similar to: [PATCH tip/core/rcu 21/21] drivers/vhost: Remove now-redundant read_barrier_depends()

On Fri, Dec 01, 2017 at 11:51:16AM -0800, Paul E. McKenney wrote: > Because READ_ONCE() now implies read_barrier_depends(), the > read_barrier_depends() in next_desc() is now redundant. This commit > therefore removes it and the related comments. > > Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com> > Cc: "Michael S. Tsirkin" <mst at

Commit-ID: 3a5db0b108e0a40f08c2bcff6a675dbf632b91e0 Gitweb: https://git.kernel.org/tip/3a5db0b108e0a40f08c2bcff6a675dbf632b91e0 Author: Paul E. McKenney <paulmck at linux.vnet.ibm.com> AuthorDate: Mon, 27 Nov 2017 09:45:10 -0800 Committer: Paul E. McKenney <paulmck at linux.vnet.ibm.com> CommitDate: Tue, 5 Dec 2017 11:57:55 -0800 drivers/vhost: Remove now-redundant

On Tue, Dec 05, 2017 at 08:31:20PM +0200, Michael S. Tsirkin wrote: > Apropos, READ_ONCE is now asymmetrical with WRITE_ONCE. > > I can read a pointer with READ_ONCE and be sure the value > is sane, but only if I also remember to put in smp_wmb before > WRITE_ONCE. Otherwise the pointer is ok but no guarantees > about the data pointed to. That was already the case on

On Tue, Dec 05, 2017 at 08:31:20PM +0200, Michael S. Tsirkin wrote: > Apropos, READ_ONCE is now asymmetrical with WRITE_ONCE. > > I can read a pointer with READ_ONCE and be sure the value > is sane, but only if I also remember to put in smp_wmb before > WRITE_ONCE. Otherwise the pointer is ok but no guarantees > about the data pointed to. That was already the case on

On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 08:17:33PM +0100, Peter Zijlstra wrote: > > On Tue, Dec 05, 2017 at 08:57:46PM +0200, Michael S. Tsirkin wrote: > > > > > I don't see WRITE_ONCE inserting any barriers, release or > > > write. > > > > Correct, never claimed there was. > > > >

On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 08:17:33PM +0100, Peter Zijlstra wrote: > > On Tue, Dec 05, 2017 at 08:57:46PM +0200, Michael S. Tsirkin wrote: > > > > > I don't see WRITE_ONCE inserting any barriers, release or > > > write. > > > > Correct, never claimed there was. > > > >

On Tue, Dec 05, 2017 at 08:57:46PM +0200, Michael S. Tsirkin wrote: > I don't see WRITE_ONCE inserting any barriers, release or > write. Correct, never claimed there was. Just saying that: obj = READ_ONCE(*foo); val = READ_ONCE(obj->val); Never needs a barrier (except on Alpha and we want to make that go away). Simply because a CPU needs to complete the load of @obj before it

On Tue, Dec 05, 2017 at 08:57:46PM +0200, Michael S. Tsirkin wrote: > I don't see WRITE_ONCE inserting any barriers, release or > write. Correct, never claimed there was. Just saying that: obj = READ_ONCE(*foo); val = READ_ONCE(obj->val); Never needs a barrier (except on Alpha and we want to make that go away). Simply because a CPU needs to complete the load of @obj before it

On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > > > WRITE_ONCE(obj->val, 1); > > > smp_wmb(); > > > WRITE_ONCE(*foo, obj); > > > > I believe Peter was instead suggesting: > > > > WRITE_ONCE(obj->val, 1); > > smp_store_release(foo, obj); > > Isn't that more expensive though? Depends on the

On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > > > WRITE_ONCE(obj->val, 1); > > > smp_wmb(); > > > WRITE_ONCE(*foo, obj); > > > > I believe Peter was instead suggesting: > > > > WRITE_ONCE(obj->val, 1); > > smp_store_release(foo, obj); > > Isn't that more expensive though? Depends on the

On Tue, Dec 05, 2017 at 10:28:38PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 08:57:52PM +0100, Peter Zijlstra wrote: > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > > > > > WRITE_ONCE(obj->val, 1); > > > > > smp_wmb(); > > > > > WRITE_ONCE(*foo, obj); > > > > > > > > I

On Tue, Dec 05, 2017 at 10:28:38PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 08:57:52PM +0100, Peter Zijlstra wrote: > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > > > > > WRITE_ONCE(obj->val, 1); > > > > > smp_wmb(); > > > > > WRITE_ONCE(*foo, obj); > > > > > > > > I

On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote: > READ_ONCE is really all over the place (some code literally replaced all > memory accesses with READ/WRITE ONCE). Yeah, so? Complain to the compiler people for forcing us into that. > Would an API like WRITE_POINTER()/smp_store_pointer make sense, > and READ_POINTER for symmetry? No, the whole point of the

On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote: > READ_ONCE is really all over the place (some code literally replaced all > memory accesses with READ/WRITE ONCE). Yeah, so? Complain to the compiler people for forcing us into that. > Would an API like WRITE_POINTER()/smp_store_pointer make sense, > and READ_POINTER for symmetry? No, the whole point of the

On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote: > > On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote: [ . . . ] > > > and this barrier is no longer paired with anything until > > > you realize there's a dependency barrier within READ_ONCE. > > > >

On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote: > > On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote: [ . . . ] > > > and this barrier is no longer paired with anything until > > > you realize there's a dependency barrier within READ_ONCE. > > > >

On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 12:08:01PM -0800, Paul E. McKenney wrote: > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > > > On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote: > > > > On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote: > >

On Tue, Dec 05, 2017 at 11:24:49PM +0200, Michael S. Tsirkin wrote: > On Tue, Dec 05, 2017 at 12:08:01PM -0800, Paul E. McKenney wrote: > > On Tue, Dec 05, 2017 at 09:51:48PM +0200, Michael S. Tsirkin wrote: > > > On Tue, Dec 05, 2017 at 11:33:39AM -0800, Paul E. McKenney wrote: > > > > On Tue, Dec 05, 2017 at 09:24:21PM +0200, Michael S. Tsirkin wrote: > >

On Tue, Dec 05, 2017 at 07:39:46PM +0100, Peter Zijlstra wrote: > On Tue, Dec 05, 2017 at 08:31:20PM +0200, Michael S. Tsirkin wrote: > > > Apropos, READ_ONCE is now asymmetrical with WRITE_ONCE. > > > > I can read a pointer with READ_ONCE and be sure the value > > is sane, but only if I also remember to put in smp_wmb before > > WRITE_ONCE. Otherwise the

On Tue, Dec 05, 2017 at 08:17:33PM +0100, Peter Zijlstra wrote: > On Tue, Dec 05, 2017 at 08:57:46PM +0200, Michael S. Tsirkin wrote: > > > I don't see WRITE_ONCE inserting any barriers, release or > > write. > > Correct, never claimed there was. > > Just saying that: > > obj = READ_ONCE(*foo); > val = READ_ONCE(obj->val); > > Never needs