fscache: Fix fscache_cookie_put() to not deref after dec

fscache_cookie_put() accesses the cookie it has just put inside the
tracepoint that monitors the change - but this is something it's not
allowed to do if we didn't reduce the count to zero.

Fix this by dropping most of those values from the tracepoint and grabbing
the cookie debug ID before doing the dec.

Also take the opportunity to switch over the usage and where arguments on
the tracepoint to put the reason last.

Fixes: a18feb55769b ("fscache: Add tracepoints")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/162431203107.2908479.3259582550347000088.stgit@warthog.procyon.org.uk/

4 files changed, 15 insertions, 23 deletions

diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index 2558814193e9..6df3732cf1b4 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -225,8 +225,8 @@ struct fscache_cookie *fscache_hash_cookie(struct fscache_cookie *candidate)
 
 collision:
 	if (test_and_set_bit(FSCACHE_COOKIE_ACQUIRED, &cursor->flags)) {
-		trace_fscache_cookie(cursor, fscache_cookie_collision,
-				     atomic_read(&cursor->usage));
+		trace_fscache_cookie(cursor->debug_id, atomic_read(&cursor->usage),
+				     fscache_cookie_collision);
 		pr_err("Duplicate cookie detected\n");
 		fscache_print_cookie(cursor, 'O');
 		fscache_print_cookie(candidate, 'N');
@@ -305,7 +305,8 @@ struct fscache_cookie *__fscache_acquire_cookie(
 
 	cookie = fscache_hash_cookie(candidate);
 	if (!cookie) {
-		trace_fscache_cookie(candidate, fscache_cookie_discard, 1);
+		trace_fscache_cookie(candidate->debug_id, 1,
+				     fscache_cookie_discard);
 		goto out;
 	}
 
@@ -866,8 +867,9 @@ void fscache_cookie_put(struct fscache_cookie *cookie,
 	_enter("%x", cookie->debug_id);
 
 	do {
+		unsigned int cookie_debug_id = cookie->debug_id;
 		usage = atomic_dec_return(&cookie->usage);
-		trace_fscache_cookie(cookie, where, usage);
+		trace_fscache_cookie(cookie_debug_id, usage, where);
 
 		if (usage > 0)
 			return;
diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index a49136c63e4b..345105dbbfd1 100644
--- a/fs/fscache/internal.h
+++ b/fs/fscache/internal.h
@@ -291,7 +291,7 @@ static inline void fscache_cookie_get(struct fscache_cookie *cookie,
 {
 	int usage = atomic_inc_return(&cookie->usage);
 
-	trace_fscache_cookie(cookie, where, usage);
+	trace_fscache_cookie(cookie->debug_id, usage, where);
 }
 
 /*
diff --git a/fs/fscache/netfs.c b/fs/fscache/netfs.c
index cce92216fa28..d6bdb7b5e723 100644
--- a/fs/fscache/netfs.c
+++ b/fs/fscache/netfs.c
@@ -37,7 +37,7 @@ int __fscache_register_netfs(struct fscache_netfs *netfs)
 	if (!cookie)
 		goto already_registered;
 	if (cookie != candidate) {
-		trace_fscache_cookie(candidate, fscache_cookie_discard, 1);
+		trace_fscache_cookie(candidate->debug_id, 1, fscache_cookie_discard);
 		fscache_free_cookie(candidate);
 	}
 
diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
index 33d1fd5d0383..19d65d9d4357 100644
--- a/include/trace/events/fscache.h
+++ b/include/trace/events/fscache.h
@@ -160,37 +160,27 @@ fscache_cookie_traces;
 
 
 TRACE_EVENT(fscache_cookie,
-	    TP_PROTO(struct fscache_cookie *cookie,
-		     enum fscache_cookie_trace where,
-		     int usage),
+	    TP_PROTO(unsigned int cookie_debug_id,
+		     int usage,
+		     enum fscache_cookie_trace where),
 
-	    TP_ARGS(cookie, where, usage),
+	    TP_ARGS(cookie_debug_id, usage, where),
 
 	    TP_STRUCT__entry(
 		    __field(unsigned int,		cookie		)
-		    __field(unsigned int,		parent		)
 		    __field(enum fscache_cookie_trace,	where		)
 		    __field(int,			usage		)
-		    __field(int,			n_children	)
-		    __field(int,			n_active	)
-		    __field(u8,				flags		)
 			     ),
 
 	    TP_fast_assign(
-		    __entry->cookie	= cookie->debug_id;
-		    __entry->parent	= cookie->parent ? cookie->parent->debug_id : 0;
+		    __entry->cookie	= cookie_debug_id;
 		    __entry->where	= where;
 		    __entry->usage	= usage;
-		    __entry->n_children	= atomic_read(&cookie->n_children);
-		    __entry->n_active	= atomic_read(&cookie->n_active);
-		    __entry->flags	= cookie->flags;
 			   ),
 
-	    TP_printk("%s c=%08x u=%d p=%08x Nc=%d Na=%d f=%02x",
+	    TP_printk("%s c=%08x u=%d",
 		      __print_symbolic(__entry->where, fscache_cookie_traces),
-		      __entry->cookie, __entry->usage,
-		      __entry->parent, __entry->n_children, __entry->n_active,
-		      __entry->flags)
+		      __entry->cookie, __entry->usage)
 	    );
 
 TRACE_EVENT(fscache_netfs,