| Message ID | 20230803032408.2514989-2-joel@joelfernandes.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/2] docs: rcu: Add cautionary note on plain-accesses to requirements | expand |
On Thu, Aug 03, 2023 at 03:24:07AM +0000, Joel Fernandes (Google) wrote: > The compiler has the ability to cause misordering by destroying > address-dependency barriers if comparison operations are used. Add a > note about this to memory-barriers.txt and point to rcu-dereference.rst > for more information. > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > --- > Documentation/memory-barriers.txt | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index 06e14efd8662..acc8ec5ce563 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties: > variables such as READ_ONCE() and rcu_dereference() provide implicit > address-dependency barriers. > > + [!] Note that address dependency barriers can be destroyed by comparison > + of a pointer obtained by a marked accessor such as READ_ONCE() or > + rcu_dereference() with some value. For an example of this, see > + rcu_dereference.rst (part where the comparison of pointers is discussed). Hmmm... Given that this is in a section marked "historical" (for the old smp_read_barrier_depends() API), why not instead add a pointer to Documentation/RCU/rcu_dereference.rst to the beginning of the section, noted as the updated material? Thanx, Paul > + > (3) Read (or load) memory barriers. > > A read barrier is an address-dependency barrier plus a guarantee that all > -- > 2.41.0.585.gd2178a4bd4-goog >
On Thu, Aug 03, 2023 at 11:52:06AM -0700, Paul E. McKenney wrote: > On Thu, Aug 03, 2023 at 03:24:07AM +0000, Joel Fernandes (Google) wrote: > > The compiler has the ability to cause misordering by destroying > > address-dependency barriers if comparison operations are used. Add a > > note about this to memory-barriers.txt and point to rcu-dereference.rst > > for more information. > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > > --- > > Documentation/memory-barriers.txt | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > > index 06e14efd8662..acc8ec5ce563 100644 > > --- a/Documentation/memory-barriers.txt > > +++ b/Documentation/memory-barriers.txt > > @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties: > > variables such as READ_ONCE() and rcu_dereference() provide implicit > > address-dependency barriers. > > > > + [!] Note that address dependency barriers can be destroyed by comparison > > + of a pointer obtained by a marked accessor such as READ_ONCE() or > > + rcu_dereference() with some value. For an example of this, see > > + rcu_dereference.rst (part where the comparison of pointers is discussed). > > Hmmm... > > Given that this is in a section marked "historical" (for the old > smp_read_barrier_depends() API), why not instead add a pointer to > Documentation/RCU/rcu_dereference.rst to the beginning of the section, > noted as the updated material? Sounds good. There's also another section in the same file on Address dependency barriers (also marked historical). So something like the following? ---8<----------------------- diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index acc8ec5ce563..ba50220716ca 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -396,6 +396,10 @@ Memory barriers come in four basic varieties: (2) Address-dependency barriers (historical). + [!] This section is marked as HISTORICAL: For more up-to-date + information, including how compiler transformations related to pointer + comparisons can sometimes cause problems, see + Documentation/RCU/rcu_dereference.rst. An address-dependency barrier is a weaker form of read barrier. In the case where two loads are performed such that the second depends on the @@ -561,6 +565,9 @@ There are certain things that the Linux kernel memory barriers do not guarantee: ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) ---------------------------------------- +[!] This section is marked as HISTORICAL: For more up-to-date information, +including how compiler transformations related to pointer comparisons can +sometimes cause problems, see Documentation/RCU/rcu_dereference.rst. As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for DEC Alpha, which means that about the only people who need to pay attention
On Fri, Aug 04, 2023 at 05:11:27AM +0000, Joel Fernandes wrote: > On Thu, Aug 03, 2023 at 11:52:06AM -0700, Paul E. McKenney wrote: > > On Thu, Aug 03, 2023 at 03:24:07AM +0000, Joel Fernandes (Google) wrote: > > > The compiler has the ability to cause misordering by destroying > > > address-dependency barriers if comparison operations are used. Add a > > > note about this to memory-barriers.txt and point to rcu-dereference.rst > > > for more information. > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > > > --- > > > Documentation/memory-barriers.txt | 5 +++++ > > > 1 file changed, 5 insertions(+) > > > > > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > > > index 06e14efd8662..acc8ec5ce563 100644 > > > --- a/Documentation/memory-barriers.txt > > > +++ b/Documentation/memory-barriers.txt > > > @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties: > > > variables such as READ_ONCE() and rcu_dereference() provide implicit > > > address-dependency barriers. > > > > > > + [!] Note that address dependency barriers can be destroyed by comparison > > > + of a pointer