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The WARN_ON_ONCE() statement in riscv's huge_pte_alloc() is susceptible
to false positives, because the pte is read twice at the C language
level, locklessly, within the same conditional statement. Depending on
compiler behavior, this can lead to generated machine code that actually
reads the pte just once, or twice. Reading twice will expose the code to
changing pte values and cause incorrect behavior.
In [1], similar code actually caused a kernel crash on 64-bit x86, when
using clang to build the kernel, but only after the conversion from *pte
reads, to ptep_get(pte). The latter uses READ_ONCE(), which forced a
double read of *pte.
Rather than waiting for the upcoming ptep_get() conversion, just convert
this part of the code now, but in a way that avoids the above problem:
take a single snapshot of the pte before using it in the WARN
conditional.
As expected, this preparatory step does not actually change the
generated code ("make mm/hugetlbpage.s"), on riscv64, when using a gcc
12.2 cross compiler.
[1] https://lore.kernel.org/20230630013203.1955064-1-jhubbard@nvidia.com
Suggested-by: James Houghton <jthoughton@google.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20230703190044.311730-1-jhubbard@nvidia.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
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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.