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SubjectRe: [PATCH] SMP race in ext2 - metadata corruption.


On Thu, 26 Apr 2001, Linus Torvalds wrote:

> Note that I think all these arguments are fairly bogus. Doing things like
> "dump" on a live filesystem is stupid and dangerous (in my opinion it is
> stupid and dangerous to use "dump" at _all_, but that's a whole 'nother
> discussion in itself), and there really are no valid uses for opening a
> block device that is already mounted. More importantly, I don't think
> anybody actually does.

Agreed.

> The fact that you _can_ do so makes the patch valid, and I do agree with
> Al on the "least surprise" issue. I've already applied the patch, in fact.
> But the fact is that nobody should ever do the thing that could cause
> problems.

IMO the real issue is in fuzzy rules for use of wait_on_buffer(). There is
one case when it's 100% correct - when we had done ll_rw_block() on that
bh at earlier point and want to make sure that it's completed. And there
is a lot of uses that are kinda-sorta correct for UP, but break on SMP.
unmap_buffer() was similar to that race. So are races in minix, sysvfs and
ufs. So is one in block_write() and here the problem is quite real - it's
not as idiotic as device/mounted fs races.

Basically, all legitimate cases are ones where we would be very unhappy
about buffer being not uptodate afterwards.
getblk(); if (!buffer_uptodate) wait_on_buffer();
is not in that class. It _is_ OK on UP as long as we don't block, but on
SMP it doesn't guarantee anything - buffer_head can be in any state
when we are done. IMO all such places require fixing.
How about adding
if (!buffer_uptodate(bh)) {
printk(KERN_ERR "IO error or racy use of wait_on_buffer()");
show_task(current);
}
in the end of wait_on_buffer() for a while?
Al
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