[Xen-changelog] [xen stable-4.2] tools: Retry blktap2 tapdisk message on interrupt.

[patchbot@xxxxxxx]

commit e5b9f23485aa3709bec753badbcb989fde390925
Author:     Dr. Greg Wettstein <greg@xxxxxxxxxxxxxxxxx>
AuthorDate: Tue Mar 19 07:26:33 2013 +0000
Commit:     Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CommitDate: Thu Apr 11 12:31:13 2013 +0100

    tools: Retry blktap2 tapdisk message on interrupt.
    
    Re-start blktap2 IPC select call on interrupt.
    
    We hunted this miserable bug for a long time.
    
    The teardown of a blktap2 tapdisk instance is being carried out
    inconsistently up to and including the 4.2.1 release.  The
    problem appears to be a classic 'Heisenbug' which disappears if a
    single function call is added to the tapdisk shutdown path.  It
    is likely this bug has been in existence for the life of the
    blktap2 code.
    
    Control messages to manipulate a tapdisk instance are sent over a
    UNIX domain socket.  A select call is used on both the read and
    write paths to wait on I/O and to set a timeout for the
    transmission and reception of the control plane messages.
    
    The existing code fails receipt or transmission of the control message
    on any type of error return from the select call.  The xl control
    process receives an interrupt while waiting in the select call which
    in turn causes an error return with SIGINT as the return code.
    
    This prematurely terminates the teardown of the tapdisk instance
    leaving it in various states of shutdown.  Since multiple messages
    are needed to implement a full teardown the tapdisk instance can be
    left in various states ranging from fully connected to only the minor
    being left allocated.
    
    The fix is straight forward.  Check the return code from the
    select call and re-try read or write of the control message if
    errno is sent to EINTR.  The problem manifests itself in the read
    path but there appears to be little reason to not add the fix to
    the write path as well.  Both paths appear to be cut-and-paste
    copies of each other.
    
    Signed-off-by: Dr. Greg Wettstein <greg@xxxxxxxxxxxx>
    Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
    (cherry picked from commit 6cffb2b469a55032a2900ccb8776c0082f346758)
---
 tools/blktap2/control/tap-ctl-ipc.c |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/tools/blktap2/control/tap-ctl-ipc.c 
b/tools/blktap2/control/tap-ctl-ipc.c
index cc6160e..c8aad1c 100644
--- a/tools/blktap2/control/tap-ctl-ipc.c
+++ b/tools/blktap2/control/tap-ctl-ipc.c
@@ -64,8 +64,11 @@ tap_ctl_read_message(int fd, tapdisk_message_t *message, int 
timeout)
                FD_SET(fd, &readfds);
 
                ret = select(fd + 1, &readfds, NULL, NULL, t);
-               if (ret == -1)
+               if (ret == -1) {
+                       if (errno == EINTR)
+                               continue;
                        break;
+               }
                else if (FD_ISSET(fd, &readfds)) {
                        ret = read(fd, message + offset, len - offset);
                        if (ret <= 0)
@@ -114,8 +117,11 @@ tap_ctl_write_message(int fd, tapdisk_message_t *message, 
int timeout)
                 * bit more time than expected. */
 
                ret = select(fd + 1, NULL, &writefds, NULL, t);
-               if (ret == -1)
+               if (ret == -1) {
+                       if (errno == EINTR)
+                               continue;
                        break;
+               }
                else if (FD_ISSET(fd, &writefds)) {
                        ret = write(fd, message + offset, len - offset);
                        if (ret <= 0)
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.2

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