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Date: Thu, 8 Oct 2015 21:32:49 -0700 From: Josef Bacik <jbacik@...com> To: Kosuke Tatsukawa <tatsu@...jp.nec.com>, Chris Mason <clm@...com>, David Sterba <dsterba@...e.com> CC: "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] btrfs: fix waitqueue_active without memory barrier in btrfs On 10/08/2015 05:35 PM, Kosuke Tatsukawa wrote: > btrfs_bio_counter_sub() seems to be missing a memory barrier which might > cause the waker to not notice the waiter and miss sending a wake_up as > in the following figure. > > btrfs_bio_counter_sub btrfs_rm_dev_replace_blocked > ------------------------------------------------------------------------ > if (waitqueue_active(&fs_info->replace_wait)) > /* The CPU might reorder the test for > the waitqueue up here, before > prior writes complete */ > /* wait_event */ > /* __wait_event */ > /* ___wait_event */ > long __int = prepare_to_wait_event(&wq, > &__wait, state); > if (!percpu_counter_sum(&fs_info->bio_counter)) > percpu_counter_sub(&fs_info->bio_counter, > amount); > schedule() percpu_counter_sub can't be reordered, in its most basic form it does preempt_disable/enable which in its most basic form does barrier(). Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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