Messages in this thread | | | Date | Wed, 13 Feb 2019 21:38:05 -0500 | From | Andrea Arcangeli <> | Subject | Re: [PATCH -mm -V7] mm, swap: fix race between swapoff and some swap operations |
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Hello everyone,
On Mon, Feb 11, 2019 at 04:38:46PM +0800, Huang, Ying wrote: > @@ -2386,7 +2463,17 @@ static void enable_swap_info(struct swap_info_struct *p, int prio, > frontswap_init(p->type, frontswap_map); > spin_lock(&swap_lock); > spin_lock(&p->lock); > - _enable_swap_info(p, prio, swap_map, cluster_info); > + setup_swap_info(p, prio, swap_map, cluster_info); > + spin_unlock(&p->lock); > + spin_unlock(&swap_lock); > + /* > + * Guarantee swap_map, cluster_info, etc. fields are used > + * between get/put_swap_device() only if SWP_VALID bit is set > + */ > + stop_machine(swap_onoff_stop, NULL, cpu_online_mask);
Should cpu_online_mask be read while holding cpus_read_lock?
cpus_read_lock(); err = __stop_machine(swap_onoff_stop, NULL, cpu_online_mask); cpus_read_unlock();
I missed what the exact motivation was for the switch from rcu_read_lock()/syncrhonize_rcu() to preempt_disable()/stop_machine().
It looks like the above stop_machine all it does is to reach a quiescent point, when you've RCU that already can reach the quiescent point without an explicit stop_machine.
The reason both implementations are basically looking the same is that stop_machine dummy call of swap_onoff_stop() { /* noop */ } will only reach a quiescent point faster than RCU, but it's otherwise functionally identical to RCU, but it's extremely more expensive. If it wasn't functionally identical stop_machine() couldn't be used as a drop in replacement of synchronize_sched() in the previous patch.
I don't see the point of worrying about the synchronize_rcu latency in swapoff when RCU is basically identical and not more complex.
So to be clear, I'm not against stop_machine() but with stop_machine() method invoked in all CPUs, you can actually do more than RCU and you can remove real locking not just reach a quiescent point.
With stop_machine() the code would need reshuffling around so that the actual p->swap_map = NULL happens inside stop_machine, not outside like with RCU.
With RCU all code stays concurrent at all times, simply the race is controlled, as opposed with stop_machine() you can make fully serialize and run like in UP temporarily (vs all preempt_disable() section at least).
For example nr_swapfiles could in theory become a constant under preempt_disable() with stop_machine() without having to take a swap_lock.
swap_onoff_stop can be implemented like this:
enum { FIRST_STOP_MACHINE_INIT, FIRST_STOP_MACHINE_START, FIRST_STOP_MACHINE_END, }; static int first_stop_machine; static int swap_onoff_stop(void *data) { struct swap_stop_machine *swsm = (struct swap_stop_machine *)data; int first;
first = cmpxchg(&first_stop_machine, FIRST_STOP_MACHINE_INIT, FIRST_STOP_MACHINE_START); if (first == FIRST_STOP_MACHINE_INIT) { swsm->p->swap_map = NULL; /* add more stuff here until swap_lock goes away */ smp_wmb(); WRITE_ONCE(first_stop_machine, FIRST_STOP_MACHINE_END); } else { do { cpu_relax(); } while (READ_ONCE(first_stop_machine) != FIRST_STOP_MACHINE_END); smp_rmb(); }
return 0; }
stop_machine invoked with a method like above, will guarantee while we set p->swap_map to NULL (and while we do nr_swapfiles++) nothing else can run, no even interrupts, so some lock may just disappear. Only NMI and SMI could possibly run concurrently with the swsm->p->swap_map = NULL operation.
If we've to keep swap_onoff_stop() a dummy function run on all CPUs just to reach a quiescent point, then I don't see why the synchronize_rcu() (or synchronize_sched or synchronize_kernel or whatever it is called right now, but still RCU) solution isn't preferable.
Thanks, Andrea
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