[PATCHv5 1/2] xenbus: fix deadlock on writes to /proc/xen/xenbus

[David Vrabel <david.vrabel@citrix.com>]

/proc/xen/xenbus does not work correctly.  A read blocked waiting for
a xenstore message holds the mutex needed for atomic file position
updates.  This blocks any writes on the same file handle, which can
deadlock if the write is needed to unblock the read.

Clear FMODE_ATOMIC_POS when opening this device to always get
character device like sematics.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 drivers/xen/xenbus/xenbus_dev_frontend.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_dev_frontend.c b/drivers/xen/xenbus/xenbus_dev_frontend.c
index 1e8be12..ce389b4 100644
--- a/drivers/xen/xenbus/xenbus_dev_frontend.c
+++ b/drivers/xen/xenbus/xenbus_dev_frontend.c
@@ -536,6 +536,8 @@ static int xenbus_file_open(struct inode *inode, struct file *filp)
 	if (xen_store_evtchn == 0)
 		return -ENOENT;
 
+	filp->f_mode &= ~FMODE_ATOMIC_POS; /* cdev-style semantics */
+
 	nonseekable_open(inode, filp);
 
 	u = kzalloc(sizeof(*u), GFP_KERNEL);
-- 
2.1.4


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