Re: [PATCH, RFC 2/2] dax: use range_lock instead of i_mmap_lock

[Boaz Harrosh]

On 08/12/2015 12:48 AM, Dave Chinner wrote:
> On Tue, Aug 11, 2015 at 04:51:22PM +0000, Wilcox, Matthew R wrote:
>> The race that you're not seeing is page fault vs page fault.  Two
>> threads each attempt to store a byte to different locations on the
>> same page.  With a read-mutex to exclude truncates, each thread
>> calls ->get_block.  One of the threads gets back a buffer marked
>> as BH_New and calls memset() to clear the page.  The other thread
>> gets back a buffer which isn't marked as BH_New and simply inserts
>> the mapping, returning to userspace, which stores the byte ...
>> just in time for the other thread's memset() to write a zero over
>> the top of it.
> 
> So, this is not a truncate race that the XFS MMAPLOCK solves.
> 
> However, that doesn't mean that the DAX code needs to add locking to
> solve it. The race here is caused by block initialisation being
> unserialised after a ->get_block call allocates the block (which the
> filesystem serialises via internal locking). Hence two simultaneous
> ->get_block calls to the same block is guaranteed to have the DAX
> block initialisation race with the second ->get_block call that says
> the block is already allocated.
> 
> IOWs, the way to handle this is to have the ->get_block call handle
> the block zeroing for new blocks instead of doing it after the fact
> in the generic DAX code where there is no fine-grained serialisation
> object available. By calling dax_clear_blocks() in the ->get_block
> callback, the filesystem can ensure that the second racing call will
> only make progress once the block has been fully initialised by the
> first call.
> 
> IMO the fix is - again - to move the functionality into the
> filesystem where we already have the necessary exclusion in place to
> avoid this race condition entirely.
> 

Exactly, thanks

> Cheers,
> 
> Dave.
> 

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