On 2026/2/2 20:51, Peter Zijlstra wrote:
On Mon, Feb 02, 2026 at 08:14:32PM +0800, Lance Yang wrote:
+ /* Pairs with smp_mb() in pt_walk_lockless_start(). */
Pairs how? The start thing does something like:
[W] active_lockless_pt_walk_mm = mm
MB
[L] page-tables
So this is:
[L] page-tables
RMB
[L] active_lockless_pt_walk_mm
?
On the walker side (pt_walk_lockless_start):
[W] active_lockless_pt_walk_mm = mm
MB
[L] page-tables (walker reads page tables)
So the walker publishes "I'm walking this mm" before reading page tables.
On the sync side we don't read page-tables. We do:
RMB
[L] active_lockless_pt_walk_mm (we read the per-CPU pointer below)
We need to observe the walker's store of active_lockless_pt_walk_mm before
we decide which CPUs to IPI.
So on the sync side we do smp_rmb(), then read active_lockless_pt_walk_mm.
That pairs with the full barrier in pt_walk_lockless_start().
No it doesn't; this is not how memory barriers work.
Hmm... we need MB rather than RMB on the sync side. Is that correct?
Walker:
[W]active_lockless_pt_walk_mm = mm -> MB -> [L]page-tables
Sync:
[W]page-tables -> MB -> [L]active_lockless_pt_walk_mm
Thanks,
Lance
