Messages in this thread |  | | Date | Fri, 14 Feb 2014 12:21:52 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH] sched/core: Create new task with twice disabled preemption |
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On Fri, Feb 14, 2014 at 11:16:09AM +0000, Kirill Tkhai wrote: > В Птн, 14/02/2014 в 10:52 +0000, Catalin Marinas пишет: > > On Thu, Feb 13, 2014 at 09:32:22PM +0400, Kirill Tkhai wrote: > > > Look at ARM64's finish_arch_post_lock_switch(). It looks a task > > > must to not be preempted between switch_mm() and this function. > > > But in case of new task this is possible. > > > > We had a thread about this at the end of last year: > > > > https://lkml.org/lkml/2013/11/15/82 > > > > There is indeed a problem on arm64, something like this (and I think > > s390 also needs a fix): > > > > 1. switch_mm() via check_and_switch_context() defers the actual mm > > switch by setting TIF_SWITCH_MM > > 2. the context switch is considered 'done' by the kernel before > > finish_arch_post_lock_switch() and therefore we can be preempted to a > > new thread before finish_arch_post_lock_switch() > > 3. The new thread has the same mm as the preempted thread but we > > actually missed the mm switching in finish_arch_post_lock_switch() > > because TIF_SWITCH_MM is per thread rather than mm > > > > > This is the problem I tried to solve. I don't know arm64, and I can't > > > say how it is serious. > > > > Have you managed to reproduce this? I don't say it doesn't exist, but I > > want to make sure that any patch actually fixes it. > > No, I have not tried. I found this place while analysing scheduler code. > But it seems with the RT technics suggested previous message it's quite > possible.
Now I think I confused myself. Looking through the __schedule() code, context_switch() and therefore finish_arch_post_lock_switch() are called with preemption disabled. So the scenario above cannot exist since the current thread cannot be preempted between switch_mm() and finish_arch_post_lock_switch(). Do I miss anything?
Now I get your point about schedule_tail() which calls finish_task_switch() with a preempt count of 0. I'll get back to your original patch.
Thanks.
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