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SubjectRe: [PATCH] fasync: Fix deadlock between task-context and interrupt-context kill_fasync()
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Hi,

almost two weeks passed, while there is no reaction.

Jeff, Bruce, what is your point of view?

Just to underline, the problem is because of rw_lock fairness, which does not
allow a reader to take a read locked lock in case of there is already a writer
called write_lock(). See queued_read_lock_slowpath() for the details. This makes
rw_lock fair, and it does not allow readers to occupy the lock forever without
a possibility for writers to take it.

Kirill

On 05.04.2018 14:58, Kirill Tkhai wrote:
> 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@virtuozzo.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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