seq_inc(struct net *net)
 {
@@ -998,33 +999,28 @@ EXPORT_SYMBOL(dev_get_by_napi_id);
  *     @net: network namespace
  *     @name: a pointer to the buffer where the name will be stored.
  *     @ifindex: the ifindex of the interface to get the name from.
- *
- *     The use of raw_seqcount_begin() and cond_resched() before
- *     retrying is required as we want to give the writers a chance
- *     to complete when CONFIG_PREEMPTION is not set.
  */
 int netdev_get_name(struct net *net, char *name, int ifindex)
 {
        struct net_device *dev;
-       unsigned int seq;
+       int ret;
 
-retry:
-       seq = raw_seqcount_begin(&devnet_rename_seq);
+       down_read(&devnet_rename_sem);
        rcu_read_lock();
+
        dev = dev_get_by_index_rcu(net, ifindex);
        if (!dev) {
-               rcu_read_unlock();
-               return -ENODEV;
+               ret = -ENODEV;
+               goto out;
        }
 
        strcpy(name, dev->name);
-       rcu_read_unlock();
-       if (read_seqcount_retry(&devnet_rename_seq, seq)) {
-               cond_resched();
-               goto retry;
-       }
 
-       return 0;
+       ret = 0;
+out:
+       rcu_read_unlock();
+       up_read(&devnet_rename_sem);
+       return ret;
 }
 
 /**
@@ -1296,10 +1292,10 @@ int dev_change_name(struct net_device *dev, const char *newname)
            likely(!(dev->priv_flags & IFF_LIVE_RENAME_OK)))
                return -EBUSY;
 
-       write_seqcount_begin(&devnet_rename_seq);
+       down_write(&devnet_rename_sem);
 
        if (strncmp(newname, dev->name, IFNAMSIZ) == 0) {
-               write_seqcount_end(&devnet_rename_seq);
+               up_write(&devnet_rename_sem);
                return 0;
        }
 
@@ -1307,7 +1303,7 @@ int dev_change_name(struct net_device *dev, const char *newname)
 
        err = dev_get_valid_name(net, dev, newname);
        if (err < 0) {
-               write_seqcount_end(&devnet_rename_seq);
+               up_write(&devnet_rename_sem);
                return err;
        }
 
@@ -1322,11 +1318,11 @@ rollback:
        if (ret) {
                memcpy(dev->name, oldname, IFNAMSIZ);
                dev->name_assign_type = old_assign_type;
-               write_seqcount_end(&devnet_rename_seq);
+               up_write(&devnet_rename_sem);
                return ret;
        }
 
-       write_seqcount_end(&devnet_rename_seq);
+       up_write(&devnet_rename_sem);
 
        netdev_adjacent_rename_links(dev, oldname);
 
@@ -1347,7 +1343,7 @@ rollback:
                /* err >= 0 after dev_alloc_name() or stores the first errno */
                if (err >= 0) {
                        err = ret;
-                       write_seqcount_begin(&devnet_rename_seq);
+                       down_write(&devnet_rename_sem);
                        memcpy(dev->name, oldname, IFNAMSIZ);
                        memcpy(oldname, newname, IFNAMSIZ);
                        dev->name_assign_type = old_assign_type;