LinuxKPI: Run irq_work callbacks under RCU protection - #2385
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LinuxKPI: Run irq_work callbacks under RCU protection#2385JustAnotherHumanBeing wants to merge 1 commit into
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Linux executes irq_work callbacks from an interrupt context that provides implicit RCU read-side protection. LinuxKPI dispatches these callbacks through a taskqueue, so provide equivalent protection explicitly. This prevents an RCU grace period from completing while an irq_work callback is still using an RCU-protected object. Suggested by: wulf Tested by: JustAnotherHumanBeing Signed-off-by: JustAnotherHumanBeing <oleglelchuk@gmail.com>
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Problem
Linux
irq_workcallbacks execute in interrupt context, which providesimplicit RCU read-side protection on non-PREEMPT_RT kernels. LinuxKPI
dispatches these callbacks through a taskqueue without equivalent RCU
protection. A grace period can therefore complete while a callback is still
using an RCU-protected object. This produced the i915/GuC use-after-free
reported in freebsd/drm-kmod#489.
Change
Wrap
irqw->func(irqw)inrcu_read_lock()andrcu_read_unlock(). Thecurrent taskqueue selection is unchanged.
The LinuxKPI-level approach was proposed by @wulf7 in
freebsd/drm-kmod#489 (comment). He
subsequently confirmed that the RCU correction is independent of the
taskqueue-selection change discussed there:
freebsd/drm-kmod#489 (comment).
Testing
irq_work/RCU test failed deterministically on theunpatched kernel:
synchronize_rcu()returned while the callback remainedactive.
GENERICkernel and module build from currentmaincompletedsuccessfully with
-Werror.drm/i915: Extend RCU protection in signal_irq_work() [FreeBSD] drm-kmod#491. Thirty controlled i915 recovery cycles were requested
while a Mesa workload was active. No panic, use-after-free, or relevant
INVARIANTS/WITNESS diagnostic occurred, and normal rendering succeeded
afterward.
Related: freebsd/drm-kmod#489
Base-system alternative to: freebsd/drm-kmod#491