Re: [Linux-fbdev-devel] [PATCH] add mutex to fbdev for fb_mmap locking

[Linus T. <tor...@li...>]

On Mon, 1 Jun 2009, Krzysztof Helt wrote:
> 
> I will fix it. 

Thanks.

> If the revert to BKL is rejected this patch may wait till 2.6.31.

Ahh, so this one helps clean up locking, but doesn't fix any actual 
regressions? I was going to ask you about that.

Btw - one thing you could try on the whole lockdep front - and I realize 
that this is a _total_ hack - is to try the patch below.

The _only_ thing it does is to hide the sysfs_mutex -> mm_lock chain from 
lockdep, by using the (incorrect) __copy_to_user_inatomic() instead of the 
(correct) copy_to_user(). But I'd like to hear if that sysfs_mutex in 
readdir is the only way you can get a chain.

So this is in _no_ way meant to be a serious patch, and is literally just 
a test to see if "ok, readdir and sysfs_mutex is really the only real 
deadlock case" is true. Or are there possibly other cases that trigger 
this?

Because if it's really just this chain through sysfs_mutex, then the 
deadlock scenario should boil down to just the "two different fbcon's and 
really unlucky activity just as you register the second of them". Which is 
still a bug, of course - it's just not ever going to bite anybody in 
practice.

		Linus

---
 fs/readdir.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/readdir.c b/fs/readdir.c
index 7723401..431f647 100644
--- a/fs/readdir.c
+++ b/fs/readdir.c
@@ -172,7 +172,7 @@ static int filldir(void * __buf, const char * name, int namlen, loff_t offset,
 		goto efault;
 	if (__put_user(reclen, &dirent->d_reclen))
 		goto efault;
-	if (copy_to_user(dirent->d_name, name, namlen))
+	if (__copy_to_user_inatomic(dirent->d_name, name, namlen))
 		goto efault;
 	if (__put_user(0, dirent->d_name + namlen))
 		goto efault;
@@ -256,7 +256,7 @@ static int filldir64(void * __buf, const char * name, int namlen, loff_t offset,
 		goto efault;
 	if (__put_user(d_type, &dirent->d_type))
 		goto efault;
-	if (copy_to_user(dirent->d_name, name, namlen))
+	if (__copy_to_user_inatomic(dirent->d_name, name, namlen))
 		goto efault;
 	if (__put_user(0, dirent->d_name + namlen))
 		goto efault;