Messages in this thread | | | Date | Thu, 27 Feb 2020 08:37:00 -0500 | From | Joel Fernandes <> | Subject | Re: [PATCH RFC] ext4: fix potential race between online resizing and write operations |
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Sorry for slightly late reply.
On Tue, Feb 25, 2020 at 07:54:00PM +0100, Uladzislau Rezki wrote: > > > > > I was thinking a 2 fold approach (just thinking out loud..): > > > > > > > > > > If kfree_call_rcu() is called in atomic context or in any rcu reader, then > > > > > use GFP_ATOMIC to grow an rcu_head wrapper on the atomic memory pool and > > > > > queue that. > > > > > > > > I am not sure if that is acceptable, i mean what to do when GFP_ATOMIC > > > gets failed in atomic context? Or we can just consider it as out of > > > memory and another variant is to say that headless object can be called > > > from preemptible context only. > > > > Yes that makes sense, and we can always put disclaimer in the API's comments > > saying if this object is expected to be freed a lot, then don't use the > > headless-API to be extra safe. > > > Agree. > > > BTW, GFP_ATOMIC the documentation says if GFP_ATOMIC reserves are depleted, > > the kernel can even panic some times, so if GFP_ATOMIC allocation fails, then > > there seems to be bigger problems in the system any way. I would say let us > > write a patch to allocate there and see what the -mm guys think. > > > OK. It might be that they can offer something if they do not like our > approach. I will try to compose something and send the patch to see. > The tree.c implementation is almost done, whereas tiny one is on hold. > > I think we should support batching as well as bulk interface there. > Another way is to workaround head-less object, just to attach the head > dynamically using kmalloc() and then call_rcu() but then it will not be > a fair headless support :) > > What is your view?
This kind of "head" will require backpointers to the original object as well right? And still wouldn't solve the "what if we run out of GFP_ATOMIC reserves". But let me know in a code snippet if possible about what you mean.
> > > > > Otherwise, grow an rcu_head on the stack of kfree_call_rcu() and call > > > > > synchronize_rcu() inline with it. > > > > > > > > > > > > > What do you mean here, Joel? "grow an rcu_head on the stack"? > > > > By "grow on the stack", use the compiler-allocated rcu_head on the > > kfree_rcu() caller's stack. > > > > I meant here to say, if we are not in atomic context, then we use regular > > GFP_KERNEL allocation, and if that fails, then we just use the stack's > > rcu_head and call synchronize_rcu() or even synchronize_rcu_expedited since > > the allocation failure would mean the need for RCU to free some memory is > > probably great. > > > Ah, i got it. I thought you meant something like recursion and then > unwinding the stack back somehow :)
Yeah something like that :) Use the compiler allocated space which you wouldn't run out of unless stack overflows.
> > > As for "task_struct's rcu_read_lock_nesting". Will it be enough just > > > have a look at preempt_count of current process? If we have for example > > > nested rcu_read_locks: > > > > > > <snip> > > > rcu_read_lock() > > > rcu_read_lock() > > > rcu_read_lock() > > > <snip> > > > > > > the counter would be 3. > > > > No, because preempt_count is not incremented during rcu_read_lock(). RCU > > reader sections can be preempted, they just cannot goto sleep in a reader > > section (unless the kernel is RT). > > > So in CONFIG_PREEMPT kernel we can identify if we are in atomic or not by > using rcu_preempt_depth() and in_atomic(). When it comes to !CONFIG_PREEMPT > then we skip it and consider as atomic. Something like: > > <snip> > static bool is_current_in_atomic()
Would be good to change this to is_current_in_rcu_reader() since rcu_preempt_depth() does not imply atomicity.
> { > #ifdef CONFIG_PREEMPT_RCU > if (!rcu_preempt_depth() && !in_atomic()) > return false;
I think use if (!rcu_preempt_depth() && preemptible()) here.
preemptible() checks for IRQ disabled section as well.
> #endif > > return true;
Otherwise LGTM.
thanks!
- Joel
> } > <snip>
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