On Thu, Jan 3, 2013 at 4:40 PM, Simon Jeons <simon.jeons@xxxxxxxxx> wrote: > On Wed, 2012-12-19 at 19:01 -0800, Michel Lespinasse wrote: >> Hi Simon, >> >> On Wed, Dec 19, 2012 at 5:56 PM, Simon Jeons <simon.jeons@xxxxxxxxx> wrote: >> > One question. >> > >> > I found that mainly callsite of expand_stack() is #PF, but it holds >> > mmap_sem each time before call expand_stack(), how can hold a *shared* >> > mmap_sem happen? >> >> the #PF handler calls down_read(&mm->mmap_sem) before calling expand_stack. >> >> I think I'm just confusing you with my terminology; shared lock == >> read lock == several readers might hold it at once (I'd say they share >> it) > > Sorry for my late response. > > Since expand_stack() will modify vma, then why hold a read lock here? Well, it'd be much nicer if we had a write lock, I think. But, we didn't know when taking the lock that we'd end up having to expand stacks. What happens is that page faults don't generally modify vmas, so they get a read lock (just to know what vma the fault is happening in) and then fault in the page. expand_stack() is the one exception to that - after getting the read lock as usual, we notice that the fault is not in any vma right now, but it's close enough to an expandable vma. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>