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Tue, 30 Jul 2019 12:11:48 +0000 (UTC) Subject: Re: [PATCH v2 0/2] Refactor snapshot vs nocow writers locking To: Valentin Schneider , Catalin Marinas Cc: andrea.parri@amarulasolutions.com, paulmck@linux.ibm.com, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org References: <20190719083949.5351-1-nborisov@suse.com> <20190729153319.GH2368@arrakis.emea.arm.com> <60eda0ab-08b3-de82-5b06-98386ee1928f@arm.com> <69ef76a2-ebd6-956e-c611-2e742606ed95@arm.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: <356e1044-b676-1028-3b80-a922acfae5b2@suse.com> Date: Tue, 30 Jul 2019 15:11:46 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <69ef76a2-ebd6-956e-c611-2e742606ed95@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 30.07.19 г. 14:03 ч., Valentin Schneider wrote: > On 29/07/2019 17:32, Valentin Schneider wrote: >> On 29/07/2019 16:33, Catalin Marinas wrote: > [...] >>> I'd say that's one of the pitfalls of PlusCal. The above is executed >>> atomically, so you'd have the lock_state read and updated in the same >>> action. Looking at the C patches, there is an >>> atomic_read(&lock->readers) followed by a >>> percpu_counter_inc(&lock->writers). Between these two, you can have >>> "readers" becoming non-zero via a different CPU. >>> >>> My suggestion would be to use procedures with labels to express the >>> non-atomicity of such sequences. >>> >> > > FYI, with a very simple and stupid modification of the spec: > > ----->8----- > macro ReadUnlock() > { > reader_count := reader_count - 1; > \* Condition variable signal is "implicit" here > } > > macro WriteUnlock() > { > writer_count := writer_count - 1; > \* Ditto on the cond var > } > > procedure ReadLock() > { > add: > reader_count := reader_count + 1; > lock: > await writer_count = 0; > return; > } > > procedure WriteLock() > { > add: > writer_count := writer_count + 1; > lock: > await reader_count = 0; > return; > }; > -----8<----- > > it's quite easy to trigger the case Paul pointed out in [1]: Yes, however, there was a bug in the original posting, in that btrfs_drw_try_write_lock should have called btrfs_drw_write_unlock instead of btrfs_drw_read_unlock if it sees that readers incremented while it has already incremented its percpu counter. Additionally the implementation doesn't await with the respective variable incremented. Is there a way to express something along the lines of : > procedure WriteLock() > { > add: > writer_count := writer_count + 1; > lock: > await reader_count = 0; If we are about to wait then also decrement writer_count? I guess the correct way to specify it would be: procedure WriteLock() { writer_count := writer_count + 1; await reader_count = 0; return; }; Because the implementation (by using barriers and percpu counters ensures all of this happens as one atomic step?) E.g. before going to sleep we decrement the write unlock. > return; > }; > > ----->8----- > Error: Deadlock reached. > Error: The behavior up to this point is: > State 1: > /\ stack = (<> :> <<>> @@ <> :> <<>>) > /\ pc = (<> :> "loop" @@ <> :> "loop_") > /\ writer_count = 0 > /\ reader_count = 0 > /\ lock_state = "idle" > > State 2: > /\ stack = ( <> :> <<>> @@ > <> :> <<[pc |-> "write_cs", procedure |-> "WriteLock"]>> ) > /\ pc = (<> :> "loop" @@ <> :> "add") > /\ writer_count = 0 > /\ reader_count = 0 > /\ lock_state = "idle" > > State 3: > /\ stack = ( <> :> <<>> @@ > <> :> <<[pc |-> "write_cs", procedure |-> "WriteLock"]>> ) > /\ pc = (<> :> "loop" @@ <> :> "lock") > /\ writer_count = 1 > /\ reader_count = 0 > /\ lock_state = "idle" > > State 4: > /\ stack = ( <> :> <<[pc |-> "read_cs", procedure |-> "ReadLock"]>> @@ > <> :> <<[pc |-> "write_cs", procedure |-> "WriteLock"]>> ) > /\ pc = (<> :> "add_" @@ <> :> "lock") > /\ writer_count = 1 > /\ reader_count = 0 > /\ lock_state = "idle" > > State 5: > /\ stack = ( <> :> <<[pc |-> "read_cs", procedure |-> "ReadLock"]>> @@ > <> :> <<[pc |-> "write_cs", procedure |-> "WriteLock"]>> ) > /\ pc = (<> :> "lock_" @@ <> :> "lock") > /\ writer_count = 1 > /\ reader_count = 1 > /\ lock_state = "idle" > -----8<----- > > Which I think is pretty cool considering the effort that was required > (read: not much). > > [1]: https://lore.kernel.org/lkml/20190607105251.GB28207@linux.ibm.com/ >