[PATCH v2 1/6] net: core: device_rename: Use rwsem instead of a seqcount

[PATCH v2 1/6] net: core: device_rename: Use rwsem instead of a seqcount

[Ahmed S. Darwish]

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>
---
 net/core/dev.c | 40 ++++++++++++++++++----------------------
 1 file changed, 18 insertions(+), 22 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 2d8aceee4284..93a279ab4e97 100644
--- 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)
 {
@@ -930,33 +931,28 @@ 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;
 }
 
 /**
@@ -1228,10 +1224,10 @@ 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;
 	}
 
@@ -1239,7 +1235,7 @@ 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;
 	}
 
@@ -1254,11 +1250,11 @@ int dev_change_name(struct net_device *dev, const char *newname)
 	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);
 
@@ -1279,7 +1275,7 @@ int dev_change_name(struct net_device *dev, const char *newname)
 		/* 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;
-- 
2.20.1

[PATCH v2 2/6] net: phy: fixed_phy: Remove unused seqcount

[Ahmed S. Darwish]

Commit bf7afb29d545 ("phy: improve safety of fixed-phy MII register
reading") protected the fixed PHY status with a sequence counter.

Two years later, commit d2b977939b18 ("net: phy: fixed-phy: remove
fixed_phy_update_state()") removed the sequence counter's write side
critical section -- neutralizing its read side retry loop.

Remove the unused seqcount.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 drivers/net/phy/fixed_phy.c | 26 ++++++++++----------------
 1 file changed, 10 insertions(+), 16 deletions(-)

diff --git a/drivers/net/phy/fixed_phy.c b/drivers/net/phy/fixed_phy.c
index 4a3d34f40cb9..c4641b1704d6 100644
--- a/drivers/net/phy/fixed_phy.c
+++ b/drivers/net/phy/fixed_phy.c
@@ -19,7 +19,6 @@
 #include <linux/slab.h>
 #include <linux/of.h>
 #include <linux/gpio/consumer.h>
-#include <linux/seqlock.h>
 #include <linux/idr.h>
 #include <linux/netdevice.h>
 #include <linux/linkmode.h>
@@ -34,7 +33,6 @@ struct fixed_mdio_bus {
 struct fixed_phy {
 	int addr;
 	struct phy_device *phydev;
-	seqcount_t seqcount;
 	struct fixed_phy_status status;
 	bool no_carrier;
 	int (*link_update)(struct net_device *, struct fixed_phy_status *);
@@ -80,19 +78,17 @@ 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) {
 			struct fixed_phy_status state;
-			int s;
 
-			do {
-				s = read_seqcount_begin(&fp->seqcount);
-				fp->status.link = !fp->no_carrier;
-				/* Issue callback if user registered it. */
-				if (fp->link_update)
-					fp->link_update(fp->phydev->attached_dev,
-							&fp->status);
-				/* Check the GPIO for change in status */
-				fixed_phy_update(fp);
-				state = fp->status;
-			} while (read_seqcount_retry(&fp->seqcount, s));
+			fp->status.link = !fp->no_carrier;
+
+			/* Issue callback if user registered it. */
+			if (fp->link_update)
+				fp->link_update(fp->phydev->attached_dev,
+						&fp->status);
+
+			/* Check the GPIO for change in status */
+			fixed_phy_update(fp);
+			state = fp->status;
 
 			return swphy_read_reg(reg_num, &state);
 		}
@@ -150,8 +146,6 @@ static int fixed_phy_add_gpiod(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;
 
-- 
2.20.1

Re: [PATCH v2 2/6] net: phy: fixed_phy: Remove unused seqcount

[Andrew Lunn]

On Wed, Jun 03, 2020 at 04:49:45PM +0200, Ahmed S. Darwish wrote:
> Commit bf7afb29d545 ("phy: improve safety of fixed-phy MII register
> reading") protected the fixed PHY status with a sequence counter.
> 
> Two years later, commit d2b977939b18 ("net: phy: fixed-phy: remove
> fixed_phy_update_state()") removed the sequence counter's write side
> critical section -- neutralizing its read side retry loop.
> 
> Remove the unused seqcount.
> 
> Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew

[PATCH v2 3/6] u64_stats: Document writer non-preemptibility requirement

[Ahmed S. Darwish]

The u64_stats mechanism uses sequence counters to protect against 64-bit
values tearing on 32-bit architectures. Updating such statistics is a
sequence counter write side critical section.

Preemption must be disabled before entering this seqcount write critical
section.  Failing to do so, the seqcount read side can preempt the write
side section and spin for the entire scheduler tick.  If that reader
belongs to a real-time scheduling class, it can spin forever and the
kernel will livelock.

Document this statistics update side non-preemptibility requirement.

