On Sat, Aug 31, 2013 at 07:01:33AM +0000, Liu, Chuansheng wrote: > My scenario is: > P1 files_struct refcount is 1, P2's is 1 also. > P1 get_files_struct(P2) > P1 install one file into P2's files_struct > P1 put_files_struct(P2) > > Then P1 and P2's files_struct refcount are 1, then when P1 is doing ioctl() and P2 is exiting > with put_files_struct(P2), the race will occur, my understanding is wrong? First of all, this wouldn't have been a problem (so you get a new reference to file inserted in P2's files_struct; file refcount had been bumped, so destruction of P2's files_struct will undo that increment of file refcount and we are still fine). _Removal_ in a similar scenario would have been a problem, with P2 doing fdget() while its table isn't shared, then P1 removing a reference from it and dropping a file - the last one, at that, since fdget() assumed that the reference would've stayed in P2's descriptor table. HOWEVER, P1 does not do get_files_struct(P2) at all - it's only done by P2 in binder_mmap(). Again, the invariant to look for is this: * if current->files had not been shared at fdget() time, it won't be shared at matching fdput() and no entries will have been removed in between. task_fd_install()/task_close_fd() are done on proc->files, which contributes to descriptor table refcount. All other modifications are done to current->files, which also contributes to refcount. If at fdget() time current->files had refcount 1, we had no other processes with task->files pointing to this descriptor table *and* no binder_proc had their ->files pointint to it. No new ones may appear, since new process could get such a reference only from do_fork() called by us and new binder_proc could get such a reference only from binder_mmap() called by us. Neither is called between fdget() and fdput(). So in that case the only reference to this descriptor table will remain current->files and all removals would have to be done by ourselves (and not via task_close_fd(), at that). And AFAICS, binder_lock() prevents proc->files being dropped under task_close_fd() and task_fd_install(). Hell knows... How reproducible it is? Do you have any more instances, or had that been a one-off panic? -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
