Messages in this thread | | | Subject | Re: [patch 6/6] mm: fix pagecache write deadlocks | From | Peter Zijlstra <> | Date | Sun, 15 Oct 2006 15:51:09 +0200 |
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> > > + /* > > > + * Must not enter the pagefault handler here, because we hold > > > + * the page lock, so we might recursively deadlock on the same > > > + * lock, or get an ABBA deadlock against a different lock, or > > > + * against the mmap_sem (which nests outside the page lock). > > > + * So increment preempt count, and use _atomic usercopies. > > > + */ > > > + inc_preempt_count(); > > > if (likely(nr_segs == 1)) > > > - copied = filemap_copy_from_user(page, offset, > > > + copied = filemap_copy_from_user_atomic(page, offset, > > > buf, bytes); > > > else > > > - copied = filemap_copy_from_user_iovec(page, offset, > > > - cur_iov, iov_offset, bytes); > > > + copied = filemap_copy_from_user_iovec_atomic(page, > > > + offset, cur_iov, iov_offset, > > > + bytes); > > > + dec_preempt_count(); > > > + > > > > Why use raw {inc,dec}_preempt_count() and not > > preempt_{disable,enable}()? Is the compiler barrier not needed here? And > > do we really want to avoid the preempt_check_resched()? > > Counter to intuition, we actually don't mind being preempted here, > but we do mind entering the (core) pagefault handler. Incrementing > the preempt count causes the arch specific handler to bail out early > before it takes any locks. > > Clear as mud? Wrapping it in a better name might be an improvement? > Or wrapping it into the copy*user_atomic functions themselves (which > is AFAIK the only place we use it).
Right, but since you do inc the preempt_count you do disable preemption, might as well check TIF_NEED_RESCHED when enabling preemption again.
Sticking it in the atomic copy functions does make sense to me.
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