Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> The TIF_RCU_QS thing is just a fancy way for synchronize_rcu() 
> (being executed on some other CPU not doing RT work) to 
> intelligently wait for the remote (RT work doing) CPU to finish 
> executing kernel code, without polling or so.

it's basically a cheap IPI being inserted on the remote CPU.

We need the TIF_RCU_QS callback not just to wait intelligently, but 
mainly to elapse a grace period, otherwise synchronize_rcu() might not 
ever make progress: think a SCHED_FIFO task doing some kernel work, 
synchronize_rcu() stumbling upon it - but the SCHED_FIFO task 
otherwise never scheduling and never getting any timer irqs either, 
and thus never entering quiescent state.

(Cc:-ed Paul too, he might be interested in this as well.)

Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/