Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin_nested()

Tetsuo Handa
2023-07-29T05:32:54Z

On 2023/07/28 0:10, Sebastian Andrzej Siewior wrote:
> On 2023-06-28 21:14:16 [+0900], Tetsuo Handa wrote:
>>> Anyway, please do not do this change only because of printk().
>>> IMHO, the current ordering is more logical and the printk() problem
>>> should be solved another way.
>>
>> Then, since [PATCH 1/2] cannot be applied, [PATCH 2/2] is automatically
>> rejected.
>
> My understanding is that this patch gets applied and your objection will
> be noted.

My preference is that zonelist_update_seq is not checked by !__GFP_DIRECT_RECLAIM
allocations, which is a low-hanging fruit towards GFP_LOCKLESS mentioned at
https://lkml.kernel.org/r/ZG3+l4qcCWTPtSMD@dhcp22.suse.cz and
https://lkml.kernel.org/r/ZJWWpGZMJIADQvRS@dhcp22.suse.cz .

Maybe we can defer checking zonelist_update_seq till retry check like below,
for this is really an infrequent event.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 7d3460c7a480..2f7b82af2590 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3642,22 +3642,27 @@ EXPORT_SYMBOL_GPL(fs_reclaim_release);
* retries the allocation if zonelist changes. Writer side is protected by the
* embedded spin_lock.
*/
-static DEFINE_SEQLOCK(zonelist_update_seq);
+static unsigned int zonelist_update_seq;

static unsigned int zonelist_iter_begin(void)
{
if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
- return read_seqbegin(&zonelist_update_seq);
+ return data_race(READ_ONCE(zonelist_update_seq));

return 0;
}

-static unsigned int check_retry_zonelist(unsigned int seq)
+static unsigned int check_retry_zonelist(gfp_t gfp, unsigned int seq)
{
- if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE))
- return read_seqretry(&zonelist_update_seq, seq);
+ if (IS_ENABLED(CONFIG_MEMORY_HOTREMOVE) && (gfp & __GFP_DIRECT_RECLAIM)) {
+ unsigned int seq2;
+
+ smp_rmb();
+ seq2 = data_race(READ_ONCE(zonelist_update_seq));
+ return unlikely(seq != seq2 || (seq2 & 1));
+ }

- return seq;
+ return 0;
}

/* Perform direct synchronous page reclaim */
@@ -4146,7 +4151,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
* a unnecessary OOM kill.
*/
if (check_retry_cpuset(cpuset_mems_cookie, ac) ||
- check_retry_zonelist(zonelist_iter_cookie))
+ check_retry_zonelist(gfp_mask, zonelist_iter_cookie))
goto restart;

/* Reclaim has failed us, start killing things */
@@ -4172,7 +4177,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
* a unnecessary OOM kill.
*/
if (check_retry_cpuset(cpuset_mems_cookie, ac) ||
- check_retry_zonelist(zonelist_iter_cookie))
+ check_retry_zonelist(gfp_mask, zonelist_iter_cookie))
goto restart;

/*
@@ -5136,22 +5141,12 @@ static void __build_all_zonelists(void *data)
int nid;
int __maybe_unused cpu;
pg_data_t *self = data;
+ static DEFINE_SPINLOCK(lock);
unsigned long flags;

- /*
- * Explicitly disable this CPU's interrupts before taking seqlock
- * to prevent any IRQ handler from calling into the page allocator
- * (e.g. GFP_ATOMIC) that could hit zonelist_iter_begin and livelock.
- */
- local_irq_save(flags);
- /*
- * Explicitly disable this CPU's synchronous printk() before taking
- * seqlock to prevent any printk() from trying to hold port->lock, for
- * tty_insert_flip_string_and_push_buffer() on other CPU might be
- * calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held.
- */
- printk_deferred_enter();
- write_seqlock(&zonelist_update_seq);
+ spin_lock_irqsave(&lock, flags);
+ data_race(zonelist_update_seq++);
+ smp_wmb();

#ifdef CONFIG_NUMA
memset(node_load, 0, sizeof(node_load));
@@ -5188,9 +5183,9 @@ static void __build_all_zonelists(void *data)
#endif
}

- write_sequnlock(&zonelist_update_seq);
- printk_deferred_exit();
- local_irq_restore(flags);
+ smp_wmb();
+ data_race(zonelist_update_seq++);
+ spin_unlock_irqrestore(&lock, flags);
}

static noinline void __init