From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <48F33543.6070303@domain.hid> Date: Mon, 13 Oct 2008 13:47:15 +0200 From: Jan Kiszka MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Subject: [Xenomai-core] Fast mutexes vs. automatic mode switch List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: xenomai-core Hi, while preparing my reworked fast mutex patches for submission, reviewing them once again, I realized a conception problem that the fast path can introduce: So far every pthread_mutex_lock or rt_mutex_acquire forced the caller into primary mode in case it was in secondary before. Now this will only happen if the mutex is contended! Let's consider the fairly typical use case of a two threads synchronizing a critical section with the help of a mutex. One thread is high-prio, always in primary mode, the other is low-prio, constantly transiting between primary (while holding the lock) and secondary (while interacting with Linux). If the low-prio acquires the uncontended lock, it will now remain in secondary mode thanks to the new mutex fast path. That means if the high-prio requests the lock as well, prio-inheritance will no longer work as the owner is not in the right mode! I guess what we need is mode detection for the caller in the mutex fast path. If the possible new owner is in secondary mode, the syscall path needs to be taken to trigger a migration reliably. That in turn means we need a syscall-less detection of the current execution mode. Any spontaneous ideas on this? Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux