9d11001420
From the department of "generic/482 keeps on giving", we bring you another tail update race condition: iclog: S1 C1 +-----------------------+-----------------------+ S2 EOIC Two checkpoints in a single iclog. One is complete, the other just contains the start record and overruns into a new iclog. Timeline: Before S1: Cache flush, log tail = X At S1: Metadata stable, write start record and checkpoint At C1: Write commit record, set NEED_FUA Single iclog checkpoint, so no need for NEED_FLUSH Log tail still = X, so no need for NEED_FLUSH After C1, Before S2: Cache flush, log tail = X At S2: Metadata stable, write start record and checkpoint After S2: Log tail moves to X+1 At EOIC: End of iclog, more journal data to write Releases iclog Not a commit iclog, so no need for NEED_FLUSH Writes log tail X+1 into iclog. At this point, the iclog has tail X+1 and NEED_FUA set. There has been no cache flush for the metadata between X and X+1, and the iclog writes the new tail permanently to the log. THis is sufficient to violate on disk metadata/journal ordering. We have two options here. The first is to detect this case in some manner and ensure that the partial checkpoint write sets NEED_FLUSH when the iclog is already marked NEED_FUA and the log tail changes. This seems somewhat fragile and quite complex to get right, and it doesn't actually make it obvious what underlying problem it is actually addressing from reading the code. The second option seems much cleaner to me, because it is derived directly from the requirements of the C1 commit record in the iclog. That is, when we write this commit record to the iclog, we've guaranteed that the metadata/data ordering is correct for tail update purposes. Hence if we only write the log tail into the iclog for the *first* commit record rather than the log tail at the last release, we guarantee that the log tail does not move past where the the first commit record in the log expects it to be. IOWs, taking the first option means that replay of C1 becomes dependent on future operations doing the right thing, not just the C1 checkpoint itself doing the right thing. This makes log recovery almost impossible to reason about because now we have to take into account what might or might not have happened in the future when looking at checkpoints in the log rather than just having to reconstruct the past... Signed-off-by: Dave Chinner <dchinner@redhat.com> Reviewed-by: Darrick J. Wong <djwong@kernel.org> Signed-off-by: Darrick J. Wong <djwong@kernel.org> |
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| fs | ||
| include | ||
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| ipc | ||
| kernel | ||
| lib | ||
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| samples | ||
| scripts | ||
| security | ||
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| tools | ||
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| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.