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Date:   Thu, 05 Apr 2018 14:58:06 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     jlayton@...nel.org, bfields@...ldses.org, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        boqun.feng@...il.com, longman@...hat.com, peterz@...radead.org,
        mingo@...hat.com, ktkhai@...tuozzo.com
Subject: [PATCH] fasync: Fix deadlock between task-context and
 interrupt-context kill_fasync()

I observed the following deadlock between them:

[task 1]                          [task 2]                         [task 3]
kill_fasync()                     mm_update_next_owner()           copy_process()
 spin_lock_irqsave(&fa->fa_lock)   read_lock(&tasklist_lock)        write_lock_irq(&tasklist_lock)
  send_sigio()                    <IRQ>                             ...
   read_lock(&fown->lock)         kill_fasync()                     ...
    read_lock(&tasklist_lock)      spin_lock_irqsave(&fa->fa_lock)  ...

Task 1 can't acquire read locked tasklist_lock, since there is
already task 3 expressed its wish to take the lock exclusive.
Task 2 holds the read locked lock, but it can't take the spin lock.

Also, there is possible another deadlock (which I haven't observed):

[task 1]                            [task 2]
f_getown()                          kill_fasync()
 read_lock(&f_own->lock)             spin_lock_irqsave(&fa->fa_lock,)
 <IRQ>                               send_sigio()                     write_lock_irq(&f_own->lock)
  kill_fasync()                       read_lock(&fown->lock)
   spin_lock_irqsave(&fa->fa_lock,)

Actually, we do not need exclusive fa->fa_lock in kill_fasync_rcu(),
as it guarantees fa->fa_file->f_owner integrity only. It may seem,
that it used to give a task a small possibility to receive two sequential
signals, if there are two parallel kill_fasync() callers, and task
handles the first signal fastly, but the behaviour won't become
different, since there is exclusive sighand lock in do_send_sig_info().

The patch converts fa_lock into rwlock_t, and this fixes two above
deadlocks, as rwlock is allowed to be taken from interrupt handler
by qrwlock design.

Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>

I used the following program for testing:

#include <unistd.h>
#include <stdlib.h>
#include <signal.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>

#ifndef F_SETSIG
#define F_SETSIG 10
#endif

void handler(int sig)
{
}

main()
{
	unsigned int flags;
	int fd;

	system("echo 8 > /proc/sys/kernel/random/read_wakeup_threshold");
	system("while :; do ls -R / > /dev/random 2>&1 ; echo 3 > /proc/sys/vm/drop_caches; done &");

	if (signal(SIGINT, handler) < 0) {
		perror("Signal");
		exit(1);
	}

	fd = open("/dev/random", O_RDWR);
	if (fd < 0) {
		perror("Can't open");
		exit(1);
	}

	flags = FASYNC | fcntl(fd, F_GETFL);
	if (fcntl(fd, F_SETFL, flags) < 0) {
		perror("Setfl");
		exit(1);
	}
	if (fcntl(fd, F_SETOWN, getpid())) {
		perror("Setown");
		exit(1);
	}
	if (fcntl(fd, F_SETSIG, SIGINT)) {
		perror("Setsig");
		exit(1);
	}

	while (1)
		sleep(100);
}
---
 fs/fcntl.c         |   15 +++++++--------
 include/linux/fs.h |    2 +-
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 1e97f1fda90c..780161a11f9d 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -865,9 +865,9 @@ int fasync_remove_entry(struct file *filp, struct fasync_struct **fapp)
 		if (fa->fa_file != filp)
 			continue;
 
-		spin_lock_irq(&fa->fa_lock);
+		write_lock_irq(&fa->fa_lock);
 		fa->fa_file = NULL;
-		spin_unlock_irq(&fa->fa_lock);
+		write_unlock_irq(&fa->fa_lock);
 
 		*fp = fa->fa_next;
 		call_rcu(&fa->fa_rcu, fasync_free_rcu);
@@ -912,13 +912,13 @@ struct fasync_struct *fasync_insert_entry(int fd, struct file *filp, struct fasy
 		if (fa->fa_file != filp)
 			continue;
 
-		spin_lock_irq(&fa->fa_lock);
+		write_lock_irq(&fa->fa_lock);
 		fa->fa_fd = fd;
-		spin_unlock_irq(&fa->fa_lock);
+		write_unlock_irq(&fa->fa_lock);
 		goto out;
 	}
 
-	spin_lock_init(&new->fa_lock);
+	rwlock_init(&new->fa_lock);
 	new->magic = FASYNC_MAGIC;
 	new->fa_file = filp;
 	new->fa_fd = fd;
@@ -981,14 +981,13 @@ static void kill_fasync_rcu(struct fasync_struct *fa, int sig, int band)
 {
 	while (fa) {
 		struct fown_struct *fown;
-		unsigned long flags;
 
 		if (fa->magic != FASYNC_MAGIC) {
 			printk(KERN_ERR "kill_fasync: bad magic number in "
 			       "fasync_struct!\n");
 			return;
 		}
-		spin_lock_irqsave(&fa->fa_lock, flags);
+		read_lock(&fa->fa_lock);
 		if (fa->fa_file) {
 			fown = &fa->fa_file->f_owner;
 			/* Don't send SIGURG to processes which have not set a
@@ -997,7 +996,7 @@ static void kill_fasync_rcu(struct fasync_struct *fa, int sig, int band)
 			if (!(sig == SIGURG && fown->signum == 0))
 				send_sigio(fown, fa->fa_fd, band);
 		}
-		spin_unlock_irqrestore(&fa->fa_lock, flags);
+		read_unlock(&fa->fa_lock);
 		fa = rcu_dereference(fa->fa_next);
 	}
 }
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c6baf767619e..297e2dcd9dd2 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1250,7 +1250,7 @@ static inline int locks_lock_file_wait(struct file *filp, struct file_lock *fl)
 }
 
 struct fasync_struct {
-	spinlock_t		fa_lock;
+	rwlock_t		fa_lock;
 	int			magic;
 	int			fa_fd;
 	struct fasync_struct	*fa_next; /* singly linked list */

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