On Mon, Feb 27, 2012 at 03:11:35PM -0800, Andrew Morton wrote: > This patch comes somewhat out of the blue and I'm unsure what's going on. > > You say there's some (potential?) deadlock with mmap, but it is > undescribed. Have people observed this deadlock? Has it caused > lockdep warnings? Please update the changelog to fully describe the > bug. There's one simple rule: never, ever take ->i_mutex under ->mmap_sem. E.g. in any ->mmap() (obvious - mmap(2) calls that under ->mmap_sem) or any ->release() of mappable file (munmap(2) does fput() under ->mmap_sem and that will call ->release() if no other references are still around). Hugetlbfs is slightly unusual since it takes ->i_mutex in read() - usually that's done in write(), while read() doesn't bother with that. In either case you do copying to/from userland buffer while holding ->i_mutex, which nests ->mmap_sem within it. > Also, the new truncate_sem is undoumented. This leaves readers to work > out for themselves what it might be for. Please let's add code > comments which completely describe the race, and how this lock prevents > it. > > We should also document our locking rules. Hell, yes. I've spent the last couple of weeks crawling through VM-related code and locking in there is _scary_. "Convoluted" doesn't even begin to cover it, especially when it gets to "what locks are required when accessing this field" ;-/ Got quite a catch out of that trawl by now... > When should code take this > lock? What are its ranking rules with respect to i_mutex, i_mmap_mutex > and possibly others? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>