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Wed, 23 Oct 2019 05:57:04 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 5794FAD54; Wed, 23 Oct 2019 09:57:01 +0000 (UTC) Subject: Re: [PATCH 4/5] btrfs: serialize blocking_writers updates To: David Sterba , linux-btrfs@vger.kernel.org References: <2a310858bfa3754f6f7a4d4b7693959b0fdd7005.1571340084.git.dsterba@suse.com> From: Nikolay Borisov Openpgp: preference=signencrypt Autocrypt: addr=nborisov@suse.com; prefer-encrypt=mutual; keydata= mQINBFiKBz4BEADNHZmqwhuN6EAzXj9SpPpH/nSSP8YgfwoOqwrP+JR4pIqRK0AWWeWCSwmZ T7g+RbfPFlmQp+EwFWOtABXlKC54zgSf+uulGwx5JAUFVUIRBmnHOYi/lUiE0yhpnb1KCA7f u/W+DkwGerXqhhe9TvQoGwgCKNfzFPZoM+gZrm+kWv03QLUCr210n4cwaCPJ0Nr9Z3c582xc bCUVbsjt7BN0CFa2BByulrx5xD9sDAYIqfLCcZetAqsTRGxM7LD0kh5WlKzOeAXj5r8DOrU2 GdZS33uKZI/kZJZVytSmZpswDsKhnGzRN1BANGP8sC+WD4eRXajOmNh2HL4P+meO1TlM3GLl EQd2shHFY0qjEo7wxKZI1RyZZ5AgJnSmehrPCyuIyVY210CbMaIKHUIsTqRgY5GaNME24w7h TyyVCy2qAM8fLJ4Vw5bycM/u5xfWm7gyTb9V1TkZ3o1MTrEsrcqFiRrBY94Rs0oQkZvunqia c+NprYSaOG1Cta14o94eMH271Kka/reEwSZkC7T+o9hZ4zi2CcLcY0DXj0qdId7vUKSJjEep c++s8ncFekh1MPhkOgNj8pk17OAESanmDwksmzh1j12lgA5lTFPrJeRNu6/isC2zyZhTwMWs k3LkcTa8ZXxh0RfWAqgx/ogKPk4ZxOXQEZetkEyTFghbRH2BIwARAQABtCNOaWtvbGF5IEJv cmlzb3YgPG5ib3Jpc292QHN1c2UuY29tPokCOAQTAQIAIgUCWIo48QIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQcb6CRuU/KFc0eg/9GLD3wTQz9iZHMFbjiqTCitD7B6dTLV1C ddZVlC8Hm/TophPts1bWZORAmYIihHHI1EIF19+bfIr46pvfTu0yFrJDLOADMDH+Ufzsfy2v HSqqWV/nOSWGXzh8bgg/ncLwrIdEwBQBN9SDS6aqsglagvwFD91UCg/TshLlRxD5BOnuzfzI Leyx2c6YmH7Oa1R4MX9Jo79SaKwdHt2yRN3SochVtxCyafDlZsE/efp21pMiaK1HoCOZTBp5 VzrIP85GATh18pN7YR9CuPxxN0V6IzT7IlhS4Jgj0NXh6vi1DlmKspr+FOevu4RVXqqcNTSS E2rycB2v6cttH21UUdu/0FtMBKh+rv8+yD49FxMYnTi1jwVzr208vDdRU2v7Ij/TxYt/v4O8 V+jNRKy5Fevca/1xroQBICXsNoFLr10X5IjmhAhqIH8Atpz/89ItS3+HWuE4BHB6RRLM0gy8 T7rN6ja+KegOGikp/VTwBlszhvfLhyoyjXI44Tf3oLSFM+8+qG3B7MNBHOt60CQlMkq0fGXd mm4xENl/SSeHsiomdveeq7cNGpHi6i6ntZK33XJLwvyf00PD7tip/GUj0Dic/ZUsoPSTF/mG EpuQiUZs8X2xjK/AS/l3wa4Kz2tlcOKSKpIpna7V1+CMNkNzaCOlbv7QwprAerKYywPCoOSC 7P25Ag0EWIoHPgEQAMiUqvRBZNvPvki34O/dcTodvLSyOmK/MMBDrzN8Cnk302XfnGlW/YAQ csMWISKKSpStc6tmD+2Y0z9WjyRqFr3EGfH1RXSv9Z1vmfPzU42jsdZn667UxrRcVQXUgoKg QYx055Q2FdUeaZSaivoIBD9WtJq/66UPXRRr4H/+Y5FaUZx+gWNGmBT6a0S/GQnHb9g3nonD jmDKGw+YO4P6aEMxyy3k9PstaoiyBXnzQASzdOi39BgWQuZfIQjN0aW+Dm8kOAfT5i/yk59h VV6v3NLHBjHVw9kHli3jwvsizIX9X2W8tb1SefaVxqvqO1132AO8V9CbE1DcVT8fzICvGi42 FoV/k0QOGwq+LmLf0t04Q0csEl+h69ZcqeBSQcIMm/Ir+NorfCr6HjrB6lW7giBkQl6hhomn l1mtDP6MTdbyYzEiBFcwQD4terc7S/8ELRRybWQHQp7sxQM/Lnuhs77MgY/e6c5AVWnMKd/z MKm4ru7A8+8gdHeydrRQSWDaVbfy3Hup0Ia76J9FaolnjB8YLUOJPdhI2vbvNCQ2ipxw3Y3c KhVIpGYqwdvFIiz0Fej7wnJICIrpJs/+XLQHyqcmERn3s/iWwBpeogrx2Lf8AGezqnv9woq7 OSoWlwXDJiUdaqPEB/HmGfqoRRN20jx+OOvuaBMPAPb+aKJyle8zABEBAAGJAh8EGAECAAkF