Messages in this thread | | | Date | Thu, 25 Apr 2013 19:24:41 +0530 | Subject | Re: [BUG] staging: android: ashmem: Deadlock during ashmem_shrink | From | Shankar Brahadeeswaran <> |
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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@google.com> wrote: > On Tue, Apr 23, 2013 at 12:20 PM, Shankar Brahadeeswaran > <shankoo77@gmail.com> 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
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