obtained by a marked accessor such as READ_ONCE() or > > > + rcu_dereference() with some value. For an example of this, see > > > + rcu_dereference.rst (part where the comparison of pointers is discussed). > > > > Hmmm... > > > > Given that this is in a section marked "historical" (for the old > > smp_read_barrier_depends() API), why not instead add a pointer to > > Documentation/RCU/rcu_dereference.rst to the beginning of the section, > > noted as the updated material? > > Sounds good. There's also another section in the same file on Address > dependency barriers (also marked historical). So something like the > following? Given a Signed-off-by and so forth, I would be happy to take this one. Thanx, Paul > ---8<----------------------- > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index acc8ec5ce563..ba50220716ca 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -396,6 +396,10 @@ Memory barriers come in four basic varieties: > > > (2) Address-dependency barriers (historical). > + [!] This section is marked as HISTORICAL: For more up-to-date > + information, including how compiler transformations related to pointer > + comparisons can sometimes cause problems, see > + Documentation/RCU/rcu_dereference.rst. > > An address-dependency barrier is a weaker form of read barrier. In the > case where two loads are performed such that the second depends on the > @@ -561,6 +565,9 @@ There are certain things that the Linux kernel memory barriers do not guarantee: > > ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) > ---------------------------------------- > +[!] This section is marked as HISTORICAL: For more up-to-date information, > +including how compiler transformations related to pointer comparisons can > +sometimes cause problems, see Documentation/RCU/rcu_dereference.rst. > > As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for > DEC Alpha, which means that about the only people who need to pay attention
On Fri, Aug 04, 2023 at 06:52:32AM -0700, Paul E. McKenney wrote: > On Fri, Aug 04, 2023 at 05:11:27AM +0000, Joel Fernandes wrote: > > On Thu, Aug 03, 2023 at 11:52:06AM -0700, Paul E. McKenney wrote: > > > On Thu, Aug 03, 2023 at 03:24:07AM +0000, Joel Fernandes (Google) wrote: > > > > The compiler has the ability to cause misordering by destroying > > > > address-dependency barriers if comparison operations are used. Add a > > > > note about this to memory-barriers.txt and point to rcu-dereference.rst > > > > for more information. > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > > > > --- > > > > Documentation/memory-barriers.txt | 5 +++++ > > > > 1 file changed, 5 insertions(+) > > > > > > > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > > > > index 06e14efd8662..acc8ec5ce563 100644 > > > > --- a/Documentation/memory-barriers.txt > > > > +++ b/Documentation/memory-barriers.txt > > > > @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties: > > > > variables such as READ_ONCE() and rcu_dereference() provide implicit > > > > address-dependency barriers. > > > > > > > > + [!] Note that address dependency barriers can be destroyed by comparison > > > > + of a pointer obtained by a marked accessor such as READ_ONCE() or > > > > + rcu_dereference() with some value. For an example of this, see > > > > + rcu_dereference.rst (part where the comparison of pointers is discussed). > > > > > > Hmmm... > > > > > > Given that this is in a section marked "historical" (for the old > > > smp_read_barrier_depends() API), why not instead add a pointer to > > > Documentation/RCU/rcu_dereference.rst to the beginning of the section, > > > noted as the updated material? > > > > Sounds good. There's also another section in the same file on Address > > dependency barriers (also marked historical). So something like the > > following? > > Given a Signed-off-by and so forth, I would be happy to take this one. Thank you for helping me improve the docs, here it goes: ---8<----------------------- From: "Joel Fernandes (Google)" <joel@joelfernandes.org> Subject: [PATCH] docs: memory-barriers: Add note on compiler transformation and address deps The compiler has the ability to cause misordering by destroying address-dependency barriers if comparison operations are used. Add a note about this to memory-barriers.txt in the beginning of both the historical address-dependency sections and point to rcu-dereference.rst for more information. Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> --- Documentation/memory-barriers.txt | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index acc8ec5ce563..ba50220716ca 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -396,6 +396,10 @@ Memory barriers come in four basic varieties: (2) Address-dependency barriers (historical). + [!] This section is marked as HISTORICAL: For more up-to-date + information, including how compiler transformations related to pointer + comparisons can sometimes cause problems, see + Documentation/RCU/rcu_dereference.rst. An address-dependency barrier is a weaker form of read barrier. In the case where two loads are performed such that the second depends on the @@ -561,6 +565,9 @@ There are certain things that the Linux kernel memory barriers do not guarantee: ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) ---------------------------------------- +[!] This section is marked as HISTORICAL: For more up-to-date information, +including how compiler transformations related to pointer comparisons can +sometimes cause problems, see Documentation/RCU/rcu_dereference.rst. As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for DEC Alpha, which means that about the only people who need to pay attention
