[haiku-bugs] [Haiku] #5340: BLOCK_CACHE_DEBUG_CHANGED is too strict

#5340: BLOCK_CACHE_DEBUG_CHANGED is too strict
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 Reporter:  romain     |       Owner:  nobody   
     Type:  bug        |      Status:  new      
 Priority:  normal     |   Milestone:  R1       
Component:  - General  |     Version:  R1/alpha1
 Keywords:             |   Blockedby:           
 Platform:  All        |    Blocking:           
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 Ticket #3690 often stops in a block cache check detecting an invalid block
 buffer change. It seems that this detected error is in fact not an error.
 Here is what seems to happen:
 - The fat filesystem is using the block cache without transactions.
 - Block n is acquire through block_cache_get. This initializes the compare
 pointer in the block cache object, is_dirty is set to false.
 - Block n is released through block_cache_put. This adds the block cache
 object to the unused_block list.
 - Block n is acquired through block_get_get_writable. This sets is_dirty
 to true.
 - block_notifier_and_writter wakes up. It will flush the block because it
 is marked "dirty", not "is_writting", not "busy" and not using
 transaction. write_cached_block sets "is_dirty" to false.
 - Block n is modified in memory.
 - Block n is released through block_cache_put: this breaks in
 put_cached_block because the block is marked as not "dirty" but has a
 compare pointer that differs from current data.

 First I am not sure if this analysis is correct. If it is right this may
 be solved by marking the block as "is_writing" via a dedicated API, but
 this field does not seem to be used. So I am not sure what is the purpose
 of this flag.
 A second option is to clear the compare data in write_cached_block. This
 what is done in the provided patch.

 In the current situation there is also the problem that we can write the
 block on disk while it is being modified in memory. I do not know if this
 has to be handle by the code using the block_cache or in the block_cache
 itself.

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Ticket URL: <http://dev.haiku-os.org/ticket/5340>
Haiku <http://dev.haiku-os.org>
Haiku - the operating system.

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