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List:       linux-kernel
Subject:    Re: [Patch] shm bug introduced with pagecache in 2.3.11
From:       Linus Torvalds <torvalds () transmeta ! com>
Date:       1999-11-25 17:57:10
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On Thu, 25 Nov 1999, Andrea Arcangeli wrote:
> 
> But I am not been trivially succesfully in fixing the shm swapin races
> with "read pte with spinlock acquired, release the spinlock, reacquire the
> spinlock and the check if the pte is changed". That's why I added the
> spinlock.

I was planning on just depending on the sanity of the page cache on this
one. Basically we have two cases:
 - paging in something new ("no_page"), for which the final test is just
   to test that the page table is still zero (ie we don't even need to
   save any "original" value).
 - paging in something old ("swap_page"), in wich case the final test is
   to check that the pte is still the same as swp_entry_to_pte(entry).

(we have the rw_page case too, but that is already protected by the
spinlock appropriately as far as I can tell, exactly because it already
has the same race wrt page_out rather than page_in).

No, I haven't checked the exact details. Maybe it's worse than I envision,
but it _looks_ like adding a simple spinlock and the test. If the test
fails, we just return and expect the fault to happen again..

		Linus


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