Messages in this thread | | | Date | Thu, 13 Jan 2005 09:56:24 -0600 | From | Chris Friesen <> | Subject | Re: write barriers - Was: Re: [RFC][PATCH] problem of cont_prepare_write() |
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Anton Altaparmakov wrote:
> If I am setting two variables in sequence and it is essential that if a > different cpu reads those variables it seems them updated in the same > order as they were written in the C code do I need a write barrier in > between the two? For example: > > ntfs_inode->allocated_size = 10; > ntfs_inode->initilized_size = 10;
I believe so. You may also need to cast them as volatile to prevent the compiler from reordering--can someone with more gcc knowledge than I state definitively whether or not it is smart enough to not reorder barriers?
> Should another CPU see initialized_size = 10 but allocated_size < 10 the > ntfs driver will blow up in some places. So does that mean I need a > write barrier, between the two?
As above.
You may also need a read barrier to ensure that they are not speculatively loaded in the wrong order--could someone more knowledgable than I comment on that?
> If yes, do I still need it if I wrap the two settings (and all accesses) > with a spin lock? And in particular with a rw-spinlock? For example: > > write_lock_irqsave(&ntfs_inode->size_lock, flags); > ntfs_inode->allocated_size = 10; > ntfs_inode->initilized_size = 10; > write_unlock_irqrestore(&ntfs_inode->size_lock, flags); > > Do I still need a write barrier or does the spinlock imply it already?
I believe the spinlock implies it.
> Thanks a lot in advance and apologies for the stupid(?) questions...
Not stupid. Concurrency is hard.
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