Subject: Re: [PATCH v6 18/24] mm: Try spin lock in speculative path
To: Matthew Wilcox <willy@infradead.org>
Cc: paulmck@linux.vnet.ibm.com, peterz@infradead.org,
        akpm@linux-foundation.org, kirill@shutemov.name,
        ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net,
        jack@suse.cz, benh@kernel.crashing.org, mpe@ellerman.id.au,
        paulus@samba.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
        Will Deacon <will.deacon@arm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        kemi.wang@intel.com, sergey.senozhatsky.work@gmail.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com,
        npiggin@gmail.com, bsingharora@gmail.com,
        Tim Chen <tim.c.chen@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org
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 <20180112181840.GA7590@bombadil.infradead.org>
From: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Date: Tue, 16 Jan 2018 14:24:39 +0100
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On 12/01/2018 19:18, Matthew Wilcox wrote:
> On Fri, Jan 12, 2018 at 06:26:02PM +0100, Laurent Dufour wrote:
>> There is a deadlock when a CPU is doing a speculative page fault and
>> another one is calling do_unmap().
>>
>> The deadlock occurred because the speculative path try to spinlock the
>> pte while the interrupt are disabled. When the other CPU in the
>> unmap's path has locked the pte then is waiting for all the CPU to
>> invalidate the TLB. As the CPU doing the speculative fault have the
>> interrupt disable it can't invalidate the TLB, and can't get the lock.
>>
>> Since we are in a speculative path, we can race with other mm action.
>> So let assume that the lock may not get acquired and fail the
>> speculative page fault.
> 
> It seems like you introduced this bug in the previous patch, and now
> you're fixing it in this patch?  Why not merge the two?

You're right this is a fix from the previous patch. Initially my idea was
to keep the original Peter's patch as is, but sounds that this is not a
good idea.
I'll merge it in the previous one.

Thanks,
Laurent.