CVE-2024-50032: rcu/nocb: Fix rcuog wake-up from offline softirq

In the Linux kernel, the following vulnerability has been resolved:

rcu/nocb: Fix rcuog wake-up from offline softirq

After a CPU has set itself offline and before it eventually calls rcutreereportcpudead(), there are still opportunities for callbacks to be enqueued, for example from a softirq. When that happens on NOCB, the rcuog wake-up is deferred through an IPI to an online CPU in order not to call into the scheduler and risk arming the RT-bandwidth after hrtimers have been migrated out and disabled.

But performing a synchronized IPI from a softirq is buggy as reported in the following scenario:

WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smpcallfunctionsingle Modules linked in: rcutorture torture CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139f93209d1 #1 Stopper: multicpustop+0x0/0x320 <- stopcpus+0xd0/0x120 RIP: 0010:smpcallfunctionsingle <IRQ> swakeuponeonline callrcunocbwake callrcucommon ? rcutortureoneread calltimerfn runtimers runtimersoftirq handlesoftirqs irqexitrcu ? tickhandleperiodic sysvecapictimerinterrupt </IRQ>

Fix this with forcing deferred rcuog wake up through the NOCB timer when the CPU is offline. The actual wake up will happen from rcutreereportcpudead().