powerpc/spufs: Rework fcheck() usage

Currently the spu coredump code triggers an RCU warning:

=============================
WARNING: suspicious RCU usage
5.7.0-rc3-01755-g7cd49f0b7ec7 #1 Not tainted
-----------------------------
include/linux/fdtable.h:95 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by spu-coredump/1343:
#0: c0000007fa22f430 (sb_writers#2){.+.+}-{0:0}, at: .do_coredump+0x1010/0x13c8

stack backtrace:
CPU: 0 PID: 1343 Comm: spu-coredump Not tainted 5.7.0-rc3-01755-g7cd49f0b7ec7 #1
Call Trace:
.dump_stack+0xec/0x15c (unreliable)
.lockdep_rcu_suspicious+0x120/0x144
.coredump_next_context+0x148/0x158
.spufs_coredump_extra_notes_size+0x54/0x190
.elf_coredump_extra_notes_size+0x34/0x50
.elf_core_dump+0xe48/0x19d0
.do_coredump+0xe50/0x13c8
.get_signal+0x864/0xd88
.do_notify_resume+0x158/0x3c8
.interrupt_exit_user_prepare+0x19c/0x208
interrupt_return+0x14/0x1c0

This comes from fcheck_files() via fcheck().

It's pretty clearly documented that fcheck() must be wrapped with
rcu_read_lock(), adding that fixes the RCU warning.

hch points out that once we've released the RCU read lock the file may
be closed and freed, which would leave us with a pointer to a freed
spu_context.

To avoid that, take a reference to the spu_context while we hold the
RCU read lock, and drop that reference later once we're done with the
context.

Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/platforms/cell/spufs/coredump.c

···
  */
 static struct spu_context *coredump_next_context(int *fd)
 {
+	struct spu_context *ctx;
 	struct file *file;
 	int n = iterate_fd(current->files, *fd, match_context, NULL);
 	if (!n)
 		return NULL;
 	*fd = n - 1;
+
+	rcu_read_lock();
 	file = fcheck(*fd);
-	return SPUFS_I(file_inode(file))->i_ctx;
+	ctx = SPUFS_I(file_inode(file))->i_ctx;
+	get_spu_context(ctx);
+	rcu_read_unlock();
+
+	return ctx;
 }
 
 int spufs_coredump_extra_notes_size(void)
···
 	fd = 0;
 	while ((ctx = coredump_next_context(&fd)) != NULL) {
 		rc = spu_acquire_saved(ctx);
-		if (rc)
+		if (rc) {
+			put_spu_context(ctx);
 			break;
+		}
+
 		rc = spufs_ctx_note_size(ctx, fd);
 		spu_release_saved(ctx);
-		if (rc < 0)
+		if (rc < 0) {
+			put_spu_context(ctx);
 			break;
+		}
 
 		size += rc;
 
 		/* start searching the next fd next time */
 		fd++;
+		put_spu_context(ctx);
 	}
 
 	return size;