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Message-Id: <1449702902-2520-6-git-send-email-paulmck@linux.vnet.ibm.com> Date: Wed, 9 Dec 2015 15:15:00 -0800 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: linux-kernel@...r.kernel.org Cc: mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Subject: [PATCH v2 tip/core/rcu 6/8] documentation: Clarify RCU memory barriers and requirements The RCU requirements do not make it absolutely clear that the memory-barrier requirements are not intended to replace the fundamental requirement that all pre-existing RCU readers complete before a grace period completes. This commit therefore pulls the memory-barrier requirements into a separate section and explicitly calls out the relationship between the memory-barrier requirements and the fundamental requirement. Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> --- .../RCU/Design/Requirements/Requirements.html | 66 +++++++++++++--------- .../RCU/Design/Requirements/Requirements.htmlx | 66 +++++++++++++--------- 2 files changed, 78 insertions(+), 54 deletions(-) diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index ab513ed229d7..96cdcf7195d5 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -80,6 +80,8 @@ These are: Grace-Period Guarantee</a> <li> <a href="#Publish-Subscribe Guarantee"> Publish-Subscribe Guarantee</a> +<li> <a href="#Memory-Barrier Guarantees"> + Memory-Barrier Guarantees</a> <li> <a href="#RCU Primitives Guaranteed to Execute Unconditionally"> RCU Primitives Guaranteed to Execute Unconditionally</a> <li> <a href="#Guaranteed Read-to-Write Upgrade"> @@ -499,9 +501,37 @@ might the compiler make use of? <br><a href="#qq4answer">Answer</a> <p> -This simple linked-data-structure scenario clearly demonstrates the need -for RCU's stringent memory-ordering guarantees on systems with more than -one CPU: +In short, RCU's publish-subscribe guarantee is provided by the combination +of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>. +This guarantee allows data elements to be safely added to RCU-protected +linked data structures without disrupting RCU readers. +This guarantee can be used in combination with the grace-period +guarantee to also allow data elements to be removed from RCU-protected +linked data structures, again without disrupting RCU readers. + +<p> +This guarantee was only partially premeditated. +DYNIX/ptx used an explicit memory barrier for publication, but had nothing +resembling <tt>rcu_dereference()</tt> for subscription, nor did it +have anything resembling the <tt>smp_read_barrier_depends()</tt> +that was later subsumed into <tt>rcu_dereference()</tt>. +The need for these operations made itself known quite suddenly at a +late-1990s meeting with the DEC Alpha architects, back in the days when +DEC was still a free-standing company. +It took the Alpha architects a good hour to convince me that any sort +of barrier would ever be needed, and it then took me a good <i>two</i> hours +to convince them that their documentation did not make this point clear. +More recent work with the C and C++ standards committees have provided +much education on tricks and traps from the compiler. +In short, compilers were much less tricky in the early 1990s, but in +2015, don't even think about omitting <tt>rcu_dereference()</tt>! + +<h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3> + +<p> +The previous section's simple linked-data-structure scenario clearly +demonstrates the need for RCU's stringent memory-ordering guarantees on +systems with more than one CPU: <ol> <li> Each CPU that has an RCU read-side critical section that @@ -554,30 +584,12 @@ Are all these memory barriers <i> really</i> required? <br><a href="#qq6answer">Answer</a> <p> -In short, RCU's publish-subscribe guarantee is provided by the combination -of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>. -This guarantee allows data elements to be safely added to RCU-protected -linked data structures without disrupting RCU readers. -This guarantee can be used in combination with the grace-period -guarantee to also allow data elements to be removed from RCU-protected -linked data structures, again without disrupting RCU readers. - -<p> -This guarantee was only partially premeditated. -DYNIX/ptx used an explicit memory barrier for publication, but had nothing -resembling <tt>rcu_dereference()</tt> for subscription, nor did it -have anything resembling the <tt>smp_read_barrier_depends()</tt> -that was later subsumed into <tt>rcu_dereference()</tt>. -The need for these operations made itself known quite suddenly at a -late-1990s meeting with the DEC Alpha architects, back in the days when -DEC was still a free-standing company. -It took the Alpha architects a good hour to convince me that any sort -of barrier would ever be needed, and it then took me a good <i>two</i> hours -to convince them that their documentation did not make this point clear. -More recent work with the C and C++ standards committees have provided -much education on tricks and traps from the compiler. -In short, compilers were much less tricky in the early 1990s, but in -2015, don't even think about omitting <tt>rcu_dereference()</tt>! +Note that these memory-barrier requirements do not replace the fundamental +RCU requirement that a grace period wait for all pre-existing readers. +On the contrary, the memory barriers called out in this section must operate in +such a way as to <i>enforce</i> this fundamental requirement. +Of course, different implementations enforce this requirement in different +ways, but enforce it they must. <h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3> diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx index f7c817f235e0..2d0cd90987f6 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx @@ -78,6 +78,8 @@ These are: Grace-Period Guarantee</a> <li> <a href="#Publish-Subscribe Guarantee"> Publish-Subscribe Guarantee</a> +<li> <a href="#Memory-Barrier Guarantees"> + Memory-Barrier Guarantees</a> <li> <a href="#RCU Primitives Guaranteed to Execute Unconditionally"> RCU Primitives Guaranteed to Execute Unconditionally</a> <li> <a href="#Guaranteed Read-to-Write Upgrade"> @@ -539,9 +541,37 @@ either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>. <p>@@QQE@@ <p> -This simple linked-data-structure scenario clearly demonstrates the need -for RCU's stringent memory-ordering guarantees on systems with more than -one CPU: +In short, RCU's publish-subscribe guarantee is provided by the combination +of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>. +This guarantee allows data elements to be safely added to RCU-protected +linked data structures without disrupting RCU readers. +This guarantee can be used in combination with the grace-period +guarantee to also allow data elements to be removed from RCU-protected +linked data structures, again without disrupting RCU readers. + +<p> +This guarantee was only partially premeditated. +DYNIX/ptx used an explicit memory barrier for publication, but had nothing +resembling <tt>rcu_dereference()</tt> for subscription, nor did it +have anything resembling the <tt>smp_read_barrier_depends()</tt> +that was later subsumed into <tt>rcu_dereference()</tt>. +The need for these operations made itself known quite suddenly at a +late-1990s meeting with the DEC Alpha architects, back in the days when +DEC was still a free-standing company. +It took the Alpha architects a good hour to convince me that any sort +of barrier would ever be needed, and it then took me a good <i>two</i> hours +to convince them that their documentation did not make this point clear. +More recent work with the C and C++ standards committees have provided +much education on tricks and traps from the compiler. +In short, compilers were much less tricky in the early 1990s, but in +2015, don't even think about omitting <tt>rcu_dereference()</tt>! + +<h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3> + +<p> +The previous section's simple linked-data-structure scenario clearly +demonstrates the need for RCU's stringent memory-ordering guarantees on +systems with more than one CPU: <ol> <li> Each CPU that has an RCU read-side critical section that @@ -653,30 +683,12 @@ adhered to the as-if rule than it is to actually adhere to it! <p>@@QQE@@ <p> -In short, RCU's publish-subscribe guarantee is provided by the combination -of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>. -This guarantee allows data elements to be safely added to RCU-protected -linked data structures without disrupting RCU readers. -This guarantee can be used in combination with the grace-period -guarantee to also allow data elements to be removed from RCU-protected -linked data structures, again without disrupting RCU readers. - -<p> -This guarantee was only partially premeditated. -DYNIX/ptx used an explicit memory barrier for publication, but had nothing -resembling <tt>rcu_dereference()</tt> for subscription, nor did it -have anything resembling the <tt>smp_read_barrier_depends()</tt> -that was later subsumed into <tt>rcu_dereference()</tt>. -The need for these operations made itself known quite suddenly at a -late-1990s meeting with the DEC Alpha architects, back in the days when -DEC was still a free-standing company. -It took the Alpha architects a good hour to convince me that any sort -of barrier would ever be needed, and it then took me a good <i>two</i> hours -to convince them that their documentation did not make this point clear. -More recent work with the C and C++ standards committees have provided -much education on tricks and traps from the compiler. -In short, compilers were much less tricky in the early 1990s, but in -2015, don't even think about omitting <tt>rcu_dereference()</tt>! +Note that these memory-barrier requirements do not replace the fundamental +RCU requirement that a grace period wait for all pre-existing readers. +On the contrary, the memory barriers called out in this section must operate in +such a way as to <i>enforce</i> this fundamental requirement. +Of course, different implementations enforce this requirement in different +ways, but enforce it they must. <h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3> -- 2.5.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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