Hi Robert, Thanks for the feedback. > This doesn't look insurmountable. It isn't necessary AFAICT to hold > ashmem_mutex across shmem_file_setup. I thought it may not be a good idea to do so and hence thought its difficult to fix. Dropping the lock in-between mmap may not be any issue if the user land code follows a given sequence. But assuming that the following sequence of event happens, it would lead to other races. Process P1 Process P2 -------------- -------------- Creates ashmem region ..... Shares the fd to P2 via binder Gets the fd Does an mmap Does an mmap Releases the mutex before Procees with ashmem_mmap since mutex is shmem_file_setup and sleeps available, checks for a asma->file, still NUL within shmem_file_setup so this also calls shmem_file_setup. The expected behavior is, one of them does the shmem_file_setup, puts it in asma->file The other process would just do get_file. With the original code (without dropping the mutex in-between) this would have been the behavior. So IMHO dropping the lock in between could lead to other race conditions. Also, there are other places in the code where ashmem_mutex is held and memory allocation functions are called, ex:- range_alloc, calls kmem_cache_zalloc Since ashmem_shrink holds the ashmem_mutex, any where from ashmem driver if a memory allocation function is called with the ashmem_mutex held && if there is a low memory condition that leads to shrinkers being called we'll hit the deadlock. I'm trying to see if the ashmem_shrink should really hold the ashmem_mutex, but looks like its necessary. Warm Regards, Shankar On Tue, Apr 23, 2013 at 10:54 PM, Robert Love <rlove@xxxxxxxxxx> wrote: > On Tue, Apr 23, 2013 at 12:20 PM, Shankar Brahadeeswaran > <shankoo77@xxxxxxxxx> wrote: > >> I'm unable to think of a straight forward way to fix this. If you have >> any suggestions please provide the same. >> If we are unable to solve this too with minor mods, as suggested by >> Dan we have to re-look at the locking in this driver. > > This doesn't look insurmountable. It isn't necessary AFAICT to hold > ashmem_mutex across shmem_file_setup. > > Patch attached (untested). > > Robert _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
