ocfs2_dentry_attach_lock() can be executed in parallel threads against the same dentry. Make that race safe. The race is like this:
thread A thread B
(A1) enter ocfs2_dentry_attach_lock, seeing dentry->d_fsdata is NULL, and no alias found by ocfs2_find_local_alias, so kmalloc a new ocfs2_dentry_lock structure to local variable "dl", dl1
.....
(B1) enter ocfs2_dentry_attach_lock, seeing dentry->d_fsdata is NULL, and no alias found by ocfs2_find_local_alias so kmalloc a new ocfs2_dentry_lock structure to local variable "dl", dl2.
......
(A2) set dentry->d_fsdata with dl1, call ocfs2_dentry_lock() and increase dl1->dl_lockres.l_ro_holders to 1 on success. ......
(B2) set dentry->d_fsdata with dl2 call ocfs2_dentry_lock() and increase dl2->dl_lockres.l_ro_holders to 1 on success.
......
(A3) call ocfs2_dentry_unlock() and decrease dl2->dl_lockres.l_ro_holders to 0 on success. ....
(B3) call ocfs2_dentry_unlock(), decreasing dl2->dl_lockres.l_ro_holders, but see it's zero now, panic
Signed-off-by: Wengang Wang wen.gang.wang@oracle.com Reported-by: Daniel Sobe daniel.sobe@nxp.com Tested-by: Daniel Sobe daniel.sobe@nxp.com Reviewed-by: Changwei Ge gechangwei@live.cn --- v4: return in place on race detection.
v3: add Reviewed-by, Reported-by and Tested-by only
v2: 1) removed lock on dentry_attach_lock at the first access of dentry->d_fsdata since it helps very little. 2) do cleanups before freeing the duplicated dl 3) return after freeing the duplicated dl found. ---
fs/ocfs2/dcache.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)
diff --git a/fs/ocfs2/dcache.c b/fs/ocfs2/dcache.c index 290373024d9d..e8ace3b54e9c 100644 --- a/fs/ocfs2/dcache.c +++ b/fs/ocfs2/dcache.c @@ -310,6 +310,18 @@ int ocfs2_dentry_attach_lock(struct dentry *dentry,
out_attach: spin_lock(&dentry_attach_lock); + if (unlikely(dentry->d_fsdata && !alias)) { + /* d_fsdata is set by a racing thread which is doing + * the same thing as this thread is doing. Leave the racing + * thread going ahead and we return here. + */ + spin_unlock(&dentry_attach_lock); + iput(dl->dl_inode); + ocfs2_lock_res_free(&dl->dl_lockres); + kfree(dl); + return 0; + } + dentry->d_fsdata = dl; dl->dl_count++; spin_unlock(&dentry_attach_lock);