Reword the introductory paragraph to highlight u64_stats raison d'être:
64-bit values tearing protection on 32-bit architectures. Divide
documentation on a basis of internal design vs. usage constraints.

Reword the u64_stats header file top comment to always mention "Reader"
or "Writer" at the start of each bullet point, making it easier to
follow which side each point is actually for.

Clarify the statement "whole thing is a NOOP on 64bit arches or UP
kernels".  For 32-bit UP kernels, preemption is always disabled for the
statistics read side section.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/u64_stats_sync.h | 43 ++++++++++++++++++----------------
 1 file changed, 23 insertions(+), 20 deletions(-)

diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
index 9de5c10293f5..c6abb79501b3 100644
--- a/include/linux/u64_stats_sync.h
+++ b/include/linux/u64_stats_sync.h
@@ -3,33 +3,36 @@
 #define _LINUX_U64_STATS_SYNC_H
 
 /*
- * To properly implement 64bits network statistics on 32bit and 64bit hosts,
- * we provide a synchronization point, that is a noop on 64bit or UP kernels.
+ * Protect against 64-bit values tearing on 32-bit architectures. This is
+ * typically used for statistics read/update in different subsystems.
  *
  * Key points :
- * 1) Use a seqcount on SMP 32bits, with low overhead.
- * 2) Whole thing is a noop on 64bit arches or UP kernels.
- * 3) Write side must ensure mutual exclusion or one seqcount update could
+ *
+ * -  Use a seqcount on 32-bit SMP, only disable preemption for 32-bit UP.
+ * -  The whole thing is a no-op on 64-bit architectures.
+ *
+ * Usage constraints:
+ *
+ * 1) Write side must ensure mutual exclusion, or one seqcount update could
  *    be lost, thus blocking readers forever.
- *    If this synchronization point is not a mutex, but a spinlock or
- *    spinlock_bh() or disable_bh() :
- * 3.1) Write side should not sleep.
- * 3.2) Write side should not allow preemption.
- * 3.3) If applicable, interrupts should be disabled.
+ *
+ * 2) Write side must disable preemption, or a seqcount reader can preempt the
+ *    writer and also spin forever.
+ *
+ * 3) Write side must use the _irqsave() variant if other writers, or a reader,
+ *    can be invoked from an IRQ context.
  *
  * 4) If reader fetches several counters, there is no guarantee the whole values
- *    are consistent (remember point 1) : this is a noop on 64bit arches anyway)
+ *    are consistent w.r.t. each other (remember point #2: seqcounts are not
+ *    used for 64bit architectures).
  *
- * 5) readers are allowed to sleep or be preempted/interrupted : They perform
- *    pure reads. But if they have to fetch many values, it's better to not allow
- *    preemptions/interruptions to avoid many retries.
+ * 5) Readers are allowed to sleep or be preempted/interrupted: they perform
+ *    pure reads.
  *
- * 6) If counter might be written by an interrupt, readers should block interrupts.
- *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could
- *     read partial values)
- *
- * 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
- *    u64_stats_fetch_retry_irq() helpers
+ * 6) Readers must use both u64_stats_fetch_{begin,retry}_irq() if the stats
+ *    might be updated from a hardirq or softirq context (remember point #1:
+ *    seqcounts are not used for UP kernels). 32-bit UP stat readers could read
+ *    corrupted 64-bit values otherwise.
  *
  * Usage :
  *
-- 
2.20.1

[PATCH v2 4/6] net: mdiobus: Disable preemption upon u64_stats update

[Ahmed S. Darwish]

The u64_stats mechanism uses sequence counters to protect against 64-bit
values tearing on 32-bit architectures. Updating u64_stats is thus a
sequence counter write side critical section where preemption must be
disabled.

For mdiobus_stats_acct(), disable preemption upon the u64_stats update.
It is called from process context through mdiobus_read() and
mdiobus_write().

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 drivers/net/phy/mdio_bus.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/phy/mdio_bus.c b/drivers/net/phy/mdio_bus.c
index 7a4eb3f2cb74..a1a4dee2a033 100644
--- a/drivers/net/phy/mdio_bus.c
+++ b/drivers/net/phy/mdio_bus.c
@@ -757,6 +757,7 @@ EXPORT_SYMBOL(mdiobus_scan);
 
 static void mdiobus_stats_acct(struct mdio_bus_stats *stats, bool op, int ret)
 {
+	preempt_disable();
 	u64_stats_update_begin(&stats->syncp);
 
 	u64_stats_inc(&stats->transfers);
@@ -771,6 +772,7 @@ static void mdiobus_stats_acct(struct mdio_bus_stats *stats, bool op, int ret)
 		u64_stats_inc(&stats->writes);
 out:
 	u64_stats_update_end(&stats->syncp);
+	preempt_enable();
 }
 
 /**
-- 
2.20.1