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[ Upstream commit c2489bb7e6 ]
There is race issue when concurrently splice_read main trace_pipe and
per_cpu trace_pipes which will result in data read out being different
from what actually writen.
As suggested by Steven:
> I believe we should add a ref count to trace_pipe and the per_cpu
> trace_pipes, where if they are opened, nothing else can read it.
>
> Opening trace_pipe locks all per_cpu ref counts, if any of them are
> open, then the trace_pipe open will fail (and releases any ref counts
> it had taken).
>
> Opening a per_cpu trace_pipe will up the ref count for just that
> CPU buffer. This will allow multiple tasks to read different per_cpu
> trace_pipe files, but will prevent the main trace_pipe file from
> being opened.
But because we only need to know whether per_cpu trace_pipe is open or
not, using a cpumask instead of using ref count may be easier.
After this patch, users will find that:
- Main trace_pipe can be opened by only one user, and if it is
opened, all per_cpu trace_pipes cannot be opened;
- Per_cpu trace_pipes can be opened by multiple users, but each per_cpu
trace_pipe can only be opened by one user. And if one of them is
opened, main trace_pipe cannot be opened.
Link: https://lore.kernel.org/linux-trace-kernel/20230818022645.1948314-1-zhengyejian1@huawei.com
Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
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.