Re: [PATCH v2 0/2] seqlock,mm: lockdep annotation + write_seqlock_irqsave()

[Tetsuo Handa]

On 2023/06/24 2:12, Sebastian Andrzej Siewior wrote:
> Hi,
> 
> this has been a single patch (2/2) but then it was pointed out that the
> lockdep annotation in seqlock needs to be adjusted to fully close the
> printk window so that there is no printing after the seq-lock has been
> acquired and before printk_deferred_enter() takes effect.
> 
> I'm sending both patches in this series so both sides (locking and mm)
> are aware of the situation. 
> I hope that both patches can be applied independently via their subsystem
> tree (the lockdep splat + deadlock is low risk).
> 
> The original thread starts at
> 	https://lore.kernel.org/20230621104034.HT6QnNkQ@xxxxxxxxxxxxx
> 
> Sebastian

The original thread is too long to read. Below is a full summary for locking
maintainers to accept [PATCH 1/2].



When commit 1ca7d67cf5d5 ("seqcount: Add lockdep functionality to
seqcount/seqlock structures") added seqcount_acquire(&s->dep_map) check to
write_seqcount_begin_nested() and seqcount_release(&s->dep_map) check to
write_seqcount_end(), the ordering of updating s->sequence and doing lockdep
annotation was not important.

But since commit 3d36424b3b58 ("mm/page_alloc: fix race condition between
build_all_zonelists and page allocation") started calling
read_seqbegin(&zonelist_update_seq)/read_seqretry(&zonelist_update_seq) from
kmalloc(GFP_ATOMIC) path, commit 1007843a9190 ("mm/page_alloc: fix potential
deadlock on zonelist_update_seq seqlock") tried to close the race window using

 __build_all_zonelists() {
+    local_irq_save(flags);
+    printk_deferred_enter();
     write_seqlock(&zonelist_update_seq);
     (...snipped...)
     write_sequnlock(&zonelist_update_seq);
+    printk_deferred_exit();
+    local_irq_restore(flags);
 }

pattern. The reason behind this ordering was to

  satisfy "printk_deferred_enter() depends on local IRQs being disabled"

and

  make sure that "no synchronous printk() (for whatever reasons, not only
  printk() from build_zonelists() from __build_all_zonelists(), but also
  including printk() from lockdep, soft-lockup, KCSAN etc.) happens between
  write_seqlock() and write_sequnlock()

. However, Sebastian Andrzej Siewior mentioned that this ordering is
problematic if CONFIG_PREEMPT_RT=y, for disabling local IRQs conflicts with
"spin_lock(&zonelist_update_seq.lock) from write_seqlock(&zonelist_update_seq)
needs to be able to sleep", and Sebastian is proposing

 __build_all_zonelists() {
-    local_irq_save(flags);
-    printk_deferred_enter();
-    write_seqlock(&zonelist_update_seq);
+    write_seqlock_irqsave(&zonelist_update_seq, flags);
+    printk_deferred_enter();
     (...snipped...)
+    printk_deferred_exit();
+    write_sequnlock_irqrestore(&zonelist_update_seq, flags);
-    write_sequnlock(&zonelist_update_seq);
-    printk_deferred_exit();
-    local_irq_restore(flags);
 }

change as [PATCH 2/2]. Since write_seqlock_irqsave() becomes write_seqlock()
if CONFIG_PREEMPT_RT=y, this change can solve the conflict.

In order to accept this proposal, we need to make sure that

  no synchronous printk() happens between

    write_seqlock_irqsave(&zonelist_update_seq, flags) made
    zonelist_update_seq.seqcount odd

  and

    printk_deferred_enter() takes effect

and

  no synchronous printk() happens between

    printk_deferred_exit() took effect

  and

    write_sequnlock_irqrestore(&zonelist_update_seq, flags) makes
    zonelist_update_seq.seqcount even

, and Sebastian is proposing

 static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
 {
-    do_raw_write_seqcount_begin(s);
     seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+    do_raw_write_seqcount_begin(s);
 }

 static inline void do_write_seqcount_end(seqcount_t *s)
 {
-    seqcount_release(&s->dep_map, _RET_IP_);
     do_raw_write_seqcount_end(s);
+    seqcount_release(&s->dep_map, _RET_IP_);
 }

as [PATCH 1/2].

With [PATCH 1/2] and [PATCH 2/2], possibility of synchronous printk() changes like below.

__build_all_zonelists() {
  write_seqlock_irqsave(&zonelist_update_seq, flags) {
    __write_seqlock_irqsave(&zonelist_update_seq) {
      spin_lock_irqsave(&zonelist_update_seq.lock, flags); // local IRQs disabled = synchronous printk() from IRQs is disabled here
      do_write_seqcount_begin(&zonelist_update_seq.seqcount.seqcount) {
        do_write_seqcount_begin_nested(&zonelist_update_seq.seqcount.seqcount, 0) {
          seqcount_acquire(&zonelist_update_seq.seqcount.seqcount.dep_map, 0, 0, _RET_IP_); // synchronous printk() from lockdep might happen here
          do_raw_write_seqcount_begin(&zonelist_update_seq.seqcount.seqcount) {
            seqcount_acquire(&zonelist_update_seq.seqcount.seqcount.dep_map, 0, 0, _RET_IP_); // zonelist_update_seq.seqcount.seqcount.sequence is guaranteed to be even = kmalloc(GFP_ATOMIC) with port lock held is safe = synchronous printk() is safe
            kcsan_nestable_atomic_begin(); // KCSAN is diabled = synchronous printk() from KCSAN is disabled here
            zonelist_update_seq.seqcount.seqcount.sequence++; // zonelist_update_seq.seqcount.seqcount.sequence is now odd = kmalloc(GFP_ATOMIC) with port lock held is not safe = synchronous printk() is not safe
          }
        }
      }
    }
  }
  printk_deferred_enter(); // synchronous printk() from whatever reason is disabled here
  (...snipped...)
  printk_deferred_exit(); // synchronous printk() from whatever reason is enabled here
  write_sequnlock_irqrestore(&zonelist_update_seq, flags) {
    do_write_seqcount_end(&zonelist_update_seq.seqcount.seqcount) {
      do_raw_write_seqcount_end(&zonelist_update_seq.seqcount.seqcount) {
        zonelist_update_seq.seqcount.seqcount.sequence++; // zonelist_update_seq.seqcount.seqcount.sequence is now even = kmalloc(GFP_ATOMIC) with port lock held is safe = synchronous printk() is safe
        kcsan_nestable_atomic_end(); // KCSAN is enabled = synchronous printk() from KCSAN is enabled here
      }
      seqcount_release(&zonelist_update_seq.seqcount.seqcount.dep_map, _RET_IP_); // synchronous printk() from lockdep might happen here
    }
    spin_unlock_irqrestore(&zonelist_update_seq.lock, flags); // local IRQs enabled = synchronous printk() from IRQs is enabled here
  }
}