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

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Date:   Thu, 14 Sep 2017 17:13:58 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill@...temov.name,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v3 04/20] mm: VMA sequence count

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