On Wed, Jul 08, 2015 at 11:32:13AM +0100, Mel Gorman wrote:
> From: Nicolai Stange <nicstange@xxxxxxxxx>
> Subject: Re: [PATCH] mm/page_alloc: deferred meminit: replace rwsem with completion
>
> Commit 0e1cc95b4cc7
> ("mm: meminit: finish initialisation of struct pages before basic setup")
> introduced a rwsem to signal completion of the initialization workers.
>
> Lockdep complains about possible recursive locking:
> =============================================
> [ INFO: possible recursive locking detected ]
> 4.1.0-12802-g1dc51b8 #3 Not tainted
> ---------------------------------------------
> swapper/0/1 is trying to acquire lock:
> (pgdat_init_rwsem){++++.+},
> at: [<ffffffff8424c7fb>] page_alloc_init_late+0xc7/0xe6
>
> but task is already holding lock:
> (pgdat_init_rwsem){++++.+},
> at: [<ffffffff8424c772>] page_alloc_init_late+0x3e/0xe6
>
> Replace the rwsem by a completion together with an atomic
> "outstanding work counter".
>
> Signed-off-by: Nicolai Stange <nicstange@xxxxxxxxx>
> Acked-by: Mel Gorman <mgorman@xxxxxxx>
> ---
> mm/page_alloc.c | 34 +++++++++++++++++++++++++++-------
> 1 file changed, 27 insertions(+), 7 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 506eac8..3886e66 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -18,7 +18,9 @@
> #include <linux/mm.h>
> #include <linux/swap.h>
> #include <linux/interrupt.h>
> -#include <linux/rwsem.h>
> +#include <linux/completion.h>
> +#include <linux/atomic.h>
> +#include <asm/barrier.h>
> #include <linux/pagemap.h>
> #include <linux/jiffies.h>
> #include <linux/bootmem.h>
> @@ -1062,7 +1064,20 @@ static void __init deferred_free_range(struct page *page,
> __free_pages_boot_core(page, pfn, 0);
> }
>
> -static __initdata DECLARE_RWSEM(pgdat_init_rwsem);
> +/* counter and completion tracking outstanding deferred_init_memmap()
> + threads */
Wrong comment style.
> +static atomic_t pgdat_init_n_undone __initdata;
> +static __initdata DECLARE_COMPLETION(pgdat_init_all_done_comp);
> +
> +static inline void __init pgdat_init_report_one_done(void)
> +{
> + /* Write barrier is paired with read barrier in
> + page_alloc_init_late(). It makes all writes visible to
> + readers seeing our decrement on pgdat_init_n_undone. */
Wrong comment style.
> + smp_wmb();
Pointless barrier, because
> + if (atomic_dec_and_test(&pgdat_init_n_undone))
implies a full memory barrier, furthermore see below.
> + complete(&pgdat_init_all_done_comp);
> +}
>
> /* Initialise remaining memory on a node */
> static int __init deferred_init_memmap(void *data)
> @@ -1187,14 +1203,18 @@ void __init page_alloc_init_late(void)
> {
> int nid;
>
> + /* There will be num_node_state(N_MEMORY) threads */
> + atomic_set(&pgdat_init_n_undone, num_node_state(N_MEMORY));
> for_each_node_state(nid, N_MEMORY) {
> - down_read(&pgdat_init_rwsem);
> kthread_run(deferred_init_memmap, NODE_DATA(nid), "pgdatinit%d", nid);
> }
>
> /* Block until all are initialised */
> - down_write(&pgdat_init_rwsem);
> - up_write(&pgdat_init_rwsem);
> + wait_for_completion(&pgdat_init_all_done_comp);
> +
> + /* Paired with write barrier in deferred_init_memmap(),
> + ensures a consistent view of all its writes. */
Wrong comment style
> + smp_rmb();
Wrong barrier, IF you want a barrier it should be before
wait_for_completion, such that if you observe complete, you then must
also observe whatever happened prior to the completion.
But I would argue a completion had better imply that anyway.
> }
> #endif /* CONFIG_DEFERRED_STRUCT_PAGE_INIT */
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