From: David Sterba <email@example.com> To: Josef Bacik <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH 1/5] btrfs: check rw_devices, not num_devices for restriping Date: Thu, 16 Jan 2020 16:59:56 +0100 Message-ID: <20200116155955.GY3929@twin.jikos.cz> (raw) In-Reply-To: <firstname.lastname@example.org> On Tue, Jan 14, 2020 at 01:07:22PM -0800, Josef Bacik wrote: > >> - num_devices = btrfs_num_devices(fs_info); > >> + /* > >> + * rw_devices can be messed with by rm_device and device replace, so > >> + * take the chunk_mutex to make sure we have a relatively consistent > >> + * view of the fs at this point. > > > > Well, what does 'relatively consistent' mean here? There are enough > > locks and exclusion that device remove or replace should not change the > > value until btrfs_balance ends, no? > > > > Again I don't have the code in front of me, but there's nothing at this point to > stop us from running in at the tail end of device replace or device rm. This should be prevented by the EXCL_OP mechanism, so even the end of device remove or replace will not be running at this time because it cannot even start. > The > mutex keeps us from getting weirdly inflated values when we increment and > decrement at the end of device replace, but there's nothing (that I can > remember) that will stop rw devices from changing right after we check it, thus > relatively. rw_devices is changed in a handful of places on a mounted filesystem, not counting device open/close. Device remove and replace are excluded from running at that time, rw_devices can't change at this point of balance. btrfs_dev_replace_finishing - when removing srcdev, rw_devices-- - when adding the target device as new, rw_devices++ btrfs_rm_device - rw_devices-- btrfs_init_new_device (called by device add) - rw_devices++ So the chunk mutex is either redundant or there's something I'm missing.
next prev parent reply index Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-01-10 16:11 [PATCH 0/5][v3] clean up how we mark block groups read only Josef Bacik 2020-01-10 16:11 ` [PATCH 1/5] btrfs: check rw_devices, not num_devices for restriping Josef Bacik 2020-01-11 9:24 ` Qu Wenruo 2020-01-14 20:56 ` David Sterba 2020-01-14 21:07 ` Josef Bacik 2020-01-16 15:59 ` David Sterba [this message] 2020-01-16 16:25 ` Josef Bacik 2020-01-10 16:11 ` [PATCH 2/5] btrfs: don't pass system_chunk into can_overcommit Josef Bacik 2020-01-14 19:56 ` David Sterba 2020-01-10 16:11 ` [PATCH 3/5] btrfs: kill min_allocable_bytes in inc_block_group_ro Josef Bacik 2020-01-10 16:11 ` [PATCH 4/5] btrfs: fix force usage " Josef Bacik 2020-01-11 6:15 ` Qu Wenruo 2020-01-10 16:11 ` [PATCH 5/5] btrfs: use btrfs_can_overcommit " Josef Bacik
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