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McKenney" , "Theodore Y. Ts'o" , Ext4 Developers List , Suraj Jitindar Singh , LKML Subject: Re: [PATCH RFC] ext4: fix potential race between online resizing and write operations Message-ID: <20200225185400.GA27919@pc636> References: <20200217193314.GA12604@mit.edu> <20200218170857.GA28774@pc636> <20200220045233.GC476845@mit.edu> <20200221003035.GC2935@paulmck-ThinkPad-P72> <20200221131455.GA4904@pc636> <20200221202250.GK2935@paulmck-ThinkPad-P72> <20200222222415.GC191380@google.com> <20200223011018.GB2935@paulmck-ThinkPad-P72> <20200224174030.GA22138@pc636> <20200225020705.GA253171@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200225020705.GA253171@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > > > > 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? > > > > 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 :) > > > > Use preemptible() andr task_struct's rcu_read_lock_nesting to differentiate > > > > between the 2 cases. > > > > > > If the current context is preemptable then we can inline synchronize_rcu() > > together with freeing to handle such corner case, i mean when we are run > > out of memory. > > Ah yes, exactly what I mean. > OK. > > 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: > > > > > > rcu_read_lock() > > rcu_read_lock() > > rcu_read_lock() > > > > > > 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: static bool is_current_in_atomic() { #ifdef CONFIG_PREEMPT_RCU if (!rcu_preempt_depth() && !in_atomic()) return false; #endif return true; } Thanks! -- Vlad Rezki