On Fri, 28 Aug 2009 18:34:00 GMT tip-bot for Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > Commit-ID: 7285dd7fd375763bfb8ab1ac9cf3f1206f503c16 > Gitweb: http://git.kernel.org/tip/7285dd7fd375763bfb8ab1ac9cf3f1206f503c16 > Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > AuthorDate: Fri, 28 Aug 2009 20:25:24 +0200 > Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > CommitDate: Fri, 28 Aug 2009 20:25:24 +0200 > > clocksource: Resolve cpu hotplug dead lock with TSC unstable > > Martin Schwidefsky analyzed it: > To register a clocksource the clocksource_mutex is acquired and if > necessary timekeeping_notify is called to install the clocksource as > the timekeeper clock. timekeeping_notify uses stop_machine which needs > to take cpu_add_remove_lock mutex. > Starting a new cpu is done with the cpu_add_remove_lock mutex held. > native_cpu_up checks the tsc of the new cpu and if the tsc is no good > clocksource_change_rating is called. Which needs the clocksource_mutex > and the deadlock is complete. > > The solution is to replace the TSC via the clocksource watchdog > mechanism. Mark the TSC as unstable and schedule the watchdog work so > it gets removed in the watchdog thread context. > > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > LKML-Reference: <new-submission> > Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx> > Cc: John Stultz <johnstul@xxxxxxxxxx> Ah, very good. I've been going round in circles to find a solution that allows to downgrade the tsc rating when the second cpu is enabled. Could not find a solution. Your approach changes semantics slightly: the tsc clock will continue with its old rating for a while until the watchdog will do the downgrade. If that is acceptable then this is a good solution. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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