[tip: locking/core] PCI/switchtec: Fix init_completion race condition with poll_wait()

[tip-bot2 for Logan Gunthorpe]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     efbdc769601f4d50018bf7ca50fc9f7c67392ece
Gitweb:        https://git.kernel.org/tip/efbdc769601f4d50018bf7ca50fc9f7c67392ece
Author:        Logan Gunthorpe <logang@xxxxxxxxxxxx>
AuthorDate:    Sat, 21 Mar 2020 12:25:45 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Sat, 21 Mar 2020 16:00:20 +01:00

PCI/switchtec: Fix init_completion race condition with poll_wait()

The call to init_completion() in mrpc_queue_cmd() can theoretically
race with the call to poll_wait() in switchtec_dev_poll().

  poll()			write()
    switchtec_dev_poll()   	  switchtec_dev_write()
      poll_wait(&s->comp.wait);      mrpc_queue_cmd()
			               init_completion(&s->comp)
				         init_waitqueue_head(&s->comp.wait)

To my knowledge, no one has hit this bug.

Fix this by using reinit_completion() instead of init_completion() in
mrpc_queue_cmd().

Fixes: 080b47def5e5 ("MicroSemi Switchtec management interface driver")

Reported-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200313183608.2646-1-logang@xxxxxxxxxxxx
---
 drivers/pci/switch/switchtec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/pci/switch/switchtec.c b/drivers/pci/switch/switchtec.c
index a823b4b..81dc7ac 100644
--- a/drivers/pci/switch/switchtec.c
+++ b/drivers/pci/switch/switchtec.c
@@ -175,7 +175,7 @@ static int mrpc_queue_cmd(struct switchtec_user *stuser)
 	kref_get(&stuser->kref);
 	stuser->read_len = sizeof(stuser->data);
 	stuser_set_state(stuser, MRPC_QUEUED);
-	init_completion(&stuser->comp);
+	reinit_completion(&stuser->comp);
 	list_add_tail(&stuser->list, &stdev->mrpc_queue);
 
 	mrpc_cmd_submit(stdev);