Messages in this thread | | | Subject | write barriers - Was: Re: [RFC][PATCH] problem of cont_prepare_write() | From | Anton Altaparmakov <> | Date | Thu, 13 Jan 2005 14:47:07 +0000 |
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Hi,
Sorry to pickup such a long thread but here goes anyway...
On Mon, 2004-11-22 at 15:43 -0800, Andrew Morton wrote: > Anton Altaparmakov <aia21@cam.ac.uk> wrote: > > > > I always have a flush_dcache_page(page) between the memset() and > > the SetPageUptodate() so I don't need the barrier, right? Or does the > > flush_dcache_page() not imply ordering? > > err, flush_dcache_page() might indeed provide a write barrier on all > architectures which need write barriers. Then again it might not ;) It's > not intended that this be the case.
What about if the page is unmapped between the memset() and the SetPageUptodate()? Does that imply ordering? I.e. do I need a write barrier in code like this:
memset(page_address(page), 0, blah); flush_dcache_page(page); kunmap(page); SetPageUptodate(page);
And a more general question:
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;
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?
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?
Thanks a lot in advance and apologies for the stupid(?) questions...
Best regards,
Anton -- Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @) Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/
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