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: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/fs/ioctl.c b/fs/ioctl.c
index 421aab4..e8e89ed 100644
--- a/fs/ioctl.c
+++ b/fs/ioctl.c
@@ -427,19 +427,11 @@
/* 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 @@
break;
case FIOASYNC:
- /* BKL needed to avoid races tweaking f_flags */
- lock_kernel();
error = ioctl_fioasync(fd, filp, argp);
- unlock_kernel();
break;
case FIOQSIZE: