Messages in this thread | | | From | Rusty Russell <> | Subject | Re: [PATCH] Alternate futex non-page-pinning and COW fix | Date | Thu, 04 Sep 2003 12:43:04 +1000 |
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In message <Pine.LNX.4.44.0309031201170.31853-100000@home.osdl.org> you write: > Private mappings that haven't been broken by COW (and a read-only mapping > never will be) will see updates as they happen on the file that backs it. > That's the fundamental difference between "mmap(MAP_PRIVATE)" and > "read()".
Right, so it would be consistent for someone doing a FUTEX_WAIT on an "intact" (not broken by COW) private mapping to see a FUTEX_WAKE done on that file.
However, Jamie's futex code will see !VM_SHARED on the mapping, and compare futexes by mm + uaddr (rather than inode + file offset), so this is NOT the case. Using VM_MAYSHARE instead would make the MAP_SHARED readonly case work as above, though.
The way futexes are used now, they're both "don't care". If you have a private mapping or read-only mapping, you'll never get woken by others with the same file mapped writable shared, but WTF were you waiting for a futex if the mapping is private anyway: the lock acquisition won't work (and sleeping forever is easier to debug than two tasks getting the lock).
Oh no, I think I'm starting to understand the VM a little.
Ick.
Rusty. -- Anyone who quotes me in their sig is an idiot. -- Rusty Russell. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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