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The swevent_htable::recursion counter is used to avoid creating an swevent while an event is processed to avoid recursion. The counter is per-CPU and preemption must be disabled to have a stable counter. perf_pending_task() disables preemption to access the counter and then signal. This is problematic on PREEMPT_RT because sending a signal uses a spinlock_t which must not be acquired in atomic on PREEMPT_RT because it becomes a sleeping lock. The atomic context can be avoided by moving the counter into the task_struct. There is a 4 byte hole between futex_state (usually always on) and the following perf pointer (perf_event_ctxp). After the recursion lost some weight it fits perfectly. Move swevent_htable::recursion into task_struct. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: Marco Elver <elver@google.com> Link: https://lore.kernel.org/r/20240704170424.1466941-6-bigeasy@linutronix.de
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.