Eric Dumazet wrote:
This patch deletes two calls to smp_mb() that were done after
mutex_unlock() that contains an implicit memory barrier.
Uh, there is nothing that says mutex_unlock or any unlock
functions contain an implicit smp_mb(). What is given is that the
lock and unlock obey aquire and release memory ordering,
respectively.
a = x;
xxx_unlock
b = y;
In this situation, the load of y can be executed before that of x.
And some architectures will even do so (i386 can, because the
unlock is an unprefixed store; ia64 can, because it uses a release
barrier in the unlock).
Whenever you rely on orderings of things *outside* locks (even
partially outside), you do need to be very careful about barriers
and can't rely on locks to do the right thing for you.
The first one in splice_to_pipe(), where 'do_wakeup' is set to true only
if pipe->inode is set (and in this case the
if (pipe->inode)
mutex_unlock(&pipe->inode->i_mutex);
is done too)
The second one in link_pipe(), following inode_double_unlock() that
contains calls to mutex_unlock() too.
It *may* be the case that these can be removed, but not by virtue
of the fact that the smp_mb is redundant.
Signed-off-by: Eric Dumazet <[email protected]>
------------------------------------------------------------------------
--- linux/fs/splice.c 2006-10-31 07:49:52.000000000 +0100
+++ linux-ed/fs/splice.c 2006-10-31 08:04:58.000000000 +0100
@@ -248,7 +248,6 @@
mutex_unlock(&pipe->inode->i_mutex);
if (do_wakeup) {
- smp_mb();
if (waitqueue_active(&pipe->wait))
wake_up_interruptible(&pipe->wait);
kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
@@ -1518,7 +1517,6 @@
* If we put data in the output pipe, wakeup any potential readers.
*/
if (ret > 0) {
- smp_mb();
if (waitqueue_active(&opipe->wait))
wake_up_interruptible(&opipe->wait);
kill_fasync(&opipe->fasync_readers, SIGIO, POLL_IN);
--
SUSE Labs, Novell Inc.
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