Commit 50755bc1c3 ("seqlock: fix raw_read_seqcount_latch()") broke
raw_read_seqcount_latch().
If you look at the comment that was modified; the thing that changes is
the seq count, not the latch pointer.
* void latch_modify(struct latch_struct *latch, ...)
* {
* smp_wmb(); <- Ensure that the last data[1] update is visible
* latch->seq++;
* smp_wmb(); <- Ensure that the seqcount update is visible
*
* modify(latch->data[0], ...);
*
* smp_wmb(); <- Ensure that the data[0] update is visible
* latch->seq++;
* smp_wmb(); <- Ensure that the seqcount update is visible
*
* modify(latch->data[1], ...);
* }
*
* The query will have a form like:
*
* struct entry *latch_query(struct latch_struct *latch, ...)
* {
* struct entry *entry;
* unsigned seq, idx;
*
* do {
* seq = lockless_dereference(latch->seq);
So here we have:
seq = READ_ONCE(latch->seq);
smp_read_barrier_depends();
Which is exactly what we want; the new code:
seq = ({ p = READ_ONCE(latch);
smp_read_barrier_depends(); p })->seq;
is just wrong; because it looses the volatile read on seq, which can now
be torn or worse 'optimized'. And the read_depend barrier is also placed
wrong, we want it after the load of seq, to match the above data[]
up-to-date wmb()s.
Such that when we dereference latch->data[] below, we're guaranteed to
observe the right data.
*
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
* smp_rmb();
* } while (seq != latch->seq);
*
* return entry;
* }
So yes, not passing a pointer is not pretty, but the code was correct,
and isn't anymore now.
Change to explicit READ_ONCE()+smp_read_barrier_depends() to avoid
confusion and allow strict lockless_dereference() checking.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 50755bc1c3 ("seqlock: fix raw_read_seqcount_latch()")
Link: http://lkml.kernel.org/r/20160527111117.GL3192@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
55eed755c6