On 14/09/2017 10:13, Sergey Senozhatsky wrote:

Hi,

On (09/14/17 09:55), Laurent Dufour wrote:
[..]

quoted

quoted

so if there are two CPUs, one doing write_seqcount() and the other one
doing read_seqcount() then what can happen is something like this

	CPU0					CPU1

						fs_reclaim_acquire()
	write_seqcount_begin()
	fs_reclaim_acquire()			read_seqcount_begin()
	write_seqcount_end()

CPU0 can't write_seqcount_end() because of fs_reclaim_acquire() from
CPU1, CPU1 can't read_seqcount_begin() because CPU0 did write_seqcount_begin()
and now waits for fs_reclaim_acquire(). makes sense?

Yes, this makes sense.

But in the case of this series, there is no call to
__read_seqcount_begin(), and the reader (the speculative page fault
handler), is just checking for (vm_seq & 1) and if this is true, simply
exit the speculative path without waiting.
So there is no deadlock possibility.

probably lockdep just knows that those locks interleave at some
point.


by the way, I think there is one path that can spin

find_vma_srcu()
 read_seqbegin()
  read_seqcount_begin()
   raw_read_seqcount_begin()
    __read_seqcount_begin()

That's right, but here this is the  sequence counter mm->mm_seq, not the
vm_seq one.

This one is held to protect walking the VMA list "locklessly"...

Cheers,
Laurent.

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