Messages in this thread | | | Date | Fri, 9 Nov 2012 09:01:07 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RESEND v2 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily |
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On Fri, Nov 09, 2012 at 04:46:56PM +0100, Oleg Nesterov wrote: > On 11/08, Andrew Morton wrote: > > > > On Thu, 8 Nov 2012 14:48:49 +0100 > > Oleg Nesterov <oleg@redhat.com> wrote: > > > > > > > > include/linux/percpu-rwsem.h | 83 +++++------------------------ > > > lib/Makefile | 2 +- > > > lib/percpu-rwsem.c | 123 ++++++++++++++++++++++++++++++++++++++++++ > > > > The patch also uninlines everything. > > > > And it didn't export the resulting symbols to modules, so it isn't an > > equivalent. We can export thing later if needed I guess. > > Yes, currently it is only used by block_dev.c > > > It adds percpu-rwsem.o to lib-y, so the CONFIG_BLOCK=n kernel will > > avoid including the code altogether, methinks? > > I am going to add another user (uprobes), this was my motivation for > this patch. And perhaps it will have more users. > > But I agree, CONFIG_PERCPU_RWSEM makes sense at least now, I'll send > the patch. > > > > +#include <linux/percpu-rwsem.h> > > > +#include <linux/rcupdate.h> > > > +#include <linux/sched.h> > > > > This list is nowhere near sufficient to support this file's > > requirements. atomic.h, percpu.h, rwsem.h, wait.h, errno.h and plenty > > more. IOW, if it compiles, it was sheer luck. > > OK, thanks, I'll send > send percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessarily.fix > > > > +/* > > > + * A writer takes ->writer_mutex to exclude other writers and to force the > > > + * readers to switch to the slow mode, note the mutex_is_locked() check in > > > + * update_fast_ctr(). > > > + * > > > + * After that the readers can only inc/dec the slow ->slow_read_ctr counter, > > > + * ->fast_read_ctr is stable. Once the writer moves its sum into the slow > > > + * counter it represents the number of active readers. > > > + * > > > + * Finally the writer takes ->rw_sem for writing and blocks the new readers, > > > + * then waits until the slow counter becomes zero. > > > + */ > > > > Some overview of how fast/slow_read_ctr are supposed to work would be > > useful. This comment seems to assume that the reader already knew > > that. > > I hate to say this, but I'll try to update this comment too ;) > > > > +void percpu_down_write(struct percpu_rw_semaphore *brw) > > > +{ > > > + /* also blocks update_fast_ctr() which checks mutex_is_locked() */ > > > + mutex_lock(&brw->writer_mutex); > > > + > > > + /* > > > + * 1. Ensures mutex_is_locked() is visible to any down_read/up_read > > > + * so that update_fast_ctr() can't succeed. > > > + * > > > + * 2. Ensures we see the result of every previous this_cpu_add() in > > > + * update_fast_ctr(). > > > + * > > > + * 3. Ensures that if any reader has exited its critical section via > > > + * fast-path, it executes a full memory barrier before we return. > > > + */ > > > + synchronize_sched(); > > > > Here's where I get horridly confused. Your patch completely deRCUifies > > this code, yes? Yet here we're using an RCU primitive. And we seem to > > be using it not as an RCU primitive but as a handy thing which happens > > to have desirable side-effects. But the implementation of > > synchronize_sched() differs considerably according to which rcu > > flavor-of-the-minute you're using. > > It is documented that synchronize_sched() should play well with > preempt_disable/enable. From the comment: > > Note that preempt_disable(), > local_irq_disable(), and so on may be used in place of > rcu_read_lock_sched(). > > But I guess this needs more discussion, I see other emails in this > thread... > > > And part 3 talks about the reader's critical section. The only > > critical sections I can see on the reader side are already covered by > > mutex_lock() and preempt_diable(). > > Yes, but we need to ensure that if we take the lock for writing, we > should see all memory modifications done under down_read/up_read(). > > IOW. Suppose that the reader does > > percpu_down_read(); > STORE; > percpu_up_read(); // no barriers in the fast path > > The writer should see the result of that STORE under percpu_down_write(). > > Part 3 tries to say that at this point we should already see the result, > so we should not worry about acquire/release semantics. > > > If this code isn't as brain damaged as it > > initially appears then please, > > I hope ;) > > > go easy on us simpletons in the next > > version? > > Well, I'll try to update the comments... but the code is simple, I do > not think I can simplify it more. The nontrivial part is the barriers, > but this is always nontrivial. > > Contrary, I am going to try to add some complications later, so that > it can have more users. In particular, I think it can replace > get_online_cpus/cpu_hotplug_begin, just we need > percpu_down_write_but_dont_deadlock_with_recursive_readers().
I must confess that I am a bit concerned about possible scalability bottlenecks in the current get_online_cpus(), so +1 from me on this one.
Thanx, Paul
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