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/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index ec6a939..4e78ce6 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -437,8 +437,11 @@
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