On Tue, 27 Feb 2007, Kostik Belousov wrote:
On Tue, Feb 27, 2007 at 02:51:41AM +0000, Greg Lehey wrote:
grog 2007-02-27 02:51:41 UTC
FreeBSD src repository
Modified files:
share/man/man9 sleep.9
Log:
Add warning about deadlocks created by use of wakeup_one.
+.Fn wakeup_one
+does not work reliably if more than one process is sleeping on the same
+address;
+in this case it is possible for an unrelated process to be woken.
+This process will ignore the wakeup, and the correct process will never be
+woken.
+.Pp
Is this a bug (that should be fixed), or some fundamental issue with
wakeup_one() ?
I'm also confused about this addition to the man page -- the sleep/wakeup
model is premised on using a synchronization name space that we, for
convenience, align with the possible pointer value name space. In general,
the approach works because we tie memory ownership to use of a synchronization
address. It sounds like wakeup_one() works entirely reliably, but that some
piece of kernel code is sleeping on an address associated with memory it
doesn't own. This is not a bug on wakeup_one(), it is a bug in a consumer of
the sleep(9) APIs. I'd like to see this change backed out, and replaced with
a note explicitly describing how the uniqueness of sleep/wakeup addresses is
tied to memory allocation, and that improper use of the APIs can result in
consumers behaving unreliably.
Robert N M Watson
Computer Laboratory
University of Cambridge
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