AliKBz4CGwwACgkQcb6CRuU/KFdacg/+M3V3Ti9JYZEiIyVhqs+yHb6NMI1R0kkAmzsGQ1jU zSQUz9AVMR6T7v2fIETTT/f5Oout0+Hi9cY8uLpk8CWno9V9eR/B7Ifs2pAA8lh2nW43FFwp IDiSuDbH6oTLmiGCB206IvSuaQCp1fed8U6yuqGFcnf0ZpJm/sILG2ECdFK9RYnMIaeqlNQm iZicBY2lmlYFBEaMXHoy+K7nbOuizPWdUKoKHq+tmZ3iA+qL5s6Qlm4trH28/fPpFuOmgP8P K+7LpYLNSl1oQUr+WlqilPAuLcCo5Vdl7M7VFLMq4xxY/dY99aZx0ZJQYFx0w/6UkbDdFLzN upT7NIN68lZRucImffiWyN7CjH23X3Tni8bS9ubo7OON68NbPz1YIaYaHmnVQCjDyDXkQoKC R82Vf9mf5slj0Vlpf+/Wpsv/TH8X32ajva37oEQTkWNMsDxyw3aPSps6MaMafcN7k60y2Wk/ TCiLsRHFfMHFY6/lq/c0ZdOsGjgpIK0G0z6et9YU6MaPuKwNY4kBdjPNBwHreucrQVUdqRRm RcxmGC6ohvpqVGfhT48ZPZKZEWM+tZky0mO7bhZYxMXyVjBn4EoNTsXy1et9Y1dU3HVJ8fod 5UqrNrzIQFbdeM0/JqSLrtlTcXKJ7cYFa9ZM2AP7UIN9n1UWxq+OPY9YMOewVfYtL8M= Message-ID: Date: Wed, 23 Oct 2019 12:57:00 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <2a310858bfa3754f6f7a4d4b7693959b0fdd7005.1571340084.git.dsterba@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 17.10.19 г. 22:39 ч., David Sterba wrote: > There's one potential memory ordering problem regarding > eb::blocking_writers, although this hasn't been hit in practice. On TSO > (total store ordering) architectures, the writes will always be seen in > right order. > > In case the calls in btrfs_set_lock_blocking_write are seen in this > (wrong) order on another CPU: > > 0: /* blocking_writers == 0 */ > 1: write_unlock() > 2: blocking_writers = 1 > > btrfs_try_tree_read_lock, btrfs_tree_read_lock_atomic or > btrfs_try_tree_write_lock would have to squeeze all of this between 1: > and 2: This is only a problem for unlocked (optimistic) accesses in those functions. Simply because from its POV the eb->lock doesn't play any role e.g. they don't know about it at all. This implies there needs to be yet another synchronization/ordering mechanism only for blocking_writer. But then further down you say that there are no read side barrier because observing the accesses in a particular order is not important for correctness. Given this I fail to see what this smp_wmb affects ordering. > > - check if blocking_writers is 0 (it is, continue) > - try read lock, read lock or write lock (succeeds after 1:) > - check blocking_writers again (continue) > > All of that assumes that the reordered writes can survive for quite some > time (unlikely if its in the internal store buffer). > > Another point against observing that in practice is that > blocking_writers and the lock are on the same cacheline (64B), so it's > more likely both values are stored in order, or some sort of pending > write flush will update blocking_writers, rwlock before the try lock > happens. Assuming the CPUs work like that and don't hide other > surprises. > > struct extent_buffer { > u64 start; /* 0 8 */ > long unsigned int len; /* 8 8 */ > long unsigned int