On Wed, Mar 11, 2015 at 3:40 PM, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote: > 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. I mean you could make mm->exe_file rcu protected pointer and use everywhere get_mm_exe_file() which grabs file refcount under rcu and returns pointer. > > 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>