LKML Archive on lore.kernel.org
 help / color / Atom feed
From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	"Sebastian A. Siewior" <bigeasy@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	Stephen Hemminger <stephen@networkplumber.org>,
	netdev@vger.kernel.org
Subject: [PATCH v2 1/6] net: core: device_rename: Use rwsem instead of a seqcount
Date: Wed,  3 Jun 2020 16:49:44 +0200
Message-ID: <20200603144949.1122421-2-a.darwish@linutronix.de> (raw)
In-Reply-To: <20200603144949.1122421-1-a.darwish@linutronix.de>

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


  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-03 14:49 [PATCH v2 0/6] seqlock: seqcount_t call sites bugfixes Ahmed S. Darwish
2020-06-03 14:49 ` Ahmed S. Darwish [this message]
2020-06-03 14:49 ` [PATCH v2 2/6] net: phy: fixed_phy: Remove unused seqcount Ahmed S. Darwish
2020-06-03 16:19   ` Andrew Lunn
2020-06-03 14:49 ` [PATCH v2 3/6] u64_stats: Document writer non-preemptibility requirement Ahmed S. Darwish
2020-06-03 14:49 ` [PATCH v2 4/6] net: mdiobus: Disable preemption upon u64_stats update Ahmed S. Darwish
2020-06-03 14:49 ` [PATCH v2 5/6] block: nr_sects_write(): Disable preemption on seqcount write Ahmed S. Darwish
2020-06-05  3:22   ` Jens Axboe
2020-06-03 14:49 ` [PATCH v2 6/6] dma-buf: Remove custom seqcount lockdep class key Ahmed S. Darwish
2020-06-04  8:49   ` Daniel Vetter
2020-06-04  7:28 ` [PATCH v2 0/6] seqlock: seqcount_t call sites bugfixes Daniel Vetter
2020-06-04  8:41   ` Ahmed S. Darwish
2020-06-04 22:50 ` David Miller

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200603144949.1122421-2-a.darwish@linutronix.de \
    --to=a.darwish@linutronix.de \
    --cc=bigeasy@linutronix.de \
    --cc=davem@davemloft.net \
    --cc=edumazet@google.com \
    --cc=kuba@kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=netdev@vger.kernel.org \
    --cc=paulmck@kernel.org \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=stephen@networkplumber.org \
    --cc=tglx@linutronix.de \
    --cc=will@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lkml.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lkml.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lkml.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lkml.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lkml.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lkml.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lkml.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lkml.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lkml.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lkml.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lkml.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git