Without this change the Windows backend needed to call usbi_fd_notification()
from within the backend's submit_transfer. This could cause deadlock
when attempting to lock the event lock if another thread was processing
events on the just-submitted transfer.
The deadlock comes about as the thread calling libusb_submit_transfer acquires
the transfer mutex before trying to acquire the event lock; this is the other
order of lock acquisition from an event thread handling activity on the just
submitted transfer. This could lead to one of two deadlocks:
1) If the transfer completes while usbi_fd_notification() is waiting for
the event lock and the callback attempts to resubmit the transfer.
2) If the transfer timeout is hit while usbi_fd_notification() is waiting
for the event lock then the attempt to cancel the transfer will deadlock.
This patch fixes both of these deadlocks by having libusb_submit_transfer()
only call usbi_fd_notification() after having released the transfer mutex.
Signed-off-by: Toby Gray <toby.g...@realvnc.com>
---
libusb/io.c | 4 ++++
libusb/libusbi.h | 3 +++
libusb/os/windows_usb.c | 6 +++---
3 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/libusb/io.c b/libusb/io.c
index d7fae7e..ab32e70 100644
--- a/libusb/io.c
+++ b/libusb/io.c
@@ -1292,6 +1292,7 @@ int API_EXPORTED libusb_submit_transfer(struct
libusb_transfer *transfer)
LIBUSB_TRANSFER_TO_USBI_TRANSFER(transfer);
int r;
int first;
+ int updated_fds;
usbi_mutex_lock(&itransfer->lock);
itransfer->transferred = 0;
@@ -1325,7 +1326,10 @@ int API_EXPORTED libusb_submit_transfer(struct
libusb_transfer *transfer)
#endif
out:
+ updated_fds = (itransfer->flags & USBI_TRANSFER_UPDATED_FDS);
usbi_mutex_unlock(&itransfer->lock);
+ if (updated_fds)
+ usbi_fd_notification(ctx);
return r;
}
diff --git a/libusb/libusbi.h b/libusb/libusbi.h
index c3d2158..0aa6bf9 100644
--- a/libusb/libusbi.h
+++ b/libusb/libusbi.h
@@ -347,6 +347,9 @@ enum usbi_transfer_flags {
/* Operation on the transfer failed because the device disappeared */
USBI_TRANSFER_DEVICE_DISAPPEARED = 1 << 3,
+
+ /* Set by backend submit_transfer() if the fds in use have been updated
*/
+ USBI_TRANSFER_UPDATED_FDS = 1 << 4,
};
#define USBI_TRANSFER_TO_LIBUSB_TRANSFER(transfer) \
diff --git a/libusb/os/windows_usb.c b/libusb/os/windows_usb.c
index 1957964..f29f84c 100644
--- a/libusb/os/windows_usb.c
+++ b/libusb/os/windows_usb.c
@@ -1770,7 +1770,7 @@ static int submit_bulk_transfer(struct usbi_transfer
*itransfer)
usbi_add_pollfd(ctx, transfer_priv->pollable_fd.fd,
(short)(IS_XFERIN(transfer) ? POLLIN : POLLOUT));
- usbi_fd_notification(ctx);
+ itransfer->flags |= USBI_TRANSFER_UPDATED_FDS;
return LIBUSB_SUCCESS;
}
@@ -1790,7 +1790,7 @@ static int submit_iso_transfer(struct usbi_transfer
*itransfer)
usbi_add_pollfd(ctx, transfer_priv->pollable_fd.fd,
(short)(IS_XFERIN(transfer) ? POLLIN : POLLOUT));
- usbi_fd_notification(ctx);
+ itransfer->flags |= USBI_TRANSFER_UPDATED_FDS;
return LIBUSB_SUCCESS;
}
@@ -1809,7 +1809,7 @@ static int submit_control_transfer(struct usbi_transfer
*itransfer)
usbi_add_pollfd(ctx, transfer_priv->pollable_fd.fd, POLLIN);
- usbi_fd_notification(ctx);
+ itransfer->flags |= USBI_TRANSFER_UPDATED_FDS;
return LIBUSB_SUCCESS;
}
--
1.7.8.msysgit.0
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