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 torvalds#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: 06220d7 ("powerpc/pseries: Introduce rwlock to gatekeep DTLB usage")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/dtl.h

@@ -1,6 +1,7 @@
 #ifndef _ASM_POWERPC_DTL_H
 #define _ASM_POWERPC_DTL_H
 
+#include <linux/rwsem.h>
 #include <asm/lppaca.h>
 #include <linux/spinlock_types.h>
 
@@ -35,7 +36,7 @@ struct dtl_entry {
 #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

@@ -191,15 +191,15 @@ static int dtl_enable(struct dtl *dtl)
 		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;
 	buf = kmem_cache_alloc_node(dtl_cache, GFP_KERNEL, cpu_to_node(dtl->cpu));
 	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;
 	}
 
@@ -217,7 +217,7 @@ static int dtl_enable(struct dtl *dtl)
 	spin_unlock(&dtl->lock);
 
 	if (rc) {
-		read_unlock(&dtl_access_lock);
+		up_read(&dtl_access_lock);
 		kmem_cache_free(dtl_cache, buf);
 	}
 
@@ -232,7 +232,7 @@ static void dtl_disable(struct dtl *dtl)
 	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

@@ -169,7 +169,7 @@ struct vcpu_dispatch_data {
  */
 #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);
@@ -463,7 +463,7 @@ static int dtl_worker_enable(unsigned long *time_limit)
 {
 	int rc = 0, state;
 
-	if (!write_trylock(&dtl_access_lock)) {
+	if (!down_write_trylock(&dtl_access_lock)) {
 		rc = -EBUSY;
 		goto out;
 	}
@@ -479,7 +479,7 @@ static int dtl_worker_enable(unsigned long *time_limit)
 		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;
 	}
@@ -494,7 +494,7 @@ static void dtl_worker_disable(unsigned long *time_limit)
 	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,