powerpc/pseries: Fix dtl_access_lock to be a rw_semaphore

The dtl_access_lock needs to be a rw_sempahore, a sleeping lock, because
the code calls kmalloc() while holding it, which can sleep:

# echo 1 > /proc/powerpc/vcpudispatch_stats
BUG: sleeping function called from invalid context at include/linux/sched/mm.h:337
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 199, name: sh
preempt_count: 1, expected: 0
3 locks held by sh/199:
#0: c00000000a0743f8 (sb_writers#3){.+.+}-{0:0}, at: vfs_write+0x324/0x438
#1: c0000000028c7058 (dtl_enable_mutex){+.+.}-{3:3}, at: vcpudispatch_stats_write+0xd4/0x5f4
#2: c0000000028c70b8 (dtl_access_lock){+.+.}-{2:2}, at: vcpudispatch_stats_write+0x220/0x5f4
CPU: 0 PID: 199 Comm: sh Not tainted 6.10.0-rc4 #152
Hardware name: IBM pSeries (emulated by qemu) POWER9 (raw) 0x4e1202 0xf000005 of:SLOF,HEAD hv:linux,kvm pSeries
Call Trace:
dump_stack_lvl+0x130/0x148 (unreliable)
__might_resched+0x174/0x410
kmem_cache_alloc_noprof+0x340/0x3d0
alloc_dtl_buffers+0x124/0x1ac
vcpudispatch_stats_write+0x2a8/0x5f4
proc_reg_write+0xf4/0x150
vfs_write+0xfc/0x438
ksys_write+0x88/0x148
system_call_exception+0x1c4/0x5a0
system_call_common+0xf4/0x258

Fixes: 06220d78f24a ("powerpc/pseries: Introduce rwlock to gatekeep DTLB usage")
Tested-by: Kajol Jain <[e-mail]>
Reviewed-by: Nysal Jan K.A <[e-mail]>
Reviewed-by: Kajol Jain <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

arch/powerpc/include/asm/dtl.h

···
 #ifndef _ASM_POWERPC_DTL_H
 #define _ASM_POWERPC_DTL_H
 
+#include <linux/rwsem.h>
 #include <asm/lppaca.h>
-#include <linux/spinlock_types.h>
 
 /*
  * Layout of entries in the hypervisor's dispatch trace log buffer.
···
 #define DTL_LOG_ALL		(DTL_LOG_CEDE | DTL_LOG_PREEMPT | DTL_LOG_FAULT)
 
 extern struct kmem_cache *dtl_cache;
-extern rwlock_t dtl_access_lock;
+extern struct rw_semaphore dtl_access_lock;
 
 extern void register_dtl_buffer(int cpu);
 extern void alloc_dtl_buffers(unsigned long *time_limit);

arch/powerpc/platforms/pseries/dtl.c

···
 		return -EBUSY;
 
 	/* ensure there are no other conflicting dtl users */
-	if (!read_trylock(&dtl_access_lock))
+	if (!down_read_trylock(&dtl_access_lock))
 		return -EBUSY;
 
 	n_entries = dtl_buf_entries;
···
 	if (!buf) {
 		printk(KERN_WARNING "%s: buffer alloc failed for cpu %d\n",
 				__func__, dtl->cpu);
-		read_unlock(&dtl_access_lock);
+		up_read(&dtl_access_lock);
 		return -ENOMEM;
 	}
 
···
 	spin_unlock(&dtl->lock);
 
 	if (rc) {
-		read_unlock(&dtl_access_lock);
+		up_read(&dtl_access_lock);
 		kmem_cache_free(dtl_cache, buf);
 	}
 
···
 	dtl->buf = NULL;
 	dtl->buf_entries = 0;
 	spin_unlock(&dtl->lock);
-	read_unlock(&dtl_access_lock);
+	up_read(&dtl_access_lock);
 }
 
 /* file interface */

arch/powerpc/platforms/pseries/lpar.c

···
  */
 #define NR_CPUS_H	NR_CPUS
 
-DEFINE_RWLOCK(dtl_access_lock);
+DECLARE_RWSEM(dtl_access_lock);
 static DEFINE_PER_CPU(struct vcpu_dispatch_data, vcpu_disp_data);
 static DEFINE_PER_CPU(u64, dtl_entry_ridx);
 static DEFINE_PER_CPU(struct dtl_worker, dtl_workers);
···
 {
 	int rc = 0, state;
 
-	if (!write_trylock(&dtl_access_lock)) {
+	if (!down_write_trylock(&dtl_access_lock)) {
 		rc = -EBUSY;
 		goto out;
 	}
···
 		pr_err("vcpudispatch_stats: unable to setup workqueue for DTL processing\n");
 		free_dtl_buffers(time_limit);
 		reset_global_dtl_mask();
-		write_unlock(&dtl_access_lock);
+		up_write(&dtl_access_lock);
 		rc = -EINVAL;
 		goto out;
 	}
···
 	cpuhp_remove_state(dtl_worker_state);
 	free_dtl_buffers(time_limit);
 	reset_global_dtl_mask();
-	write_unlock(&dtl_access_lock);
+	up_write(&dtl_access_lock);
 }
 
 static ssize_t vcpudispatch_stats_write(struct file *file, const char __user *p,