Re: kern/39206: ffs um_lock handling isn't great

[Antti Kantee <pooka%cs.hut.fi@localhost>]

On Fri Jul 25 2008 at 15:24:44 +0000, David Holland wrote:
> On Fri, Jul 25, 2008 at 02:15:00PM +0000, Simon Burge wrote:
>  >         One example pointed out by pooka@ is at the top of
>  >         ffs_alloccg().  It appears that once the free block check at
>  >         the top of this function succeeds, this function isn't allowed
>  >         to fail.  This is noted in the "XXX fvdl mapsearch ..." comment
>  >         further down.  This function is entered with um_lock held, and
>  >         once the free block check has passed um_lock is dropped.  This
>  >         then allows another thread to reach the same point, and could
>  >         lead to problems if there was only one block free in the CG
>  >         before the first thread get there.
>  > 
>  >         This PR is entered as priority "medium" and not "high" since no
>  >         actual problems have been observed in practice yet.
> 
> Unless this is the source of those occasional "ffs_alloccg: map
> corrupted" panics...

If we are talking about pre-vmlocking corruption, I doubt it.  Unless,
of course, this code is entered from interrupt context with the help of
softdep (which I am not sure of and won't bother to read the code now).