Messages in this thread | | | Date | Mon, 12 Nov 2012 10:31:28 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH -mm] percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessari ly.fix |
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On Sun, Nov 11, 2012 at 07:27:44PM +0100, Oleg Nesterov wrote: > More include's and more comments, no changes in code. > > To remind, once/if I am sure you agree with this patch I'll send 2 additional > and simple patches: > > 1. lockdep annotations > > 2. CONFIG_PERCPU_RWSEM > > It seems that we can do much more improvements to a) speedup the writers and > b) make percpu_rw_semaphore more useful, but not right now. > > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Looks good to me!
Thanx, Paul
> --- > lib/percpu-rwsem.c | 35 +++++++++++++++++++++++++++++++++-- > 1 files changed, 33 insertions(+), 2 deletions(-) > > diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c > index 0e3bc0f..02bd157 100644 > --- a/lib/percpu-rwsem.c > +++ b/lib/percpu-rwsem.c > @@ -1,6 +1,11 @@ > +#include <linux/mutex.h> > +#include <linux/rwsem.h> > +#include <linux/percpu.h> > +#include <linux/wait.h> > #include <linux/percpu-rwsem.h> > #include <linux/rcupdate.h> > #include <linux/sched.h> > +#include <linux/errno.h> > > int percpu_init_rwsem(struct percpu_rw_semaphore *brw) > { > @@ -21,6 +26,29 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw) > brw->fast_read_ctr = NULL; /* catch use after free bugs */ > } > > +/* > + * This is the fast-path for down_read/up_read, it only needs to ensure > + * there is no pending writer (!mutex_is_locked() check) and inc/dec the > + * fast per-cpu counter. The writer uses synchronize_sched() to serialize > + * with the preempt-disabled section below. > + * > + * The nontrivial part is that we should guarantee acquire/release semantics > + * in case when > + * > + * R_W: down_write() comes after up_read(), the writer should see all > + * changes done by the reader > + * or > + * W_R: down_read() comes after up_write(), the reader should see all > + * changes done by the writer > + * > + * If this helper fails the callers rely on the normal rw_semaphore and > + * atomic_dec_and_test(), so in this case we have the necessary barriers. > + * > + * But if it succeeds we do not have any barriers, mutex_is_locked() or > + * __this_cpu_add() below can be reordered with any LOAD/STORE done by the > + * reader inside the critical section. See the comments in down_write and > + * up_write below. > + */ > static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val) > { > bool success = false; > @@ -98,6 +126,7 @@ void percpu_down_write(struct percpu_rw_semaphore *brw) > * > * 3. Ensures that if any reader has exited its critical section via > * fast-path, it executes a full memory barrier before we return. > + * See R_W case in the comment above update_fast_ctr(). > */ > synchronize_sched(); > > @@ -116,8 +145,10 @@ void percpu_up_write(struct percpu_rw_semaphore *brw) > /* allow the new readers, but only the slow-path */ > up_write(&brw->rw_sem); > > - /* insert the barrier before the next fast-path in down_read */ > + /* > + * Insert the barrier before the next fast-path in down_read, > + * see W_R case in the comment above update_fast_ctr(). > + */ > synchronize_sched(); > - > mutex_unlock(&brw->writer_mutex); > } > -- > 1.5.5.1 > >
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