From dad6d5e7e613e51b2584c447378a044ccc2fdc81 Mon Sep 17 00:00:00 2001 From: Greg Kurz Date: Mon, 25 May 2020 10:38:03 +0200 Subject: [PATCH] 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 Message-Id: <158981894794.109297.3530035833368944254.stgit@bahia.lan> Signed-off-by: Greg Kurz (cherry picked from commit ed463454efd0ac3042ff772bfe1b1d846dc281a5) Signed-off-by: Michael Roth --- 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 3904f82901..069c86333f 100644 --- a/hw/9pfs/9p.h +++ b/hw/9pfs/9p.h @@ -186,22 +186,22 @@ 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); } /* -- GitLab