[qemu-xen staging-4.14] 9p: Lock directory streams with a CoMutex

[patchbot@xxxxxxx]

commit 250322bfbdec098699b414422008aa8ae65f7d4d
Author:     Greg Kurz <groug@xxxxxxxx>
AuthorDate: Mon May 25 10:38:03 2020 +0200
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Mon Aug 24 18:55:51 2020 -0500

    9p: Lock directory streams with a CoMutex
    
    Locking was introduced in QEMU 2.7 to address the deprecation of
    readdir_r(3) in glibc 2.24. It turns out that the frontend code is
    the worst place to handle a critical section with a pthread mutex:
    the code runs in a coroutine on behalf of the QEMU mainloop and then
    yields control, waiting for the fsdev backend to process the request
    in a worker thread. If the client resends another readdir request for
    the same fid before the previous one finally unlocked the mutex, we're
    deadlocked.
    
    This never bit us because the linux client serializes readdir requests
    for the same fid, but it is quite easy to demonstrate with a custom
    client.
    
    A good solution could be to narrow the critical section in the worker
    thread code and to return a copy of the dirent to the frontend, but
    this causes quite some changes in both 9p.c and codir.c. So, instead
    of that, in order for people to easily backport the fix to older QEMU
    versions, let's simply use a CoMutex since all the users for this
    sit in coroutines.
    
    Fixes: 7cde47d4a89d ("9p: add locking to V9fsDir")
    Reviewed-by: Christian Schoenebeck <qemu_oss@xxxxxxxxxxxxx>
    Message-Id: <158981894794.109297.3530035833368944254.stgit@xxxxxxxxx>
    Signed-off-by: Greg Kurz <groug@xxxxxxxx>
    (cherry picked from commit ed463454efd0ac3042ff772bfe1b1d846dc281a5)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 hw/9pfs/9p.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
index b8f72a3bd9..c381fe091a 100644
--- a/hw/9pfs/9p.h
+++ b/hw/9pfs/9p.h
@@ -197,22 +197,22 @@ typedef struct V9fsXattr
 
 typedef struct V9fsDir {
     DIR *stream;
-    QemuMutex readdir_mutex;
+    CoMutex readdir_mutex;
 } V9fsDir;
 
 static inline void v9fs_readdir_lock(V9fsDir *dir)
 {
-    qemu_mutex_lock(&dir->readdir_mutex);
+    qemu_co_mutex_lock(&dir->readdir_mutex);
 }
 
 static inline void v9fs_readdir_unlock(V9fsDir *dir)
 {
-    qemu_mutex_unlock(&dir->readdir_mutex);
+    qemu_co_mutex_unlock(&dir->readdir_mutex);
 }
 
 static inline void v9fs_readdir_init(V9fsDir *dir)
 {
-    qemu_mutex_init(&dir->readdir_mutex);
+    qemu_co_mutex_init(&dir->readdir_mutex);
 }
 
 /*
--
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