bflags; /* 16 8 */ > struct btrfs_fs_info * fs_info; /* 24 8 */ > spinlock_t refs_lock; /* 32 4 */ > atomic_t refs; /* 36 4 */ > atomic_t io_pages; /* 40 4 */ > int read_mirror; /* 44 4 */ > struct callback_head callback_head __attribute__((__aligned__(8))); /* 48 16 */ > /* --- cacheline 1 boundary (64 bytes) --- */ > pid_t lock_owner; /* 64 4 */ > int blocking_writers; /* 68 4 */ > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > atomic_t blocking_readers; /* 72 4 */ > bool lock_nested; /* 76 1 */ > > /* XXX 1 byte hole, try to pack */ > > short int log_index; /* 78 2 */ > rwlock_t lock; /* 80 8 */ > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > > wait_queue_head_t write_lock_wq; /* 88 24 */ > wait_queue_head_t read_lock_wq; /* 112 24 */ > /* --- cacheline 2 boundary (128 bytes) was 8 bytes ago --- */ > struct page * pages[16]; /* 136 128 */ > > /* size: 264, cachelines: 5, members: 18 */ > /* sum members: 263, holes: 1, sum holes: 1 */ > /* forced alignments: 1 */ > /* last cacheline: 8 bytes */ > } __attribute__((__aligned__(8))); > > Add the barriers for correctness sake. > > Signed-off-by: David Sterba > --- > fs/btrfs/locking.c | 16 +++++++++++++++- > 1 file changed, 15 insertions(+), 1 deletion(-) > > diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c > index a12f3edc3505..e0e0430577aa 100644 > --- a/fs/btrfs/locking.c > +++ b/fs/btrfs/locking.c > @@ -110,6 +110,18 @@ void btrfs_set_lock_blocking_write(struct extent_buffer *eb) > btrfs_assert_spinning_writers_put(eb); > btrfs_assert_tree_locked(eb); > WRITE_ONCE(eb->blocking_writers, 1); > + /* > + * Writers must be be updated before the lock is released so > + * that other threads see that in order. Otherwise this could > + * theoretically lead to blocking_writers be still set to 1 but > + * this would be unexpected eg. for spinning locks. > + * > + * There are no pairing read barriers for unlocked access as we > + * don't strictly need them for correctness (eg. in > + * btrfs_try_tree_read_lock), and the unlock/lock is an implied > + * barrier in other cases. > + */ > + smp_wmb(); > write_unlock(&eb->lock); That comment sounds to me as if you only care about the readers of blocking_writers _after_ they have acquired the eb::lock for reading. In this case this smp_wmb is pointless because write_unlock/write_lock imply release/acquire semantics. Unlocked readers are not affected by this wmb. > } > } > @@ -316,7 +328,9 @@ void btrfs_tree_unlock(struct extent_buffer *eb) > /* > * We need to order modifying blocking_writers above with > * actually waking up the sleepers to ensure they see the > - * updated value of blocking_writers > + * updated value of blocking_writers. > + * > + * cond_wake_up calls smp_mb > */ > cond_wake_up(&eb->write_lock_wq); > } else { >