On Fri, Aug 04, 2023 at 04:27:45PM +0000, Joel Fernandes wrote: > On Fri, Aug 04, 2023 at 06:52:32AM -0700, Paul E. McKenney wrote: > > On Fri, Aug 04, 2023 at 05:11:27AM +0000, Joel Fernandes wrote: > > > On Thu, Aug 03, 2023 at 11:52:06AM -0700, Paul E. McKenney wrote: > > > > On Thu, Aug 03, 2023 at 03:24:07AM +0000, Joel Fernandes (Google) wrote: > > > > > The compiler has the ability to cause misordering by destroying > > > > > address-dependency barriers if comparison operations are used. Add a > > > > > note about this to memory-barriers.txt and point to rcu-dereference.rst > > > > > for more information. > > > > > > > > > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> > > > > > --- > > > > > Documentation/memory-barriers.txt | 5 +++++ > > > > > 1 file changed, 5 insertions(+) > > > > > > > > > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > > > > > index 06e14efd8662..acc8ec5ce563 100644 > > > > > --- a/Documentation/memory-barriers.txt > > > > > +++ b/Documentation/memory-barriers.txt > > > > > @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties: > > > > > variables such as READ_ONCE() and rcu_dereference() provide implicit > > > > > address-dependency barriers. > > > > > > > > > > + [!] Note that address dependency barriers can be destroyed by comparison > > > > > + of a pointer obtained by a marked accessor such as READ_ONCE() or > > > > > + rcu_dereference() with some value. For an example of this, see > > > > > + rcu_dereference.rst (part where the comparison of pointers is discussed). > > > > > > > > Hmmm... > > > > > > > > Given that this is in a section marked "historical" (for the old > > > > smp_read_barrier_depends() API), why not instead add a pointer to > > > > Documentation/RCU/rcu_dereference.rst to the beginning of the section, > > > > noted as the updated material? > > > > > > Sounds good. There's also another section in the same file on Address > > > dependency barriers (also marked historical). So something like the > > > following? > > > > Given a Signed-off-by and so forth, I would be happy to take this one. > > Thank you for helping me improve the docs, here it goes: > > ---8<----------------------- > > From: "Joel Fernandes (Google)" <joel@joelfernandes.org> > Subject: [PATCH] docs: memory-barriers: Add note on compiler transformation > and address deps > > The compiler has the ability to cause misordering by destroying > address-dependency barriers if comparison operations are used. Add a > note about this to memory-barriers.txt in the beginning of both the > historical address-dependency sections and point to rcu-dereference.rst > for more information. > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Queued and pushed, thank you! Thanx, Paul > --- > Documentation/memory-barriers.txt | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index acc8ec5ce563..ba50220716ca 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -396,6 +396,10 @@ Memory barriers come in four basic varieties: > > > (2) Address-dependency barriers (historical). > + [!] This section is marked as HISTORICAL: For more up-to-date > + information, including how compiler transformations related to pointer > + comparisons can sometimes cause problems, see > + Documentation/RCU/rcu_dereference.rst. > > An address-dependency barrier is a weaker form of read barrier. In the > case where two loads are performed such that the second depends on the > @@ -561,6 +565,9 @@ There are certain things that the Linux kernel memory barriers do not guarantee: > > ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) > ---------------------------------------- > +[!] This section is marked as HISTORICAL: For more up-to-date information, > +including how compiler transformations related to pointer comparisons can > +sometimes cause problems, see Documentation/RCU/rcu_dereference.rst. > > As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for > DEC Alpha, which means that about the only people who need to pay attention > -- > 2.41.0.585.gd2178a4bd4-goog >
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 06e14efd8662..acc8ec5ce563 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -435,6 +435,11 @@ Memory barriers come in four basic varieties: variables such as READ_ONCE() and rcu_dereference() provide implicit address-dependency barriers. + [!] Note that address dependency barriers can be destroyed by comparison + of a pointer obtained by a marked accessor such as READ_ONCE() or + rcu_dereference() with some value. For an example of this, see + rcu_dereference.rst (part where the comparison of pointers is discussed). + (3) Read (or load) memory barriers. A read barrier is an address-dependency barrier plus a guarantee that all
The compiler has the ability to cause misordering by destroying address-dependency barriers if comparison operations are used. Add a note about this to memory-barriers.txt and point to rcu-dereference.rst for more information. Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> --- Documentation/memory-barriers.txt | 5 +++++ 1 file changed, 5 insertions(+)