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The RSEQ critical section mechanism only clears the event mask when a critical section is registered, otherwise it is stale and collects bits. That means once a critical section is installed the first invocation of that code when TIF_NOTIFY_RESUME is set will abort the critical section, even when the TIF bit was not raised by the rseq preempt/migrate/signal helpers. This also has a performance implication because TIF_NOTIFY_RESUME is a multiplexing TIF bit, which is utilized by quite some infrastructure. That means every invocation of __rseq_notify_resume() goes unconditionally through the heavy lifting of user space access and consistency checks even if there is no reason to do so. Keeping the stale event mask around when exiting to user space also prevents it from being utilized by the upcoming time slice extension mechanism. Avoid this by reading and clearing the event mask before doing the user space critical section access with interrupts or preemption disabled, which ensures that the read and clear operation is CPU local atomic versus scheduling and the membarrier IPI. This is correct as after re-enabling interrupts/preemption any relevant event will set the bit again and raise TIF_NOTIFY_RESUME, which makes the user space exit code take another round of TIF bit clearing. If the event mask was non-zero, invoke the slow path. On debug kernels the slow path is invoked unconditionally and the result of the event mask evaluation is handed in. Add a exit path check after the TIF bit loop, which validates on debug kernels that the event mask is zero before exiting to user space. While at it reword the convoluted comment why the pt_regs pointer can be NULL under certain circumstances. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Ingo Molnar <mingo@kernel.org> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Link: https://patch.msgid.link/20251027084306.022571576@linutronix.de |
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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 reStructuredText 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.