Re: [PATCH v3 04/20] mm: VMA sequence count

[Sergey Senozhatsky]

Hi,

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

	-ss