target: Fix percpu_ref_put race in transport_lun_remove_cmd

Nicholas Bellinger · Nicholas Bellinger · commit 5259a06ef970 · 2014-01-30T03:58:34.000-08:00
This patch fixes a percpu_ref_put race for se_lun-&gt;lun_ref in
transport_lun_remove_cmd() where -&gt;lun_ref could end up being
put more than once per command via different target completion
and fabric release contexts.

It adds a cmpxchg() for se_cmd-&gt;lun_ref_active to ensure that
percpu_ref_put() is only ever called once per se_cmd.

This bug was manifesting itself as a LUN shutdown regression
bug in &gt;= v3.13 code, where percpu_ref_kill() would end up
hanging indefinately due to the incorrect percpu_ref count.

(Change se_cmd-&gt;lun_ref_active from bool -&gt; int to force at
 least a 4-byte cmpxchg with MIPS ll/sc ins. - Fengguang)

Reported-by: Tommy Apel &lt;tommyapeldk@gmail.com&gt;
Cc: Tommy Apel &lt;tommyapeldk@gmail.com&gt;
Cc: &lt;stable@vger.kernel.org&gt; #3.13+
Signed-off-by: Nicholas Bellinger &lt;nab@linux-iscsi.org&gt;
diff --git a/drivers/target/target_core_transport.c b/drivers/target/target_core_transport.c
@@ -594,10 +594,11 @@ static void transport_lun_remove_cmd(struct se_cmd *cmd)
 {
 	struct se_lun *lun = cmd->se_lun;
 
-	if (!lun || !cmd->lun_ref_active)
+	if (!lun)
 		return;
 
-	percpu_ref_put(&lun->lun_ref);
+	if (cmpxchg(&cmd->lun_ref_active, true, false))
+		percpu_ref_put(&lun->lun_ref);
 }
 
 void transport_cmd_finish_abort(struct se_cmd *cmd, int remove)
diff --git a/include/target/target_core_base.h b/include/target/target_core_base.h
@@ -552,7 +552,7 @@ struct se_cmd {
 	void			*priv;
 
 	/* Used for lun->lun_ref counting */
-	bool			lun_ref_active;
+	int			lun_ref_active;
 
 	/* DIF related members */
 	enum target_prot_op	prot_op;

Original file line number	Diff line number	Diff line change
`@@ -594,10 +594,11 @@ static void transport_lun_remove_cmd(struct se_cmd *cmd)`
`594`	`594`	`{`
`595`	`595`	`struct se_lun *lun = cmd->se_lun;`
`596`	`596`
`597`		`- if (!lun \|\| !cmd->lun_ref_active)`
	`597`	`+ if (!lun)`
`598`	`598`	`return;`
`599`	`599`
`600`		`- percpu_ref_put(&lun->lun_ref);`
	`600`	`+ if (cmpxchg(&cmd->lun_ref_active, true, false))`
	`601`	`+ percpu_ref_put(&lun->lun_ref);`
`601`	`602`	`}`
`602`	`603`
`603`	`604`	`void transport_cmd_finish_abort(struct se_cmd *cmd, int remove)`