Lee Revell wrote:
On Wed, 2006-01-25 at 16:49 -0800, Howard Chu wrote:
Basic "fairness" isn't the issue. Fairness is concerned with which of
*multiple waiting threads* gets the mutex, and that is certainly
irrelevant here. The issue is that the releasing thread should not be
a candidate.
You seem to be making 2 controversial assertions:
1. pthread_mutex_unlock must cause an immediate reschedule if other
threads are blocked on the mutex, and
2. if the unlocking thread immediately tries to relock the mutex,
another thread must get it first
I disagree with #1, which makes #2 irrelevant. It would lead to
obviously incorrect behavior, pthread_mutex_unlock would no longer be an
RT safe operation for example.
Actually no, I see that #1 is unnecessary, and already acknowledged as such
http://groups.google.com/group/fa.linux.kernel/msg/89da66017d53d496
But #2 still holds.
Also consider a SCHED_FIFO policy - static priorities and the scheduler
always runs the highest priority runnable thread - under your
interpretation of POSIX a high priority thread unlocking a mutex would
require the scheduler to run a lower priority thread which violates
SCHED_FIFO semantics
See the Mutex Initialization Scheduling Attributes section which
specifically addresses priority inversion:
http://www.opengroup.org/onlinepubs/000095399/xrat/xsh_chap02.html#tag_03_02_09
If point #2 were not true, then there would be no need to bother with
any of that. Instead that text ends with "it is important that
IEEE Std 1003.1-2001 provide these interfaces for those cases in which
it is necessary."
--
-- Howard Chu
Chief Architect, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc
OpenLDAP Core Team http://www.openldap.org/project/
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