Messages in this thread | | | Date | Mon, 21 Mar 2016 16:33:05 +0100 | From | Jan Kara <> | Subject | Re: [RFC][PATCH v5 1/2] printk: Make printk() completely async |
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On Mon 21-03-16 23:58:32, Sergey Senozhatsky wrote: > Hello Jan, > > On (03/21/16 15:32), Jan Kara wrote: > [..] > > > we have 2 spin locks in vprintk_emit() -- logbuf_lock and sem->lock. and N > > > CPUs can concurrently lockup on those two locks, which already makes a > > > single static pointer in spiun_dump() questionable. > > > > > > logbug_lock *theoretically* can detect and handle recursive printk()s, > > > there is no way to catch sem->lock spin_dump() at the moment (but that's > > > not the point). > > > > > > there are 2 new spin locks in vprintk_emit() -- p->pi_lock and rq->lock. > > > > Actually, this is not true. These locks are already in vprintk_emit() via > > the up(&console_sem) call from console_unlock() since up() can call > > wake_up() which needs the same locks as wake_up_process(). > > true. I meant new locks (which come with printk_kthread). the already > existing locks and problems were not addressed. > > > And by calling wake_up_process() under logbuf_lock, you actually introduce > > recursion issues for printk_deferred() messages which are supposed to be > > working from under rq->lock and similar. So I think you have to keep this > > section outside of logbuf_lock. > > hm, in_sched (printk_deferred()) messages are printed by > irq work->wake_up_klogd_work_func(), not by wake_up_process() > from vprintk_emit(). or am I missing something?
Think of following:
some function printk() vprintk_emit() spin_lock(&logbuf_lock); ... wake_up_process() printk_deferred() vprintk_emit() -> recursion on logbuf_lock
Previously scheduler code was allowed to call printk_deferred() wherever it wanted...
So we are not supposed to call into the scheduler from under logbuf_lock...
Honza -- Jan Kara <jack@suse.com> SUSE Labs, CR
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