rcu-tasks: Fix race in schedule and flush work

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

rcu-tasks: Fix race in schedule and flush work

While booting secondary CPUs, cpus_read_[lock/unlock] is not keeping

online cpumask stable. The transient online mask results in below

calltrace.

[ 0.324121] CPU1: Booted secondary processor 0x0000000001 [0x410fd083]

[ 0.346652] Detected PIPT I-cache on CPU2

[ 0.347212] CPU2: Booted secondary processor 0x0000000002 [0x410fd083]

[ 0.377255] Detected PIPT I-cache on CPU3

[ 0.377823] CPU3: Booted secondary processor 0x0000000003 [0x410fd083]

[ 0.379040] ------------[ cut here ]------------

[ 0.383662] WARNING: CPU: 0 PID: 10 at kernel/workqueue.c:3084 __flush_work+0x12c/0x138

[ 0.384850] Modules linked in:

[ 0.385403] CPU: 0 PID: 10 Comm: rcu_tasks_rude_ Not tainted 5.17.0-rc3-v8+ #13

[ 0.386473] Hardware name: Raspberry Pi 4 Model B Rev 1.4 (DT)

[ 0.387289] pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)

[ 0.388308] pc : __flush_work+0x12c/0x138

[ 0.388970] lr : __flush_work+0x80/0x138

[ 0.389620] sp : ffffffc00aaf3c60

[ 0.390139] x29: ffffffc00aaf3d20 x28: ffffffc009c16af0 x27: ffffff80f761df48

[ 0.391316] x26: 0000000000000004 x25: 0000000000000003 x24: 0000000000000100

[ 0.392493] x23: ffffffffffffffff x22: ffffffc009c16b10 x21: ffffffc009c16b28

[ 0.393668] x20: ffffffc009e53861 x19: ffffff80f77fbf40 x18: 00000000d744fcc9

[ 0.394842] x17: 000000000000000b x16: 00000000000001c2 x15: ffffffc009e57550

[ 0.396016] x14: 0000000000000000 x13: ffffffffffffffff x12: 0000000100000000

[ 0.397190] x11: 0000000000000462 x10: ffffff8040258008 x9 : 0000000100000000

[ 0.398364] x8 : 0000000000000000 x7 : ffffffc0093c8bf4 x6 : 0000000000000000

[ 0.399538] x5 : 0000000000000000 x4 : ffffffc00a976e40 x3 : ffffffc00810444c

[ 0.400711] x2 : 0000000000000004 x1 : 0000000000000000 x0 : 0000000000000000

[ 0.401886] Call trace:

[ 0.402309] __flush_work+0x12c/0x138

[ 0.402941] schedule_on_each_cpu+0x228/0x278

[ 0.403693] rcu_tasks_rude_wait_gp+0x130/0x144

[ 0.404502] rcu_tasks_kthread+0x220/0x254

[ 0.405264] kthread+0x174/0x1ac

[ 0.405837] ret_from_fork+0x10/0x20

[ 0.406456] irq event stamp: 102

[ 0.406966] hardirqs last enabled at (101): [<ffffffc0093c8468>] _raw_spin_unlock_irq+0x78/0xb4

[ 0.408304] hardirqs last disabled at (102): [<ffffffc0093b8270>] el1_dbg+0x24/0x5c

[ 0.409410] softirqs last enabled at (54): [<ffffffc0081b80c8>] local_bh_enable+0xc/0x2c

[ 0.410645] softirqs last disabled at (50): [<ffffffc0081b809c>] local_bh_disable+0xc/0x2c

[ 0.411890] ---[ end trace 0000000000000000 ]---

[ 0.413000] smp: Brought up 1 node, 4 CPUs

[ 0.413762] SMP: Total of 4 processors activated.

[ 0.414566] CPU features: detected: 32-bit EL0 Support

[ 0.415414] CPU features: detected: 32-bit EL1 Support

[ 0.416278] CPU features: detected: CRC32 instructions

[ 0.447021] Callback from call_rcu_tasks_rude() invoked.

[ 0.506693] Callback from call_rcu_tasks() invoked.

This commit therefore fixes this issue by applying a single-CPU

optimization to the RCU Tasks Rude grace-period process. The key point

here is that the purpose of this RCU flavor is to force a schedule on

each online CPU since some past event. But the rcu_tasks_rude_wait_gp()

function runs in the context of the RCU Tasks Rude's grace-period kthread,

so there must already have been a context switch on the current CPU since

the call to either synchronize_rcu_tasks_rude() or call_rcu_tasks_rude().

So if there is only a single CPU online, RCU Tasks Rude's grace-period

kthread does not need to anything at all.

It turns out that the rcu_tasks_rude_wait_gp() function's call to

schedule_on_each_cpu() causes problems during early boot. During that

time, there is only one online CPU, namely the boot CPU. Therefore,

applying this single-CPU optimization fixes early-boot instances of

this problem.