On 09/19/2017 10:38 AM, Christoph Hellwig wrote:

On Tue, Sep 19, 2017 at 08:49:12AM -0400, Waiman Long wrote:

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On 09/18/2017 08:01 PM, Christoph Hellwig wrote:

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Taking a look at this it seems like using a lock in struct block_devi=

ce

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isn't the right thing to do anyway - all the action is on fields in
struct blk_trace, so having a lock inside that would make a lot more
sense.

It would also help to document what exactly we're actually protecting=

=2E

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I think I documented in the patch that the lock has to protect changes=

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in the blktrace structure as well as the allocation and destruction of=

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it. Because of that, it can't be put inside the blktrace structure. Th=

e

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original code use the bd_mutex of the block_device structure. I just
change the code to use another bd_fsfreeze_mutex in the same structure=

=2E

Either way it has absolutely nothing to do with struct block_device,
given that struct blk_trace hangs off the request_queue.

Reusing a mutex just because it happens to live in a structure also
generally is a bad idea if the protected data is in no way related.

I was trying not to add a new mutex to a structure just for blktrace as
it is an optional feature that is enabled only if the
CONFIG_BLK_DEV_IO_TRACE config option is defined and it will only need
to be taken occasionally.

As filesystem freeze looks orthogonal to blktrace and the mutex also
looks to be used sparingly, I think it is a good match to overload it to
control blktrace as well.

I could modify the patch to use a mutex in the request_queue structure.
The current sysfs_lock mutex has about 74 references. So I am not
totally sure if it is safe to reuse. So the only option is to add
something like

#ifdef CONFIG_BLK_DEV_IO_TRACE
struct mutex blktrace_mutex;
#endif

to the request_queue structure. That structure is large enough that
adding a mutex won't increase the size much percentage-wise.

I would like to keep the current patch as is as I don't see any problem
with it. However, I can revise the patch as discussed above if you guys
prefer that alternative.

Cheers,
Longman