Commit 76398425 authored by Jonathan Corbet's avatar Jonathan Corbet
Browse files

Move FASYNC bit handling to f_op->fasync() 



Removing the BKL from FASYNC handling ran into the challenge of keeping the
setting of the FASYNC bit in filp->f_flags atomic with regard to calls to
the underlying fasync() function.  Andi Kleen suggested moving the handling
of that bit into fasync(); this patch does exactly that.  As a result, we
have a couple of internal API changes: fasync() must now manage the FASYNC
bit, and it will be called without the BKL held.

As it happens, every fasync() implementation in the kernel with one
exception calls fasync_helper().  So, if we make fasync_helper() set the
FASYNC bit, we can avoid making any changes to the other fasync()
functions - as long as those functions, themselves, have proper locking.
Most fasync() implementations do nothing but call fasync_helper() - which
has its own lock - so they are easily verified as correct.  The BKL had
already been pushed down into the rest.

The networking code has its own version of fasync_helper(), so that code
has been augmented with explicit FASYNC bit handling.

Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: David Miller <davem@davemloft.net>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent db1dd4d3

Documentation/filesystems/Locking

+5 −2
Original line number Diff line number Diff line
@@ -437,8 +437,11 @@ grab BKL for cases when we close a file that had been opened r/w, but that 
can and should be done using the internal locking with smaller critical areas).

Current worst offender is ext2_get_block()...



->fasync() is a mess. This area needs a big cleanup and that will probably

affect locking.

->fasync() is called without BKL protection, and is responsible for

maintaining the FASYNC bit in filp->f_flags.  Most instances call

fasync_helper(), which does that maintenance, so it's not normally

something one needs to worry about.  Return values > 0 will be mapped to

zero in the VFS layer.



->readdir() and ->ioctl() on directories must be changed. Ideally we would

move ->readdir() to inode_operations and use a separate method for directory

fs/fcntl.c

+16 −13
Original line number Diff line number Diff line
@@ -141,7 +141,7 @@ SYSCALL_DEFINE1(dup, unsigned int, fildes) 
	return ret;

}



#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | FASYNC | O_DIRECT | O_NOATIME)

#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME)



static int setfl(int fd, struct file * filp, unsigned long arg)

{
@@ -177,23 +177,19 @@ static int setfl(int fd, struct file * filp, unsigned long arg) 
		return error;



	/*

	 * We still need a lock here for now to keep multiple FASYNC calls

	 * from racing with each other.

	 * ->fasync() is responsible for setting the FASYNC bit.

	 */

	lock_kernel();

	if ((arg ^ filp->f_flags) & FASYNC) {

		if (filp->f_op && filp->f_op->fasync) {

	if (((arg ^ filp->f_flags) & FASYNC) && filp->f_op &&

			filp->f_op->fasync) {

		error = filp->f_op->fasync(fd, filp, (arg & FASYNC) != 0);

		if (error < 0)

			goto out;

	}

	}



	spin_lock(&filp->f_lock);

	filp->f_flags = (arg & SETFL_MASK) | (filp->f_flags & ~SETFL_MASK);

	spin_unlock(&filp->f_lock);



 out:

	unlock_kernel();

	return error;

}
@@ -518,7 +514,7 @@ static DEFINE_RWLOCK(fasync_lock); 
static struct kmem_cache *fasync_cache __read_mostly;



/*

 * fasync_helper() is used by some character device drivers (mainly mice)

 * fasync_helper() is used by almost all character device drivers

 * to set up the fasync queue. It returns negative on error, 0 if it did

 * no changes and positive if it added/deleted the entry.

 */
@@ -557,6 +553,13 @@ int fasync_helper(int fd, struct file * filp, int on, struct fasync_struct **fap 
		result = 1;

	}

out:

	/* Fix up FASYNC bit while still holding fasync_lock */

	spin_lock(&filp->f_lock);

	if (on)

		filp->f_flags |= FASYNC;

	else

		filp->f_flags &= ~FASYNC;

	spin_unlock(&filp->f_lock);

	write_unlock_irq(&fasync_lock);

	return result;

}

fs/ioctl.c

+1 −12
Original line number Diff line number Diff line
@@ -427,19 +427,11 @@ static int ioctl_fioasync(unsigned int fd, struct file *filp, 
	/* Did FASYNC state change ? */

	if ((flag ^ filp->f_flags) & FASYNC) {

		if (filp->f_op && filp->f_op->fasync)

			/* fasync() adjusts filp->f_flags */

			error = filp->f_op->fasync(fd, filp, on);

		else

			error = -ENOTTY;

	}

	if (error)

		return error;



	spin_lock(&filp->f_lock);

	if (on)

		filp->f_flags |= FASYNC;

	else

		filp->f_flags &= ~FASYNC;

	spin_unlock(&filp->f_lock);

	return error;

}
@@ -507,10 +499,7 @@ int do_vfs_ioctl(struct file *filp, unsigned int fd, unsigned int cmd, 
		break;



	case FIOASYNC:

		/* BKL needed to avoid races tweaking f_flags */

		lock_kernel();

		error = ioctl_fioasync(fd, filp, argp);

		unlock_kernel();

		break;



	case FIOQSIZE:

net/socket.c

+7 −0
Original line number Diff line number Diff line
@@ -1030,6 +1030,13 @@ static int sock_fasync(int fd, struct file *filp, int on) 


	lock_sock(sk);



	spin_lock(&filp->f_lock);

	if (on)

		filp->f_flags |= FASYNC;

	else

		filp->f_flags &= ~FASYNC;

	spin_unlock(&filp->f_lock);



	prev = &(sock->fasync_list);



	for (fa = *prev; fa != NULL; prev = &fa->fa_next, fa = *prev)

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