On Thu, 8 Nov 2007, Bernhard Kuhn wrote: > Luís Henriques wrote: > > > Maybe I didn't completely understood your question, but... if there is a > > problem with SMP efficiency (I do not really know, but believe you :) ) with > > priority ceiling (PC) implementation, this problem should have been already > > identified (and possibly solved) in the current PI implementation, right? > > To make it clear: I'm not aware of any efficiency issues with > the current PI implementation :-) > > > The (potential) efficiency issue with PC on SMP derives from the > dead-lock avoidance mechanism that you mentioned: If a task > locks a resource, then it's priority is raised to the ceiling > (which is typically the highest priority of all task sharing > that resource). Once that happens, no other task (sharing the > same resource) could then lock another resource that potentially > leads to a dead-lock situation. > > Example: > > * A low priority Task A shall share resources X and Y > with a high priority Task B. > > * Task A shall execute the following fraction of code: > > 1. lock(X) > 2. lock(Y) > > * Task B shall execute the following fraction of code: > > 1. lock(Y) > 2. lock(X) Classic ABBA deadlock :) Priority ceiling is not designed to allow that. It's just a coincidence that it does not lead to a deadlock on UP, but thats not a semantical property. Thanks, tglx
