On Wed, 2015-03-11 at 15:21 +0300, Konstantin Khlebnikov wrote: > On Fri, Feb 27, 2015 at 9:34 PM, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote: > > On Fri, 2015-02-27 at 18:36 +0100, Oleg Nesterov wrote: > >> On 02/26, Cyrill Gorcunov wrote: > >> > > >> > On Thu, Feb 26, 2015 at 11:36:57AM -0800, Davidlohr Bueso wrote: > >> > > We currently use the mmap_sem to serialize the mm exe_file. > >> > > This is atrocious and a clear example of the misuses this > >> > > lock has all over the place, making any significant changes > >> > > to the address space locking that much more complex and tedious. > >> > > This also has to do of how we used to check for the vma's vm_file > >> > > being VM_EXECUTABLE (much of which was replaced by 2dd8ad81e31). > >> > > > >> > > This patch, therefore, removes the mmap_sem dependency and > >> > > introduces a specific lock for the exe_file (rwlock_t, as it is > >> > > read mostly and protects a trivial critical region). As mentioned, > >> > > the motivation is to cleanup mmap_sem (as opposed to exe_file > >> > > performance). > >> > >> Well, I didn't see the patch, can't really comment. > >> > >> But I have to admit that this looks as atrocious and a clear example of > >> "lets add yet another random lock which we will regret about later" ;) > >> > >> rwlock_t in mm_struct just to serialize access to exe_file? > > > > I don't see why this is a random lock nor how would we regret this > > later. I regret having to do these kind of patches because people were > > lazy and just relied on mmap_sem without thinking beyond their use case. > > That's history: exe_file had direct relation to mm->mmap_sem, > that was file from first executable vma. After my patch it's less > related to vmas. Indeed. Yet I'm not changing the exe_file address space semantics at all. > > > As mentioned I'm also planning on creating an own sort of > > exe_file_struct, which would be an isolated entity (still in the mm > > though), with its own locking and prctl bits, that would tidy mm_struct > > a bit. RCU was something else I considered, but it doesn't suite well in > > all paths and we would still need a spinlock when updating the file > > anyway. > > Please don't. What's wrong with mmap_sem? > > Do you want optimize reading mm->exe_file? No, I want to get rid of certain things being done under mmap_sem, that's all. This is not performance motivated, it's to allow future work on lock breaking. I've just yesterday explained this at lsfmm (and not only related to exe_file). In any case I've clean up this patch and added more on top to create a friendlier interface, I'll send that out a bit later. > Then you should use rcu for that: struct file is rcu-protected thing. > See fget(), you could do something like that. As mentioned, not all exe paths are RCU friendly ;) We'd at least need srcu, but that's neither here nor there. A rwlock is suficient to get the job done and we really need not care much about optimizing this particular file further. Thanks, Davidlohr -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>