From 4a1360d01d7b6bb9d9c295378850b6d1ae134a0f Mon Sep 17 00:00:00 2001 From: Aurelien Aptel Date: Thu, 25 Jan 2018 18:47:52 +0100 Subject: [PATCH] CIFS: document tcon/ses/server refcount dance Signed-off-by: Aurelien Aptel Signed-off-by: Steve French Reviewed-by: Ronnie Sahlberg --- fs/cifs/connect.c | 32 +++++++++++++++++++++++++++++++- 1 file changed, 31 insertions(+), 1 deletion(-) diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c index ee2ab86bff5b..a726f524fb84 100644 --- a/fs/cifs/connect.c +++ b/fs/cifs/connect.c @@ -2704,6 +2704,13 @@ cifs_set_cifscreds(struct smb_vol *vol __attribute__((unused)), } #endif /* CONFIG_KEYS */ +/** + * cifs_get_smb_ses - get a session matching @volume_info data from @server + * + * This function assumes it is being called from cifs_mount() where we + * already got a server reference (server refcount +1). See + * cifs_get_tcon() for refcount explanations. + */ static struct cifs_ses * cifs_get_smb_ses(struct TCP_Server_Info *server, struct smb_vol *volume_info) { @@ -2877,6 +2884,26 @@ cifs_put_tcon(struct cifs_tcon *tcon) cifs_put_smb_ses(ses); } +/** + * cifs_get_tcon - get a tcon matching @volume_info data from @ses + * + * - tcon refcount is the number of mount points using the tcon. + * - ses refcount is the number of tcon using the session. + * + * 1. This function assumes it is being called from cifs_mount() where + * we already got a session reference (ses refcount +1). + * + * 2. Since we're in the context of adding a mount point, the end + * result should be either: + * + * a) a new tcon already allocated with refcount=1 (1 mount point) and + * its session refcount incremented (1 new tcon). This +1 was + * already done in (1). + * + * b) an existing tcon with refcount+1 (add a mount point to it) and + * identical ses refcount (no new tcon). Because of (1) we need to + * decrement the ses refcount. + */ static struct cifs_tcon * cifs_get_tcon(struct cifs_ses *ses, struct smb_vol *volume_info) { @@ -2885,8 +2912,11 @@ cifs_get_tcon(struct cifs_ses *ses, struct smb_vol *volume_info) tcon = cifs_find_tcon(ses, volume_info); if (tcon) { + /* + * tcon has refcount already incremented but we need to + * decrement extra ses reference gotten by caller (case b) + */ cifs_dbg(FYI, "Found match on UNC path\n"); - /* existing tcon already has a reference */ cifs_put_smb_ses(ses); return tcon; }